Re: [PATCH] x86/entry/64: Remove %ebx handling from error_entry/exit

Greg KH
2018-07-23T07:25:11Z

On Sun, Jul 22, 2018 at 11:05:09AM -0700, Andy Lutomirski wrote:
> 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.
>
> I suspect that malicious userspace could use this bug to crash the
> kernel even without the offending patch applied, though.
>
> [Historical note: I wrote this patch as a cleanup before I was aware
> of the bug it fixed.]
>
> [Note to stable maintainers: this should probably get applied to all
> kernels. If you're nervous about that, a more conservative fix to
> add xorl %ebx,%ebx; incl %ebx before the jump to error_exit should
> also fix the problem.]
>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: xen-devel@lists.xenproject.org
> Cc: x86@kernel.org
> Cc: stable@vger.kernel.org
> 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@runbox.com>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>
> I could also submit the conservative fix tagged for -stable and respin
> this on top of it. Ingo, Greg, what do you prefer?

I don't care, this patch looks good to me to take as-is for the stable
trees. If you trust it in Linus's tree, it should be fine for others :)

thanks,

greg k-h