From: Andy Lutomirski <luto@kernel.org>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Borislav Petkov <bp@alien8.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Brian Gerst <brgerst@gmail.com>,
Dominik Brodowski <linux@dominikbrodowski.net>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Juergen Gross <jgross@suse.com>,
xen-devel@lists.xenproject.org, stable <stable@vger.kernel.org>
Subject: Re: [PATCH] x86/entry/64: Remove %ebx handling from error_entry/exit
Date: Mon, 23 Jul 2018 19:31:55 -0700 [thread overview]
Message-ID: <CALCETrXiQkR+k8Dz9-sOpU_J7VK1uokC0Tor3dw704n=jMepMA@mail.gmail.com> (raw)
In-Reply-To: <20180723072505.GA24222@kroah.com>
On Mon, Jul 23, 2018 at 12:25 AM, Greg KH <gregkh@linuxfoundation.org> 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@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 :)
>
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.
next prev parent reply other threads:[~2018-07-24 2:32 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-22 18:05 [PATCH] x86/entry/64: Remove %ebx handling from error_entry/exit Andy Lutomirski
2018-07-23 7:25 ` Greg KH
2018-07-24 2:31 ` Andy Lutomirski [this message]
2018-07-24 15:03 ` [tip:x86/urgent] " tip-bot for Andy Lutomirski
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