On Wed, Oct 30, 2024 at 08:23:50PM +0000, Mark Brown wrote:
When we update the in memory register state in sve_init_regs() we neglect to flush the task's CPU binding, meaning if the task is rescheduled to the last CPU it ran on it is possible for the check for current state in fpsimd_thread_switch() to falsely determine that up to date register state is present on the CPU. This results in it incorrectly clearing TIF_FOREIGN_FPSTATE and suppress reloading.
This will also suppress the sve_user_enable() done in fpsimd_bind_task_to_cpu() as part of return to userspace, causing spurious SVE access traps.
Call fpsimd_flush_task_state() to invalidate the last loaded CPU recorded in the task.
Fixes: cccb78ce89c4 ("arm64/sve: Rework SVE access trap to convert state in registers") Reported-by: Mark Rutlamd mark.rutland@arm.com Signed-off-by: Mark Brown broonie@kernel.org Cc: stable@vger.kernel.org
How about the following:
| arm64/sve: Discard stale CPU state when handling SVE traps | | 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)) { | | ... | | fpsimd_bind_task_to_cpu(); | | } 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 | | } | | In the case where sve_init_regs() is called while the state is live and | TIF_FOREIGN_FPSTATE is clear, the state is correctly modified and the | call to fpsimd_bind_task_to_cpu() disables the SVE trap. | | 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
Note: I fixed the typo (s/Rutlamd/Rutland)
With that:
Reviewed-by: Mark Rutland mark.rutland@arm.com
Mark.
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 77006df20a75aee7c991cf116b6d06bfe953d1a4..6d21971ae5594f32947480cfa168db400a69a283 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -1367,6 +1367,7 @@ static void sve_init_regs(void) } else { fpsimd_to_sve(current); current->thread.fp_type = FP_STATE_SVE;
fpsimd_flush_task_state(current);}
-- 2.39.2