From 5b7c4cabbb65f5c469464da6c5f614cbd7f730f2 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Tue, 21 Feb 2023 18:24:12 -0800 Subject: Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next Pull networking updates from Jakub Kicinski: "Core: - Add dedicated kmem_cache for typical/small skb->head, avoid having to access struct page at kfree time, and improve memory use. - Introduce sysctl to set default RPS configuration for new netdevs. - Define Netlink protocol specification format which can be used to describe messages used by each family and auto-generate parsers. Add tools for generating kernel data structures and uAPI headers. - Expose all net/core sysctls inside netns. - Remove 4s sleep in netpoll if carrier is instantly detected on boot. - Add configurable limit of MDB entries per port, and port-vlan. - Continue populating drop reasons throughout the stack. - Retire a handful of legacy Qdiscs and classifiers. Protocols: - Support IPv4 big TCP (TSO frames larger than 64kB). - Add IP_LOCAL_PORT_RANGE socket option, to control local port range on socket by socket basis. - Track and report in procfs number of MPTCP sockets used. - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path manager. - IPv6: don't check net.ipv6.route.max_size and rely on garbage collection to free memory (similarly to IPv4). - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986). - ICMP: add per-rate limit counters. - Add support for user scanning requests in ieee802154. - Remove static WEP support. - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate reporting. - WiFi 7 EHT channel puncturing support (client & AP). BPF: - Add a rbtree data structure following the "next-gen data structure" precedent set by recently added linked list, that is, by using kfunc + kptr instead of adding a new BPF map type. - Expose XDP hints via kfuncs with initial support for RX hash and timestamp metadata. - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to better support decap on GRE tunnel devices not operating in collect metadata. - Improve x86 JIT's codegen for PROBE_MEM runtime error checks. - Remove the need for trace_printk_lock for bpf_trace_printk and bpf_trace_vprintk helpers. - Extend libbpf's bpf_tracing.h support for tracing arguments of kprobes/uprobes and syscall as a special case. - Significantly reduce the search time for module symbols by livepatch and BPF. - Enable cpumasks to be used as kptrs, which is useful for tracing programs tracking which tasks end up running on which CPUs in different time intervals. - Add support for BPF trampoline on s390x and riscv64. - Add capability to export the XDP features supported by the NIC. - Add __bpf_kfunc tag for marking kernel functions as kfuncs. - Add cgroup.memory=nobpf kernel parameter option to disable BPF memory accounting for container environments. Netfilter: - Remove the CLUSTERIP target. It has been marked as obsolete for years, and we still have WARN splats wrt races of the out-of-band /proc interface installed by this target. - Add 'destroy' commands to nf_tables. They are identical to the existing 'delete' commands, but do not return an error if the referenced object (set, chain, rule...) did not exist. Driver API: - Improve cpumask_local_spread() locality to help NICs set the right IRQ affinity on AMD platforms. - Separate C22 and C45 MDIO bus transactions more clearly. - Introduce new DCB table to control DSCP rewrite on egress. - Support configuration of Physical Layer Collision Avoidance (PLCA) Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of shared medium Ethernet. - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing preemption of low priority frames by high priority frames. - Add support for controlling MACSec offload using netlink SET. - Rework devlink instance refcounts to allow registration and de-registration under the instance lock. Split the code into multiple files, drop some of the unnecessarily granular locks and factor out common parts of netlink operation handling. - Add TX frame aggregation parameters (for USB drivers). - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning messages with notifications for debug. - Allow offloading of UDP NEW connections via act_ct. - Add support for per action HW stats in TC. - Support hardware miss to TC action (continue processing in SW from a specific point in the action chain). - Warn if old Wireless Extension user space interface is used with modern cfg80211/mac80211 drivers. Do not support Wireless Extensions for Wi-Fi 7 devices at all. Everyone should switch to using nl80211 interface instead. - Improve the CAN bit timing configuration. Use extack to return error messages directly to user space, update the SJW handling, including the definition of a new default value that will benefit CAN-FD