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Lu" , Paul Walmsley , Palmer Dabbelt , Albert Ou , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, Mark Brown X-Mailer: b4 0.13-dev-034f2 X-Developer-Signature: v=1; a=openpgp-sha256; l=10578; i=broonie@kernel.org; h=from:subject:message-id; bh=kzDW22+Ip55uKZ9DLH3zSCFrVc4e69ndgeGIo6Fkn1s=; b=owEBbQGS/pANAwAKASTWi3JdVIfQAcsmYgBk5L/ylMhJ5pTw69sly3WFG/1VzJ6x9vUBrkBCcmX9 7B/ELc+JATMEAAEKAB0WIQSt5miqZ1cYtZ/in+ok1otyXVSH0AUCZOS/8gAKCRAk1otyXVSH0OHdB/ 0b9r4N4jkywqRms6FpCjH8HyGlNDdo2N8KvzIIuuTICSMWlsKqo33mD6ce16VCaR3Vj9hHA60HDmpl 8/NaFAj6lbwo9NoTenZjpfdkXCcy5zsgjv1iCVfWaGrujA5/UKYtCE5gbH2RDNiM6r2RW3Uy1Qfl3M yYkqWNqY/EFY/5MelCrOszeCcL3Q5pIsjc5RByLVt6SJCkyEJOTluyN6RWZljshqpzvTr/HEktkf4T mjVJQNNobXqAddvEdd8jCAge/gHRe8jiozgDpjadyJfEMuyeVnLaR7GFfSTyuNAPTxEWObJKh1xAit 4sX54t6rU3gIEEpUOadZnPARJmwjSN X-Developer-Key: i=broonie@kernel.org; a=openpgp; fpr=3F2568AAC26998F9E813A1C5C3F436CA30F5D8EB X-Rspamd-Queue-Id: 939E61C003B X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: zrfe5txzttfxfwgnhxnsjn94g44ch7xp X-HE-Tag: 1692713009-992913 X-HE-Meta: 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 zcyfXPKP 3I79mdy8uhbM1Ku/5FZSj54pDX0GvEdCmDl8XSF5zN1iTjT9hUi4tx6l0x3Ln6bcno8/gKYrgD2R+16s9ozG+feoU0xH9VmgK6TPrbSjyHpXEEYNOuPvZYMSON4tmpNNUeYjPcDVSX4jhPe8xChhz3mj6giSj3SqL0CdEdnr6m5XICbKbboq50LA9WOla4OkXAa5oi+gZm4L0nYuGEcCSy/vgTSKnx0MU/cqEJNntQfVUeN2O0A2ktlZLN4rbniQuwZVvbLsvniPNqWE5lN1LFyOsmpBEj7xFcGrcbcxe122Tqve7j105BqvA3ViRdDFHHDy4hzQZXvtIttAzryc9hKGB6lce1sxF9aRkm8OfAbtXdL9iqdej3Qg7DA1kiXN2cEt/8roGZrwcMJy375l6ZAj8uciuSqdqHZE4TAdh6MYCPgE= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Add some documentation of the userspace ABI for Guarded Control Stacks. Signed-off-by: Mark Brown --- Documentation/arch/arm64/gcs.rst | 233 +++++++++++++++++++++++++++++++++++++ Documentation/arch/arm64/index.rst | 1 + 2 files changed, 234 insertions(+) diff --git a/Documentation/arch/arm64/gcs.rst b/Documentation/arch/arm64/gcs.rst new file mode 100644 index 000000000000..3ded850aa182 --- /dev/null +++ b/Documentation/arch/arm64/gcs.rst @@ -0,0 +1,233 @@ +=============================================== +Guarded Control Stack support for AArch64 Linux +=============================================== + +This document outlines briefly the interface provided to userspace by Linux in +order to support use of the ARM Guarded Control Stack (GCS) feature. + +This is an outline of the most important features and issues only and not +intended to be exhaustive. + + + +1. General +----------- + +* GCS is an architecture feature intended to provide greater protection + against return oriented programming (ROP) attacks and to simplify the + implementation of features that need to collect stack traces such as + profiling. + +* When GCS is enabled a separate guarded control stack is maintained by the + PE which is writeable only through specific GCS operations. This + stores the call stack only, when a procedure call instruction is + performed the current PC is pushed onto the GCS and on RET the + address in the LR is verified against that on the top of the GCS. + +* When active current GCS pointer is stored in the system register + GCSPR_EL0. This is readable by userspace but can only be updated + via specific GCS instructions. + +* The architecture provides instructions for switching between guarded + control stacks with checks to ensure that the new stack is a valid + target for switching. + +* The functionality of GCS is similar to that provided by the x86 Shadow + Stack feature, due to sharing of userspace interfaces the ABI refers to + shadow stacks rather than GCS. + +* Support for GCS is reported to userspace via HWCAP2_GCS in the aux vector + AT_HWCAP2 entry. + +* GCS is enabled per thread. While there is support for disabling GCS + at runtime this should be done with great care. + +* GCS memory access faults are reported as normal memory