On 6/21/21 7:29 AM, Borislav Petkov wrote:

On Mon, Jun 21, 2021 at 12:51:46PM +0200, gregkh@linuxfoundation.org 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 stable@vger.kernel.org.

thanks,

greg k-h

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

From d8778e393afa421f1f117471144f8ce6deb6953a Mon Sep 17 00:00:00 2001 From: Andy Lutomirski luto@kernel.org 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 luto@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Borislav Petkov bp@suse.de Acked-by: Dave Hansen dave.hansen@linux.intel.com Acked-by: Rik van Riel riel@surriel.com Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20210608144345.758116583@linutronix.de

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index d5bc96a536c2..4ab9aeb9a963 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -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.
  

•  *
  

•  * 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.