controllers, by increasing their oscillator tolerance. New hardware / drivers: - Ethernet: - nVidia BlueField-3 support (control traffic driver) - Ethernet support for imx93 SoCs - Motorcomm yt8531 gigabit Ethernet PHY - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA) - Microchip LAN8841 PHY (incl. cable diagnostics and PTP) - Amlogic gxl MDIO mux - WiFi: - RealTek RTL8188EU (rtl8xxxu) - Qualcomm Wi-Fi 7 devices (ath12k) - CAN: - Renesas R-Car V4H Drivers: - Bluetooth: - Set Per Platform Antenna Gain (PPAG) for Intel controllers. - Ethernet NICs: - Intel (1G, igc): - support TSN / Qbv / packet scheduling features of i226 model - Intel (100G, ice): - use GNSS subsystem instead of TTY - multi-buffer XDP support - extend support for GPIO pins to E823 devices - nVidia/Mellanox: - update the shared buffer configuration on PFC commands - implement PTP adjphase function for HW offset control - TC support for Geneve and GRE with VF tunnel offload - more efficient crypto key management method - multi-port eswitch support - Netronome/Corigine: - add DCB IEEE support - support IPsec offloading for NFP3800 - Freescale/NXP (enetc): - support XDP_REDIRECT for XDP non-linear buffers - improve reconfig, avoid link flap and waiting for idle - support MAC Merge layer - Other NICs: - sfc/ef100: add basic devlink support for ef100 - ionic: rx_push mode operation (writing descriptors via MMIO) - bnxt: use the auxiliary bus abstraction for RDMA - r8169: disable ASPM and reset bus in case of tx timeout - cpsw: support QSGMII mode for J721e CPSW9G - cpts: support pulse-per-second output - ngbe: add an mdio bus driver - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing - r8152: handle devices with FW with NCM support - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation - virtio-net: support multi buffer XDP - virtio/vsock: replace virtio_vsock_pkt with sk_buff - tsnep: XDP support - Ethernet high-speed switches: - nVidia/Mellanox (mlxsw): - add support for latency TLV (in FW control messages) - Microchip (sparx5): - separate explicit and implicit traffic forwarding rules, make the implicit rules always active - add support for egress DSCP rewrite - IS0 VCAP support (Ingress Classification) - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS etc.) - ES2 VCAP support (Egress Access Control) - support for Per-Stream Filtering and Policing (802.1Q, 8.6.5.1) - Ethernet embedded switches: - Marvell (mv88e6xxx): - add MAB (port auth) offload support - enable PTP receive for mv88e6390 - NXP (ocelot): - support MAC Merge layer - support for the the vsc7512 internal copper phys - Microchip: - lan9303: convert to PHYLINK - lan966x: support TC flower filter statistics - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x - lan937x: support Credit Based Shaper configuration - ksz9477: support Energy Efficient Ethernet - other: - qca8k: convert to regmap read/write API, use bulk operations - rswitch: Improve TX timestamp accuracy - Intel WiFi (iwlwifi): - EHT (Wi-Fi 7) rate reporting - STEP equalizer support: transfer some STEP (connection to radio on platforms with integrated wifi) related parameters from the BIOS to the firmware. - Qualcomm 802.11ax WiFi (ath11k): - IPQ5018 support - Fine Timing Measurement (FTM) responder role support - channel 177 support - MediaTek WiFi (mt76): - per-PHY LED support - mt7996: EHT (Wi-Fi 7) support - Wireless Ethernet Dispatch (WED) reset support - switch to using page pool allocator - RealTek WiFi (rtw89): - support new version of Bluetooth co-existance - Mobile: - rmnet: support TX aggregation" * tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits) page_pool: add a comment explaining the fragment counter usage net: ethtool: fix __ethtool_dev_mm_supported() implementation ethtool: pse-pd: Fix double word in comments xsk: add linux/vmalloc.h to xsk.c sefltests: netdevsim: wait for devlink instance after netns removal selftest: fib_tests: Always cleanup before exit net/mlx5e: Align IPsec ASO result memory to be as required by hardware net/mlx5e: TC, Set CT miss to the specific ct action instance net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG net/mlx5: Refactor tc miss handling to a single function net/mlx5: Kconfig: Make tc offload depend on tc skb extension net/sched: flower: Support hardware miss to tc action net/sched: flower: Move filter handle initialization earlier net/sched: cls_api: Support hardware miss to tc action net/sched: Rename user cookie and act cookie sfc: fix builds without CONFIG_RTC_LIB sfc: clean up some inconsistent indentings net/mlx4_en: Introduce flexible array to silence overflow warning net: lan966x: Fix possible deadlock inside PTP net/ulp: Remove redundant ->clone() test in inet_clone_ulp(). ... --- Documentation/security/self-protection.rst | 