6.15-stable review patch. If anyone has any objections, please let me know.
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From: Mark Brown broonie@kernel.org
[ Upstream commit d3eaab3c70905c5467e5c4ea403053d67505adeb ]
The logic for handling SME traps manipulates saved FPSIMD/SVE/SME state incorrectly, and a race with preemption can result in a task having TIF_SME set and TIF_FOREIGN_FPSTATE clear even though the live CPU state is stale (e.g. with SME traps enabled). This can result in warnings from do_sme_acc() where SME traps are not expected while TIF_SME is set:
| /* With TIF_SME userspace shouldn't generate any traps */ | if (test_and_set_thread_flag(TIF_SME)) | WARN_ON(1);
This is very similar to the SVE issue we fixed in commit:
751ecf6afd6568ad ("arm64/sve: Discard stale CPU state when handling SVE traps")
The race can occur when the SME trap handler is preempted before and after manipulating the saved FPSIMD/SVE/SME state, starting and ending on the same CPU, e.g.
| void do_sme_acc(unsigned long esr, struct pt_regs *regs) | { | // Trap on CPU 0 with TIF_SME clear, SME traps enabled | // task->fpsimd_cpu is 0. | // per_cpu_ptr(&fpsimd_last_state, 0) is task. | | ... | | // Preempted; migrated from CPU 0 to CPU 1. | // TIF_FOREIGN_FPSTATE is set. | | get_cpu_fpsimd_context(); | | /* With TIF_SME userspace shouldn't generate any traps */ | if (test_and_set_thread_flag(TIF_SME)) | WARN_ON(1); | | if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) { | unsigned long vq_minus_one = | sve_vq_from_vl(task_get_sme_vl(current)) - 1; | sme_set_vq(vq_minus_one); | | fpsimd_bind_task_to_cpu(); | } | | put_cpu_fpsimd_context(); | | // Preempted; migrated from CPU 1 to CPU 0. | // task->fpsimd_cpu is still 0 | // If per_cpu_ptr(&fpsimd_last_state, 0) is still task then: | // - Stale HW state is reused (with SME traps enabled) | // - TIF_FOREIGN_FPSTATE is cleared | // - A return to userspace skips HW state restore | }
Fix the case where the state is not live and TIF_FOREIGN_FPSTATE is set by calling fpsimd_flush_task_state() to detach from the saved CPU state. This ensures that a subsequent context switch will not reuse the stale CPU state, and will instead set TIF_FOREIGN_FPSTATE, forcing the new state to be reloaded from memory prior to a return to userspace.
Note: this was originallly posted as [1].
Fixes: 8bd7f91c03d8 ("arm64/sme: Implement traps and syscall handling for SME") Reported-by: Mark Rutland mark.rutland@arm.com Signed-off-by: Mark Brown broonie@kernel.org Link: https://lore.kernel.org/linux-arm-kernel/20241204-arm64-sme-reenable-v2-1-ba... [ Rutland: rewrite commit message ] Signed-off-by: Mark Rutland mark.rutland@arm.com Cc: Marc Zyngier maz@kernel.org Cc: Will Deacon will@kernel.org Link: https://lore.kernel.org/r/20250409164010.3480271-6-mark.rutland@arm.com Signed-off-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm64/kernel/fpsimd.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 1ee5f330b8ed3..c92c0a08370b2 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -1460,6 +1460,8 @@ void do_sme_acc(unsigned long esr, struct pt_regs *regs) sme_set_vq(vq_minus_one);
fpsimd_bind_task_to_cpu(); + } else { + fpsimd_flush_task_state(current); }
put_cpu_fpsimd_context();