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FROMGIT: arm64/fpsimd: Avoid clobbering kernel FPSIMD state with SMSTOP

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On system with SME, a thread's kernel FPSIMD state may be erroneously
clobbered during a context switch immediately after that state is
restored. Systems without SME are unaffected.

If the CPU happens to be in streaming SVE mode before a context switch
to a thread with kernel FPSIMD state, fpsimd_thread_switch() will
restore the kernel FPSIMD state using fpsimd_load_kernel_state() while
the CPU is still in streaming SVE mode. When fpsimd_thread_switch()
subsequently calls fpsimd_flush_cpu_state(), this will execute an
SMSTOP, causing an exit from streaming SVE mode. The exit from
streaming SVE mode will cause the hardware to reset a number of
FPSIMD/SVE/SME registers, clobbering the FPSIMD state.

Fix this by calling fpsimd_flush_cpu_state() before restoring the kernel
FPSIMD state.

Fixes: e92bee9f86 ("arm64/fpsimd: Avoid erroneous elide of user state reload")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409164010.3480271-8-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 01098d893fa8a6edb2b56e178b798e3e6b674f02
 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: I9c214065a66f7d29ec736a26f543f8aff3012634
This commit is contained in:
Mark Rutland
2025-04-09 17:40:04 +01:00
committed by Will Deacon
parent 2d33087d98
commit 32dbf4add0
1 changed files with 1 additions and 1 deletions
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2
arch/arm64/kernel/fpsimd.c
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@@ -1550,8 +1550,8 @@ void fpsimd_thread_switch(struct task_struct *next)
fpsimd_save_user_state();
if (test_tsk_thread_flag(next, TIF_KERNEL_FPSTATE)) {
fpsimd_load_kernel_state(next);
fpsimd_flush_cpu_state();
fpsimd_load_kernel_state(next);
} else {
/*
* Fix up TIF_FOREIGN_FPSTATE to correctly describe next's