Re: [Xen-devel] backport of XSA-274 patch to 4.9.x kernel (could use a review)

[George Dunlap <dunlapg@xxxxxxxxx>]

On Mon, Aug 6, 2018 at 8:10 PM, Chris Brannon <cmb@xxxxxxxxx> wrote:
> I just got the following patch from a colleague.  It's a backport of
> the XSA 274 kernel patch to 4.9.x kernels.  The kernel patch given in
> the XSA would not apply cleanly.  Would someone mind reviewing it?  It
> would be much appreciated.
>
> commit b3681dd548d06deb2e1573890829dff4b15abf46 upstream.
>
> This version applies to v4.9.
>
> error_entry and error_exit communicate the user vs kernel status of
> the frame using %ebx.  This is unnecessary -- the information is in
> regs->cs.  Just use regs->cs.
>
> This makes error_entry simpler and makes error_exit more robust.
>
> It also fixes a nasty bug.  Before all the Spectre nonsense, The
> xen_failsafe_callback entry point returned like this:
>
>         ALLOC_PT_GPREGS_ON_STACK
>         SAVE_C_REGS
>         SAVE_EXTRA_REGS
>         ENCODE_FRAME_POINTER
>         jmp     error_exit
>
> And it did not go through error_entry.  This was bogus: RBX
> contained garbage, and error_exit expected a flag in RBX.
> Fortunately, it generally contained *nonzero* garbage, so the
> correct code path was used.  As part of the Spectre fixes, code was
> added to clear RBX to mitigate certain speculation attacks.  Now,
> depending on kernel configuration, RBX got zeroed and, when running
> some Wine workloads, the kernel crashes.  This was introduced by:
>
>     commit 3ac6d8c787b8 ("x86/entry/64: Clear registers for
>     exceptions/interrupts, to reduce speculation attack surface")
>
> With this patch applied, RBX is no longer needed as a flag, and the
> problem goes away.
>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: x86@xxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for 
> exceptions/interrupts, to reduce speculation attack surface")
> Reported-and-tested-by: "M. Vefa Bicakci" <m.v.b@xxxxxxxxxx>
> Signed-off-by: Sarah Newman <srn@xxxxxxxxx>

I think you need to retain Andy's SoB, and add your own underneath.

This looks plausible to me -- Andy / Boris, any opinions?

 -George

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