access faults. + +* GCS specific errors (those reported with EC 0x2d) will be reported as + SIGSEGV with a si_code of SEGV_CPERR (control protection error). + +* GCS is supported only for AArch64. + +* On systems where GCS is supported GCSPR_EL0 is always readable by EL0 + regardless of the GCS configuration for the thread. + +* The architecture supports enabling GCS without verifying that return values + in LR match those in the GCS, the LR will be ignored. This is not supported + by Linux. + +* EL0 GCS entries with bit 63 set are reserved for use, one such use is defined + below for signals and should be ignored when parsing the stack if not + understood. + + +2. Enabling and disabling Guarded Control Stacks +------------------------------------------------- + +* GCS is enabled and disabled for a thread via the PR_SET_SHADOW_STACK_STATUS + prctl(), this takes a single flags argument specifying which GCS features + should be used. + +* When set PR_SHADOW_STACK_ENABLE flag allocates a Guarded Control Stack + and enables GCS for the thread, enabling the functionality controlled by + GCSCRE0_EL1.{nTR, RVCHKEN, PCRSEL}. + +* When set the PR_SHADOW_STACK_PUSH flag enables the functionality controlled + by GCSCRE0_EL1.PUSHMEn, allowing explicit GCS pushes. + +* When set the PR_SHADOW_STACK_WRITE flag enables the functionality controlled + by GCSCRE0_EL1.STREn, allowing explicit stores to the Guarded Control Stack. + +* Any unknown flags will cause PR_SET_SHADOW_STACK_STATUS to return -EINVAL. + +* PR_LOCK_SHADOW_STACK_STATUS is passed a bitmask of features with the same + values as used for PR_SET_SHADOW_STACK_STATUS. Any future changes to the + status of the specified GCS mode bits will be rejected. + +* PR_LOCK_SHADOW_STACK_STATUS allows any bit to be locked, this allows + userspace to prevent changes to any future features. + +* There is no support for a process to remove a lock that has been set for + it. + +* PR_SET_SHADOW_STACK_STATUS and PR_LOCK_SHADOW_STACK_STATUS affect only the + thread that called them, any other running threads will be unaffected. + +* New threads inherit the GCS configuration of the thread that created them. + +* GCS is disabled on exec(). + +* The current GCS configuration for a thread may be read with the + PR_GET_SHADOW_STACK_STATUS prctl(), this returns the same flags that + are passed to PR_SET_SHADOW_STACK_STATUS. + +* If GCS is disabled for a thread after having previously been enabled then + the stack will remain allocated for the lifetime of the thread. At present + any attempt to reenable GCS for the thread will be rejected, this may be + revisited in future. + +* It should be noted that since enabling GCS will result in GCS becoming + active immediately it is not normally possible to return from the function + that invoked the prctl() that enabled GCS. It is expected that the normal + usage will be that GCS is enabled very early in execution of a program. + + + +3. Allocation of Guarded Control Stacks +---------------------------------------- + +* When GCS is enabled for a thread a new Guarded Control Stack will be + allocated for it of size RLIMIT_STACK or 4 gigabytes, whichever is + smaller. + +* When a new thread is created by a thread which has GCS enabled then a + new Guarded Control Stack will be allocated for the new thread with + half the size of the standard stack. + +* When a stack is allocated by enabling GCS or during thread creation then + the top 8 bytes of the stack will be initialised to 0 and GCSPR_EL0 will + be set to point to the address of this 0 value, this can be used to + detect the top of the stack. + +* Additional Guarded Control Stacks can be allocated using the + map_shadow_stack() system call. + +* Stacks allocated using map_shadow_stack() can optionally have an end of + stack marker and cap placed at the top of the stack. If the flag + SHADOW_STACK_SET_TOKEN is specified a cap will be placed on