Patchwork [11/93] xen: Fix stack corruption in xen_failsafe_callback for 32bit PVOPS guests.

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Submitter Herton Ronaldo Krzesinski
Date Feb. 5, 2013, 10:06 p.m.
Message ID <1360102042-10732-12-git-send-email-herton.krzesinski@canonical.com>
Download mbox | patch
Permalink /patch/218375/
State New
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Herton Ronaldo Krzesinski - Feb. 5, 2013, 10:06 p.m.
3.5.7.5 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrew Cooper <andrew.cooper3@citrix.com>

commit 9174adbee4a9a49d0139f5d71969852b36720809 upstream.

This fixes CVE-2013-0190 / XSA-40

There has been an error on the xen_failsafe_callback path for failed
iret, which causes the stack pointer to be wrong when entering the
iret_exc error path.  This can result in the kernel crashing.

In the classic kernel case, the relevant code looked a little like:

        popl %eax      # Error code from hypervisor
        jz 5f
        addl $16,%esp
        jmp iret_exc   # Hypervisor said iret fault
5:      addl $16,%esp
                       # Hypervisor said segment selector fault

Here, there are two identical addls on either option of a branch which
appears to have been optimised by hoisting it above the jz, and
converting it to an lea, which leaves the flags register unaffected.

In the PVOPS case, the code looks like:

        popl_cfi %eax         # Error from the hypervisor
        lea 16(%esp),%esp     # Add $16 before choosing fault path
        CFI_ADJUST_CFA_OFFSET -16
        jz 5f
        addl $16,%esp         # Incorrectly adjust %esp again
        jmp iret_exc

It is possible unprivileged userspace applications to cause this
behaviour, for example by loading an LDT code selector, then changing
the code selector to be not-present.  At this point, there is a race
condition where it is possible for the hypervisor to return back to
userspace from an interrupt, fault on its own iret, and inject a
failsafe_callback into the kernel.

This bug has been present since the introduction of Xen PVOPS support
in commit 5ead97c84 (xen: Core Xen implementation), in 2.6.23.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
---
 arch/x86/kernel/entry_32.S |    1 -
 1 file changed, 1 deletion(-)
Frediano Ziglio - Feb. 6, 2013, 10:18 a.m.
At stated before I'm the author of this patch.

Please change the From: to

From: Frediano Ziglio <frediano.ziglio@citrix.com>

Frediano


On Tue, 2013-02-05 at 20:06 -0200, Herton Ronaldo Krzesinski wrote:
> 3.5.7.5 -stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Andrew Cooper <andrew.cooper3@citrix.com>
> 
> commit 9174adbee4a9a49d0139f5d71969852b36720809 upstream.
> 
> This fixes CVE-2013-0190 / XSA-40
> 
> There has been an error on the xen_failsafe_callback path for failed
> iret, which causes the stack pointer to be wrong when entering the
> iret_exc error path.  This can result in the kernel crashing.
> 
> In the classic kernel case, the relevant code looked a little like:
> 
>         popl %eax      # Error code from hypervisor
>         jz 5f
>         addl $16,%esp
>         jmp iret_exc   # Hypervisor said iret fault
> 5:      addl $16,%esp
>                        # Hypervisor said segment selector fault
> 
> Here, there are two identical addls on either option of a branch which
> appears to have been optimised by hoisting it above the jz, and
> converting it to an lea, which leaves the flags register unaffected.
> 
> In the PVOPS case, the code looks like:
> 
>         popl_cfi %eax         # Error from the hypervisor
>         lea 16(%esp),%esp     # Add $16 before choosing fault path
>         CFI_ADJUST_CFA_OFFSET -16
>         jz 5f
>         addl $16,%esp         # Incorrectly adjust %esp again
>         jmp iret_exc
> 
> It is possible unprivileged userspace applications to cause this
> behaviour, for example by loading an LDT code selector, then changing
> the code selector to be not-present.  At this point, there is a race
> condition where it is possible for the hypervisor to return back to
> userspace from an interrupt, fault on its own iret, and inject a
> failsafe_callback into the kernel.
> 
> This bug has been present since the introduction of Xen PVOPS support
> in commit 5ead97c84 (xen: Core Xen implementation), in 2.6.23.
> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
> ---
>  arch/x86/kernel/entry_32.S |    1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
> index 8f8e8ee..2a6919e 100644
> --- a/arch/x86/kernel/entry_32.S
> +++ b/arch/x86/kernel/entry_32.S
> @@ -1065,7 +1065,6 @@ ENTRY(xen_failsafe_callback)
>  	lea 16(%esp),%esp
>  	CFI_ADJUST_CFA_OFFSET -16
>  	jz 5f
> -	addl $16,%esp
>  	jmp iret_exc
>  5:	pushl_cfi $-1 /* orig_ax = -1 => not a system call */
>  	SAVE_ALL
Herton Ronaldo Krzesinski - Feb. 6, 2013, 1:16 p.m.
On Wed, Feb 06, 2013 at 10:18:54AM +0000, Frediano Ziglio wrote:
> At stated before I'm the author of this patch.
> 
> Please change the From: to
> 
> From: Frediano Ziglio <frediano.ziglio@citrix.com>
> 
> Frediano

