FAILED: patch "[PATCH] arm64/sve: Discard stale CPU state when handling SVE traps" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 751ecf6afd6568adc98f2a6052315552c0483d18
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024110929-splendid-engine-1d44@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 751ecf6afd6568adc98f2a6052315552c0483d18 Mon Sep 17 00:00:00 2001
From: Mark Brown <[email protected]>
Date: Wed, 30 Oct 2024 20:23:50 +0000
Subject: [PATCH] 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+L7NKsLYUomGsgqpdBj+QwDLeSg=JhGg@mail.gmail.com/
https://lore.kernel.org/linux-arm-kernel/[email protected]/

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.

index 77006df20a75..6d21971ae559 100644
@@ -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);
 	}
 }