316 +++++++++++++++++++++++++++++ 1 file changed, 316 insertions(+) create mode 100644 Documentation/security/self-protection.rst (limited to 'Documentation/security/self-protection.rst') diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst new file mode 100644 index 000000000..910668e66 --- /dev/null +++ b/Documentation/security/self-protection.rst @@ -0,0 +1,316 @@ +====================== +Kernel Self-Protection +====================== + +Kernel self-protection is the design and implementation of systems and +structures within the Linux kernel to protect against security flaws in +the kernel itself. This covers a wide range of issues, including removing +entire classes of bugs, blocking security flaw exploitation methods, +and actively detecting attack attempts. Not all topics are explored in +this document, but it should serve as a reasonable starting point and +answer any frequently asked questions. (Patches welcome, of course!) + +In the worst-case scenario, we assume an unprivileged local attacker +has arbitrary read and write access to the kernel's memory. In many +cases, bugs being exploited will not provide this level of access, +but with systems in place that defend against the worst case we'll +cover the more limited cases as well. A higher bar, and one that should +still be kept in mind, is protecting the kernel against a _privileged_ +local attacker, since the root user has access to a vastly increased +attack surface. (Especially when they have the ability to load arbitrary +kernel modules.) + +The goals for successful self-protection systems would be that they +are effective, on by default, require no opt-in by developers, have no +performance impact, do not impede kernel debugging, and have tests. It +is uncommon that all these goals can be met, but it is worth explicitly +mentioning them, since these aspects need to be explored, dealt with, +and/or accepted. + + +Attack Surface Reduction +======================== + +The most fundamental defense against security exploits is to reduce the +areas of the kernel that can be used to redirect execution. This ranges +from limiting the exposed APIs available to userspace, making in-kernel +APIs hard to use incorrectly, minimizing the areas of writable kernel +memory, etc. + +Strict kernel memory permissions +-------------------------------- + +When all of kernel memory is writable, it becomes trivial for attacks +to redirect execution flow. To reduce the availability of these targets +the kernel needs to protect its memory with a tight set of permissions. + +Executable code and read-only data must not be writable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Any areas of the kernel with executable memory must not be writable. +While this obviously includes the kernel text itself, we must consider +all additional places too: kernel modules, JIT memory, etc. (There are +temporary exceptions to this rule to support things like instruction +alternatives, breakpoints, kprobes, etc. If these must exist in a +kernel, they are implemented in a way where the memory is temporarily +made writable during the update, and then returned to the original +permissions.) + +In support of this are ``CONFIG_STRICT_KERNEL_RWX`` and +``CONFIG_STRICT_MODULE_RWX``, which seek to make sure that code is not +writable, data is not executable, and read-only data is neither writable +nor executable. + +Most architectures have these options on by default and not user selectable. +For some architectures like arm that wish to have these be selectable, +the architecture Kconfig can select ARCH_OPTIONAL_KERNEL_RWX to enable +a Kconfig prompt. ``CONFIG_ARCH_OPTIONAL_KERNEL_RWX_DEFAULT`` determines +the default setting when ARCH_OPTIONAL_KERNEL_RWX is enabled. + +Function pointers and sensitive variables must not be writable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Vast areas of kernel memory contain function pointers that are looked +up by the kernel and used to continue execution (e.g. descriptor/vector +tables, file/network/etc operation structures, etc). The number of these +variables must be reduced to an absolute minimum. + +Many such variables can be made read-only by setting them "const" +so that they live in the .rodata section instead of the .data section +of the kernel, gaining the protection of the kernel's strict memory +permissions as described above. + +For variables that are initialized once at ``__init`` time, these can +be marked with the ``__ro_after_init`` attribute. + +What remains are variables that are updated rarely (e.g. GDT). These +will need another infrastructure (similar to the temporary exceptions +made to kernel code mentioned above) that allow them to spend the rest +of their lifetime read-only. (For example, when being updated, only the +CPU