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SubjectRe: [PATCH] x86/entry/64: Remove %ebx handling from error_entry/exit
On Mon, Jul 23, 2018 at 12:25 AM, Greg KH <> 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:
>> 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 <>
>> Cc: Borislav Petkov <>
>> Cc: Dominik Brodowski <>
>> Cc: Ingo Molnar <>
>> Cc: "H. Peter Anvin" <>
>> Cc: Thomas Gleixner <>
>> Cc: Boris Ostrovsky <>
>> Cc: Juergen Gross <>
>> Cc:
>> Cc:
>> Cc:
>> Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface")
>> Reported-and-tested-by: "M. Vefa Bicakci" <>
>> Signed-off-by: Andy Lutomirski <>
>> ---
>> 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

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