I fixed this in my tree now. But note that the real problem is that this
got submitted, included and pushed in mainline with the wrong author, and
that's not going to be fixed, since would involve rebasing/rewriting the
history there. I just cherry-picked the commit as is from mainline
(Linus tree).

> 
> 
> On Tue, 2013-02-05 at 20:06 -0200, Herton Ronaldo Krzesinski wrote:
> > 3.5.7.5 -stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Andrew Cooper <andrew.cooper3@citrix.com>
> > 
> > commit 9174adbee4a9a49d0139f5d71969852b36720809 upstream.
> > 
> > This fixes CVE-2013-0190 / XSA-40
> > 
> > There has been an error on the xen_failsafe_callback path for failed
> > iret, which causes the stack pointer to be wrong when entering the
> > iret_exc error path.  This can result in the kernel crashing.
> > 
> > In the classic kernel case, the relevant code looked a little like:
> > 
> >         popl %eax      # Error code from hypervisor
> >         jz 5f
> >         addl $16,%esp
> >         jmp iret_exc   # Hypervisor said iret fault
> > 5:      addl $16,%esp
> >                        # Hypervisor said segment selector fault
> > 
> > Here, there are two identical addls on either option of a branch which
> > appears to have been optimised by hoisting it above the jz, and
> > converting it to an lea, which leaves the flags register unaffected.
> > 
> > In the PVOPS case, the code looks like:
> > 
> >         popl_cfi %eax         # Error from the hypervisor
> >         lea 16(%esp),%esp     # Add $16 before choosing fault path
> >         CFI_ADJUST_CFA_OFFSET -16
> >         jz 5f
> >         addl $16,%esp         # Incorrectly adjust %esp again
> >         jmp iret_exc
> > 
> > It is possible unprivileged userspace applications to cause this
> > behaviour, for example by loading an LDT code selector, then changing
> > the code selector to be not-present.  At this point, there is a race
> > condition where it is possible for the hypervisor to return back to
> > userspace from an interrupt, fault on its own iret, and inject a
> > failsafe_callback into the kernel.
> > 
> > This bug has been present since the introduction of Xen PVOPS support
> > in commit 5ead97c84 (xen: Core Xen implementation), in 2.6.23.
> > 
> > Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
> > Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
> > ---
> >  arch/x86/kernel/entry_32.S |    1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
> > index 8f8e8ee..2a6919e 100644
> > --- a/arch/x86/kernel/entry_32.S
> > +++ b/arch/x86/kernel/entry_32.S
> > @@ -1065,7 +1065,6 @@ ENTRY(xen_failsafe_callback)
> >  	lea 16(%esp),%esp
> >  	CFI_ADJUST_CFA_OFFSET -16
> >  	jz 5f
> > -	addl $16,%esp
> >  	jmp iret_exc
> >  5:	pushl_cfi $-1 /* orig_ax = -1 => not a system call */
> >  	SAVE_ALL
>

Patch

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 8f8e8ee..2a6919e 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1065,7 +1065,6 @@  ENTRY(xen_failsafe_callback)
 	lea 16(%esp),%esp
 	CFI_ADJUST_CFA_OFFSET -16
 	jz 5f
-	addl $16,%esp
 	jmp iret_exc
 5:	pushl_cfi $-1 /* orig_ax = -1 => not a system call */
 	SAVE_ALL