The logic for handling SVE traps manipulates saved FPSIMD/SVE state incorrectly, and a race with preemption can result in a task having TIF_SVE set and TIF_FOREIGN_FPSTATE clear even though the live CPU state is stale (e.g. with SVE traps enabled). This has been observed to result in warnings from do_sve_acc() where SVE traps are not expected while TIF_SVE is set:

| if (test_and_set_thread_flag(TIF_SVE)) | WARN_ON(1); /* SVE access shouldn't have trapped */

Warnings of this form have been reported intermittently, e.g.

https://lore.kernel.org/linux-arm-kernel/CA+G9fYtEGe_DhY2Ms7+L7NKsLYUomGsgqp... https://lore.kernel.org/linux-arm-kernel/000000000000511e9a060ce5a45c@google...

The race can occur when the SVE trap handler is preempted before and after manipulating the saved FPSIMD/SVE state, starting and ending on the same CPU, e.g.

| void do_sve_acc(unsigned long esr, struct pt_regs *regs) | { | // Trap on CPU 0 with TIF_SVE clear, SVE 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(); | | if (test_and_set_thread_flag(TIF_SVE)) | WARN_ON(1); /* SVE access shouldn't have trapped */ | | sve_init_regs() { | if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) { | ... | } else { | fpsimd_to_sve(current); | current->thread.fp_type = FP_STATE_SVE; | } | } | | 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 SVE 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.

Fixes: cccb78ce89c4 ("arm64/sve: Rework SVE access trap to convert state in registers") Reported-by: Mark Rutland mark.rutland@arm.com Signed-off-by: Mark Brown broonie@kernel.org Cc: stable@vger.kernel.org Reviewed-by: Mark Rutland mark.rutland@arm.com Link: https://lore.kernel.org/r/20241030-arm64-fpsimd-foreign-flush-v1-1-bd7bd6690... Signed-off-by: Will Deacon will@kernel.org [Backported to 5.15 -- broonie] (cherry picked from commit 751ecf6afd6568adc98f2a6052315552c0483d18) --- arch/arm64/kernel/fpsimd.c | 1 + 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 7a3fcf21b18a..e22571e57ae1 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -964,6 +964,7 @@ void do_sve_acc(unsigned long esr, struct pt_regs *regs) fpsimd_bind_task_to_cpu(); } else { fpsimd_to_sve(current); + fpsimd_flush_task_state(current); }

put_cpu_fpsimd_context();