FAILED: patch "[PATCH] x86/fpu: Invalidate FPU state after a failed XRSTOR from a" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-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]>.

thanks,

greg k-h

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

From d8778e393afa421f1f117471144f8ce6deb6953a Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <[email protected]>
Date: Tue, 8 Jun 2021 16:36:19 +0200
Subject: [PATCH] x86/fpu: Invalidate FPU state after a failed XRSTOR from a
user buffer

Both Intel and AMD consider it to be architecturally valid for XRSTOR to fail with #PF but nonetheless change the register state. The actual conditions under which this might occur are unclear [1], but it seems plausible that this might be triggered if one sibling thread unmaps a page and invalidates the shared TLB while another sibling thread is executing XRSTOR on the page in question.

__fpu__restore_sig() can execute XRSTOR while the hardware registers are preserved on behalf of a different victim task (using the fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but modify the registers.

If this happens, then there is a window in which __fpu__restore_sig() could schedule out and the victim task could schedule back in without reloading its own FPU registers. This would result in part of the FPU state that __fpu__restore_sig() was attempting to load leaking into the victim task's user-visible state.

Invalidate preserved FPU registers on XRSTOR failure to prevent this situation from corrupting any state.

[1] Frequent readers of the errata lists might imagine "complex microarchitectural conditions".

Fixes 1d731e731c4c ("x86/fpu: Add a fastpath to __fpu__restore_sig()")

Signed-off-by Andy Lutomirski <[email protected]>

Signed-off-by Thomas Gleixner <[email protected]>

Signed-off-by Borislav Petkov <[email protected]>

Acked-by Dave Hansen <[email protected]>

Acked-by Rik van Riel <[email protected]>

Cc [email protected]

Link https://lkml.kernel.org/r/[email protected] open here

arch/x86/kernel/fpu/signal.c +11 -0

index d5bc96a536c2..4ab9aeb9a963 100644
@@ -369,6 +369,25 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 			fpregs_unlock();
 			return 0;
 		}
+
+		/*
+		 * The above did an FPU restore operation, restricted to
+		 * the user portion of the registers, and failed, but the
+		 * microcode might have modified the FPU registers
+		 * nevertheless.
+		 *
+		 * If the FPU registers do not belong to current, then
+		 * invalidate the FPU register state otherwise the task might
+		 * preempt current and return to user space with corrupted
+		 * FPU registers.

Quoted 70 lines · On Mon, Jun 21, 2021 at 12:51:46PM +0200, [email protected] wrote:

The patch below does not apply to the 5.4-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]>.

thanks,

greg k-h

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

From d8778e393afa421f1f117471144f8ce6deb6953a Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <[email protected]>
Date: Tue, 8 Jun 2021 16:36:19 +0200
Subject: [PATCH] x86/fpu: Invalidate FPU state after a failed XRSTOR from a
user buffer

Both Intel and AMD consider it to be architecturally valid for XRSTOR to fail with #PF but nonetheless change the register state. The actual conditions under which this might occur are unclear [1], but it seems plausible that this might be triggered if one sibling thread unmaps a page and invalidates the shared TLB while another sibling thread is executing XRSTOR on the page in question.

__fpu__restore_sig() can execute XRSTOR while the hardware registers are preserved on behalf of a different victim task (using the fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but modify the registers.

If this happens, then there is a window in which __fpu__restore_sig() could schedule out and the victim task could schedule back in without reloading its own FPU registers. This would result in part of the FPU state that __fpu__restore_sig() was attempting to load leaking into the victim task's user-visible state.

Invalidate preserved FPU registers on XRSTOR failure to prevent this situation from corrupting any state.

[1] Frequent readers of the errata lists might imagine "complex microarchitectural conditions".

Fixes 1d731e731c4c ("x86/fpu: Add a fastpath to __fpu__restore_sig()")

Signed-off-by Andy Lutomirski <[email protected]>

Signed-off-by Thomas Gleixner <[email protected]>

Signed-off-by Borislav Petkov <[email protected]>

Acked-by Dave Hansen <[email protected]>

Acked-by Rik van Riel <[email protected]>

Cc [email protected]

Link https://lkml.kernel.org/r/[email protected] open here

arch/x86/kernel/fpu/signal.c +11 -0

index d5bc96a536c2..4ab9aeb9a963 100644
@@ -369,6 +369,25 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 			fpregs_unlock();
 			return 0;
 		}
+
+		/*
+		 * The above did an FPU restore operation, restricted to
+		 * the user portion of the registers, and failed, but the
+		 * microcode might have modified the FPU registers
+		 * nevertheless.
+		 *
+		 * If the FPU registers do not belong to current, then
+		 * invalidate the FPU register state otherwise the task might
+		 * preempt current and return to user space with corrupted
+		 * FPU registers.

arch/x86/kernel/fpu/signal.c static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) · line 392

Quoted 10 lines

arch/x86/kernel/fpu/signal.c +8 -0

+		 *
+		 * In case current owns the FPU registers then no further
+		 * action is required. The fixup below will handle it
+		 * correctly.
+		 */
+		if (test_thread_flag(TIF_NEED_FPU_LOAD))
+			__cpu_invalidate_fpregs_state();
+
 		fpregs_unlock();
 	} else {

So I'm looking at this and 5.4.127 has:

                if (!ret) {
                        fpregs_mark_activate();
                        fpregs_unlock();
                        return 0;
                }
                fpregs_deactivate(fpu);		<---
                fpregs_unlock();

i.e., an unconditional fpu invalidation there. Which got removed by:

98265c17efa9 ("x86/fpu/xstate: Preserve supervisor states for the slow path in __fpu__restore_sig()")

in 5.7.

so that Fixes: commit above which points to a 5.1 kernel is probably wrong-ish.

amluto?

I agree. The fixes line is indeed wrong, and the (horribly misnamed) fpu_deactivate() call did the right thing.