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FROMGIT: arm64/fpsimd: Consistently preserve FPSIMD state during clone()

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In arch_dup_task_struct() we try to ensure that the child task inherits
the FPSIMD state of its parent, but this depends on the parent task's
saved state being in FPSIMD format, which is not always the case.
Consequently the child task may inherit stale FPSIMD state in some
cases.

This can happen when the parent's state has been modified by ptrace
since syscall entry, as writes to the NT_ARM_SVE regset may save state
in SVE format. This has been possible since commit:

  bc0ee47603 ("arm64/sve: Core task context handling")

More recently it has been possible for a task's FPSIMD/SVE state to be
saved before lazy discarding was guaranteed to occur, in which case
preemption could cause the effective FPSIMD state to be saved in SVE
format non-deterministically. This has been possible since commit:

  f130ac0ae4 ("arm64: syscall: unmask DAIF earlier for SVCs")

Fix this by saving the parent task's effective FPSIMD state into FPSIMD
format before copying the task_struct. As this requires modifying the
parent's fpsimd_state, we must save+flush the state to avoid racing with
concurrent manipulation.

Similar issues exist when the parent has streaming mode state, and will
be addressed by subsequent patches.

Fixes: bc0ee47603 ("arm64/sve: Core task context handling")
Fixes: f130ac0ae4 ("arm64: syscall: unmask DAIF earlier for SVCs")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-12-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit e0cb0f26594c644c71ee7f48ebaae6b26bf56a12
 https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git
 for-next/sme-fixes)
Bug: 393087661
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I8331aa3a2222208d8ce508bc21cd3c65c8be82f8
This commit is contained in:
Mark Rutland
2025-05-08 14:26:31 +01:00
committed by Will Deacon
parent f5db1f9a3b
commit 370e80e212
1 changed files with 8 additions and 1 deletions
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9
arch/arm64/kernel/process.c
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@@ -299,7 +299,14 @@ void arch_release_task_struct(struct task_struct *tsk)
int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
{
fpsimd_preserve_current_state();
/*
* The current/src task's FPSIMD state may or may not be live, and may
* have been altered by ptrace after entry to the kernel. Save the
* effective FPSIMD state so that this will be copied into dst.
*/
fpsimd_save_and_flush_current_state();
fpsimd_sync_from_effective_state(src);
*dst = *src;
/*