All of lore.kernel.org
 help / color / mirror / Atom feed
From: Sarah Newman <srn@prgmr.com>
To: David Woodhouse <dwmw2@infradead.org>,
	stable@vger.kernel.org, "Durand Wesolowski,
	Jimmy" <jdw@amazon.de>
Cc: Juergen Gross <jgross@suse.com>, Brian Gerst <brgerst@gmail.com>,
	x86@kernel.org, Dominik Brodowski <linux@dominikbrodowski.net>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Andy Lutomirski <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	xen-devel@lists.xenproject.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH] x86/entry/64: Remove %ebx handling from error_entry/exit
Date: Fri, 10 Aug 2018 00:23:45 -0700	[thread overview]
Message-ID: <1f15d219-46fd-fc21-84d5-877a7af4c87e__36183.8674791672$1533885766$gmane$org@prgmr.com> (raw)
In-Reply-To: <1533818501.5775.4.camel@infradead.org>

On 08/09/2018 05:41 AM, David Woodhouse wrote:
> On Wed, 2018-08-08 at 10:35 -0700, Sarah Newman wrote:
>> commit b3681dd548d06deb2e1573890829dff4b15abf46 upstream.
>>
>> This version applies to v4.9.
> 
> I think you can kill the 'xorl %ebx,%ebx' from error_entry too but yes,
> this does want to go to 4.9 and earlier because the 'Fixes:' tag is a
> bit of a lie — the problem existed before that, at least in theory.

The commit 2140a9942 "x86/entry/64: Relax pvops stub clobber
specifications" was what removed the "movl %ebx, %eax" line later on
originally, but it was the commit 3ac6d8c787b8 that removed the
'xorl %ebx,%ebx'. So these weren't matched.

I don't know if it's a concern, but if someone had gone to the effort of
backporting the original commit 3ac6d8c787b83, adding the removal of
'xorl %ebx,%ebx' to this patch would create merge conflicts.
For that reason and given the line is harmless, should it be left in?

> 
>> From Andy Lutomirski, original author:
>>
>> 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.]
>>
>> 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
>> Cc: Andy Lutomirski <luto@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>
>> Signed-off-by: Sarah Newman <srn@prgmr.com>
>> ---
>>  arch/x86/entry/entry_64.S | 19 ++++---------------
>>  1 file changed, 4 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index d58d8dc..0dab47a 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -774,7 +774,7 @@ ENTRY(\sym)
>>  
>>  	call	\do_sym
>>  
>> -	jmp	error_exit			/* %ebx: no
>> swapgs flag */
>> +	jmp	error_exit
>>  	.endif
>>  END(\sym)
>>  .endm
>> @@ -1043,7 +1043,6 @@ END(paranoid_exit)
>>  
>>  /*
>>   * Save all registers in pt_regs, and switch gs if needed.
>> - * Return: EBX=0: came from user mode; EBX=1: otherwise
>>   */
>>  ENTRY(error_entry)
>>  	cld
>> @@ -1087,7 +1086,6 @@ ENTRY(error_entry)
>>  	 * for these here too.
>>  	 */
>>  .Lerror_kernelspace:
>> -	incl	%ebx
>>  	leaq	native_irq_return_iret(%rip), %rcx
>>  	cmpq	%rcx, RIP+8(%rsp)
>>  	je	.Lerror_bad_iret
>> @@ -1119,28 +1117,19 @@ ENTRY(error_entry)
>>  
>>  	/*
>>  	 * Pretend that the exception came from user mode: set up
>> pt_regs
>> -	 * as if we faulted immediately after IRET and clear EBX so
>> that
>> -	 * error_exit knows that we will be returning to user mode.
>> +	 * as if we faulted immediately after IRET.
>>  	 */
>>  	mov	%rsp, %rdi
>>  	call	fixup_bad_iret
>>  	mov	%rax, %rsp
>> -	decl	%ebx
>>  	jmp	.Lerror_entry_from_usermode_after_swapgs
>>  END(error_entry)
>>  
>> -
>> -/*
>> - * On entry, EBX is a "return to kernel mode" flag:
>> - *   1: already in kernel mode, don't need SWAPGS
>> - *   0: user gsbase is loaded, we need SWAPGS and standard
>> preparation for return to usermode
>> - */
>>  ENTRY(error_exit)
>> -	movl	%ebx, %eax
>>  	DISABLE_INTERRUPTS(CLBR_NONE)
>>  	TRACE_IRQS_OFF
>> -	testl	%eax, %eax
>> -	jnz	retint_kernel
>> +	testb	$3, CS(%rsp)
>> +	jz	retint_kernel
>>  	jmp	retint_user
>>  END(error_exit)
>>  
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xenproject.org
>> https://lists.xenproject.org/mailman/listinfo/xen-devel


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

  reply	other threads:[~2018-08-10  7:23 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-06 19:10 backport of XSA-274 patch to 4.9.x kernel (could use a review) Chris Brannon
2018-08-07 17:20 ` George Dunlap
2018-08-07 18:49   ` Boris Ostrovsky
2018-08-07 22:57     ` Andy Lutomirski
2018-08-08 17:35       ` [PATCH] x86/entry/64: Remove %ebx handling from error_entry/exit Sarah Newman
2018-08-08 17:35         ` Sarah Newman
2018-08-09 12:41         ` David Woodhouse
2018-08-09 12:41         ` [Xen-devel] " David Woodhouse
2018-08-10  7:23           ` Sarah Newman [this message]
2018-08-10  7:23           ` Sarah Newman
2018-08-16 15:19             ` Greg KH
2018-08-16 15:35               ` Andy Lutomirski
2018-08-16 15:35               ` Andy Lutomirski
2018-08-16 15:19             ` Greg KH
  -- strict thread matches above, loose matches on Subject: below --
2018-07-22 18:05 Andy Lutomirski
2018-07-23  7:25 ` Greg KH
2018-07-24  2:31   ` Andy Lutomirski
2018-07-24  2:31   ` Andy Lutomirski
2018-07-23  7:25 ` Greg KH
2018-07-22 18:05 Andy Lutomirski

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='1f15d219-46fd-fc21-84d5-877a7af4c87e__36183.8674791672$1533885766$gmane$org@prgmr.com' \
    --to=srn@prgmr.com \
    --cc=boris.ostrovsky@oracle.com \
    --cc=bp@alien8.de \
    --cc=brgerst@gmail.com \
    --cc=dwmw2@infradead.org \
    --cc=hpa@zytor.com \
    --cc=jdw@amazon.de \
    --cc=jgross@suse.com \
    --cc=linux@dominikbrodowski.net \
    --cc=luto@kernel.org \
    --cc=mingo@redhat.com \
    --cc=stable@vger.kernel.org \
    --cc=tglx@linutronix.de \
    --cc=x86@kernel.org \
    --cc=xen-devel@lists.xenproject.org \
    --subject='Re: [PATCH] x86/entry/64: Remove %ebx handling from error_entry/exit' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.