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

[Andy Lutomirski <luto@xxxxxxxxxx>]

On Mon, Jul 23, 2018 at 12:25 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> 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@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
>> 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: Andy Lutomirski <luto@xxxxxxxxxx>
>> ---
>>
>> 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 :)
>

My concern is more that something may work differently in older
kernels and there might be some subtle issue.  I'd be surprised, but
still.

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