linux-kernel - Re: [PATCH stable 5.4,5.10] x86/fpu: Correct pkru/xstate inconsistency

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Date:   Fri, 25 Feb 2022 13:01:02 +0100
From:   Greg KH <greg@...ah.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Brian Geffon <bgeffon@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Willis Kung <williskung@...gle.com>,
        Guenter Roeck <groeck@...gle.com>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH stable 5.4,5.10] x86/fpu: Correct pkru/xstate
 inconsistency

On Thu, Feb 24, 2022 at 07:16:17AM -0800, Dave Hansen wrote:
> On 2/15/22 11:22, Brian Geffon wrote:
> > When eagerly switching PKRU in switch_fpu_finish() it checks that
> > current is not a kernel thread as kernel threads will never use PKRU.
> > It's possible that this_cpu_read_stable() on current_task
> > (ie. get_current()) is returning an old cached value. To resolve this
> > reference next_p directly rather than relying on current.
> > 
> > As written it's possible when switching from a kernel thread to a
> > userspace thread to observe a cached PF_KTHREAD flag and never restore
> > the PKRU. And as a result this issue only occurs when switching
> > from a kernel thread to a userspace thread, switching from a non kernel
> > thread works perfectly fine because all that is considered in that
> > situation are the flags from some other non kernel task and the next fpu
> > is passed in to switch_fpu_finish().
> > 
> > This behavior only exists between 5.2 and 5.13 when it was fixed by a
> > rewrite decoupling PKRU from xstate, in:
> >   commit 954436989cc5 ("x86/fpu: Remove PKRU handling from switch_fpu_finish()")
> > 
> > Unfortunately backporting the fix from 5.13 is probably not realistic as
> > it's part of a 60+ patch series which rewrites most of the PKRU handling.
> > 
> > Fixes: 0cecca9d03c9 ("x86/fpu: Eager switch PKRU state")
> > Signed-off-by: Brian Geffon <bgeffon@...gle.com>
> > Signed-off-by: Willis Kung <williskung@...gle.com>
> > Tested-by: Willis Kung <williskung@...gle.com>
> > Cc: <stable@...r.kernel.org> # v5.4.x
> > Cc: <stable@...r.kernel.org> # v5.10.x
> 
> I don't like forking the stable code from mainline.  But I also think
> that backporting the FPU reworking that changed the PKRU handling is
> likely to cause more bugs in stable than it fixes.
> 
> This fix is at least isolated to the protection keys code.
> 
> Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>

now queued up, thanks.

greg k-h