thread performing the update would be given uninterruptible write +access to the memory.) + +Segregation of kernel memory from userspace memory +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The kernel must never execute userspace memory. The kernel must also never +access userspace memory without explicit expectation to do so. These +rules can be enforced either by support of hardware-based restrictions +(x86's SMEP/SMAP, ARM's PXN/PAN) or via emulation (ARM's Memory Domains). +By blocking userspace memory in this way, execution and data parsing +cannot be passed to trivially-controlled userspace memory, forcing +attacks to operate entirely in kernel memory. + +Reduced access to syscalls +-------------------------- + +One trivial way to eliminate many syscalls for 64-bit systems is building +without ``CONFIG_COMPAT``. However, this is rarely a feasible scenario. + +The "seccomp" system provides an opt-in feature made available to +userspace, which provides a way to reduce the number of kernel entry +points available to a running process. This limits the breadth of kernel +code that can be reached, possibly reducing the availability of a given +bug to an attack. + +An area of improvement would be creating viable ways to keep access to +things like compat, user namespaces, BPF creation, and perf limited only +to trusted processes. This would keep the scope of kernel entry points +restricted to the more regular set of normally available to unprivileged +userspace. + +Restricting access to kernel modules +------------------------------------ + +The kernel should never allow an unprivileged user the ability to +load specific kernel modules, since that would provide a facility to +unexpectedly extend the available attack surface. (The on-demand loading +of modules via their predefined subsystems, e.g. MODULE_ALIAS_*, is +considered "expected" here, though additional consideration should be +given even to these.) For example, loading a filesystem module via an +unprivileged socket API is nonsense: only the root or physically local +user should trigger filesystem module loading. (And even this can be up +for debate in some scenarios.) + +To protect against even privileged users, systems may need to either +disable module loading entirely (e.g. monolithic kernel builds or +modules_disabled sysctl), or provide signed modules (e.g. +``CONFIG_MODULE_SIG_FORCE``, or dm-crypt with LoadPin), to keep from having +root load arbitrary kernel code via the module loader interface. + + +Memory integrity +================ + +There are many memory structures in the kernel that are regularly abused +to gain execution control during an attack, By far the most commonly +understood is that of the stack buffer overflow in which the return +address stored on the stack is overwritten. Many other examples of this +kind of attack exist, and protections exist to defend against them. + +Stack buffer overflow +--------------------- + +The classic stack buffer overflow involves writing past the expected end +of a variable stored on the stack, ultimately writing a controlled value +to the stack frame's stored return address. The most widely used defense +is the presence of a stack canary between the stack variables and the +return address (``CONFIG_STACKPROTECTOR``), which is verified just before +the function returns. Other defenses include things like shadow stacks. + +Stack depth overflow +-------------------- + +A less well understood attack is using a bug that triggers the +kernel to consume stack memory with deep function calls or large stack +allocations. With this attack it is possible to write beyond the end of +the kernel's preallocated stack space and into sensitive structures. Two +important changes need to be made for better protections: moving the +sensitive thread_info structure elsewhere, and adding a faulting memory +hole at the bottom of the stack to catch these overflows. + +Heap memory integrity +--------------------- + +The structures used to track heap free lists can be sanity-checked during +allocation and freeing to make sure they aren't being used to manipulate +other memory areas. + +Counter integrity +----------------- + +Many places in the kernel use atomic counters to track object references +or perform similar lifetime management. When these counters can be made +to wrap (over or under) this traditionally exposes a use-after-free +flaw. By trapping atomic wrapping, this class of bug vanishes. + +Size calculation overflow detection +----------------------------------- + +Similar to counter overflow, integer overflows (usually size calculations) +need to be detected at runtime to kill this class of bug, which +traditionally leads to being able to write past the end of kernel buffers. + + +Probabilistic defenses +====================== + +While many protections can be considered deterministic (e.g. read-only +memory cannot be written to), some protections provide only statistical +defense, in that an attack must gather enough information about a +running system to overcome the defense. While not perfect, these do +provide meaningful defenses. + +Canaries, blinding, and other secrets +------------------------------------- + +It should be noted that things like the stack canary discussed earlier +are technically statistical defenses, since they rely on a secret value, +and such values may become discoverable through an information exposure +flaw. + +Blinding literal values for things like JITs, where the executable +contents may be partially under the control of userspace, need a similar +secret value. + +It is critical that the secret values used must be separate (e.g. +different canary per stack) and high entropy (e.g. is the RNG actually +working?) in order to maximize their success. + +Kernel Address Space Layout Randomization (KASLR) +------------------------------------------------- + +Since the location of kernel memory is almost always instrumental in +mounting a successful attack, making the location non-deterministic +raises the difficulty of an exploit. (Note that this in turn makes +the value of information exposures higher, since they may be used to +discover desired memory locations.) + +Text and module base +~~~~~~~~~~~~~~~~~~~~ + +By relocating the physical and virtual base address of the kernel at +boot-time (``CONFIG_RANDOMIZE_BASE``), attacks needing kernel code will be +frustrated. Additionally, offsetting the module loading base address +means that even systems that load the same set of modules in the same +order every boot will not share a common base address with the rest of +the kernel text. + +Stack base +~~~~~~~~~~ + +If the base address of the kernel stack is not the same between processes, +or even not the same between syscalls, targets on or beyond the stack +become more difficult to locate. + +Dynamic memory base +~~~~~~~~~~~~~~~~~~~ + +Much of the kernel's dynamic memory (e.g. kmalloc, vmalloc, etc) ends up +being relatively deterministic in layout due to the order of early-boot +initializations. If the base address of these areas is not the same +between boots, targeting them is frustrated, requiring an information +exposure specific to the region. + +Structure layout +~~~~~~~~~~~~~~~~ + +By performing a per-build randomization of the layout of sensitive +structures, attacks must either be tuned to known kernel builds or expose +enough kernel memory to determine structure layouts before manipulating +them. + + +Preventing Information Exposures +================================ + +Since the locations of sensitive structures are the primary target for +attacks, it is important to defend against exposure of both kernel memory +addresses and kernel memory contents (since they may contain kernel +addresses or other sensitive things like canary values). + +Kernel addresses +---------------- + +Printing kernel addresses to userspace leaks sensitive information about +the kernel memory layout. Care should be exercised when using any printk +specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb] +in certain circumstances [*]). Any file written to using one of these +specifiers should be readable only by privileged processes. + +Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1 +addresses printed with the specifier %p are hashed before printing. + +[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is +printed. If KALLSYMS is not enabled the raw address is printed. + +Unique identifiers +------------------ + +Kernel memory addresses must never be used as identifiers exposed to +userspace. Instead, use an atomic counter, an idr, or similar unique +identifier. + +Memory initialization +--------------------- + +Memory copied to userspace must always be fully initialized. If not +explicitly memset(), this will require changes to the compiler to make +sure structure holes are cleared. + +Memory poisoning +---------------- + +When releasing memory, it is best to poison the contents, to avoid reuse +attacks that rely on the old contents of memory. E.g., clear stack on a +syscall return (``CONFIG_GCC_PLUGIN_STACKLEAK``), wipe heap memory on a +free. This frustrates many uninitialized variable attacks, stack content +exposures, heap content exposures, and use-after-free attacks. + +Destination tracking +-------------------- + +To help kill classes of bugs that result in kernel addresses being +written to userspace, the destination of writes needs to be tracked. If +the buffer is destined for userspace (e.g. seq_file backed ``/proc`` files), +it should automatically censor sensitive values. -- cgit v1.2.3