the stack, + if SHADOW_STACK_SET_MARKER is not specified the cap will be the top 8 + bytes of the stack and if it is specified then the cap will be the next + 8 bytes. While specifying just SHADOW_STACK_SET_MARKER by itself is + valid since the marker is all bits 0 it has no observable effect. + +* Stacks allocated using map_shadow_stack() must be larger than 8 bytes and + must be 8 bytes aligned. + +* An address can be specified to map_shadow_stack(), if one is provided then + it must be aligned to a page boundary. + +* When a thread is freed the Guarded Control Stack initially allocated for + that thread will be freed. Note carefully that if the stack has been + switched this may not be the stack currently in use by the thread. + + +4. Signal handling +-------------------- + +* A new signal frame record gcs_context encodes the current GCS mode and + pointer for the interrupted context on signal delivery. This will always + be present on systems that support GCS. + +* The record contains a flag field which reports the current GCS configuration + for the interrupted context as PR_GET_SHADOW_STACK_STATUS would. + +* The signal handler is run with the same GCS configuration as the interrupted + context. + +* When GCS is enabled for the interrupted thread a signal handling specific + GCS cap token will be written to the GCS, this is an architectural GCS cap + token with bit 63 set. The GCSPR_EL0 reported in the signal frame will + point to this cap token. + +* The signal handler will use the same GCS as the interrupted context. + +* When GCS is enabled on signal entry a frame with the address of the signal + return handler will be pushed onto the GCS, allowing return from the signal + handler via RET as normal. This will not be reported in the gcs_context in + the signal frame. + + +5. Signal return +----------------- + +When returning from a signal handler: + +* If there is a gcs_context record in the signal frame then the GCS flags + and GCSPR_EL0 will be restored from that context prior to further + validation. + +* If there is no gcs_context record in the signal frame then the GCS + configuration will be unchanged. + +* If GCS is enabled on return from a signal handler then GCSPR_EL0 must + point to a valid GCS signal cap record, this will be popped from the + GCS prior to signal return. + +* If the GCS configuration is locked when returning from a signal then any + attempt to change the GCS configuration will be treated as an error. This + is true even if GCS was not enabled prior to signal entry. + +* GCS may be disabled via signal return but any attempt to enable GCS via + signal return will be rejected. + + +7. ptrace extensions +--------------------- + +* A new regset NT_ARM_GCS is defined for use with PTRACE_GETREGSET and + PTRACE_SETREGSET. + +* Due to the complexity surrounding allocation and deallocation of stacks and + lack of practical application it is not possible to enable GCS via ptrace. + GCS may be disabled via the ptrace interface. + +* Other GCS modes may be configured via ptrace. + +* Configuration via ptrace ignores locking of GCS mode bits. + + +8. ELF coredump extensions +--------------------------- + +* NT_ARM_GCS notes will be added to each coredump for each thread of the + dumped process. The contents will be equivalent to the data that would + have been read if a PTRACE_GETREGSET of the corresponding type were + executed for each thread when the coredump was generated. + + + +9. /proc extensions +-------------------- + +* Guarded Control Stack pages will include "ss" in their VmFlags in + /proc//smaps. diff --git a/Documentation/arch/arm64/index.rst b/Documentation/arch/arm64/index.rst index d08e924204bf..dcf3ee3eb8c0 100644 --- a/Documentation/arch/arm64/index.rst +++ b/Documentation/arch/arm64/index.rst @@ -14,6 +14,7 @@ ARM64 Architecture booting cpu-feature-registers elf_hwcaps + gcs hugetlbpage kdump legacy_instructions -- 2.30.2