Re: [ 049/184] x86/xen: dont assume %ds is usable in xen_iret for

[Ben Hutchings]

On Tue, 2013-06-04 at 19:22 +0200, Willy Tarreau wrote:
> 2.6.32-longterm review patch.  If anyone has any objections, please let me know.
> 
> ------------------
>  32-bit PVOPS.
> 
> From: Jan Beulich <JBeulich@xxxxxxxx>

commit 13d2b4d11d69a92574a55bfd985cfb0ca77aebdc upstream.

> This fixes CVE-2013-0228 / XSA-42
> 
> Drew Jones while working on CVE-2013-0190 found that that unprivileged guest user
> in 32bit PV guest can use to crash the > guest with the panic like this:
> 
> -------------
> general protection fault: 0000 [#1] SMP
> last sysfs file: /sys/devices/vbd-51712/block/xvda/dev
> Modules linked in: sunrpc ipt_REJECT nf_conntrack_ipv4 nf_defrag_ipv4
> iptable_filter ip_tables ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6
> xt_state nf_conntrack ip6table_filter ip6_tables ipv6 xen_netfront ext4
> mbcache jbd2 xen_blkfront dm_mirror dm_region_hash dm_log dm_mod [last
> unloaded: scsi_wait_scan]
> 
> Pid: 1250, comm: r Not tainted 2.6.32-356.el6.i686 #1
> EIP: 0061:[<c0407462>] EFLAGS: 00010086 CPU: 0
> EIP is at xen_iret+0x12/0x2b
> EAX: eb8d0000 EBX: 00000001 ECX: 08049860 EDX: 00000010
> ESI: 00000000 EDI: 003d0f00 EBP: b77f8388 ESP: eb8d1fe0
>  DS: 0000 ES: 007b FS: 0000 GS: 00e0 SS: 0069
> Process r (pid: 1250, ti=eb8d0000 task=c2953550 task.ti=eb8d0000)
> Stack:
>  00000000 0027f416 00000073 00000206 b77f8364 0000007b 00000000 00000000
> Call Trace:
> Code: c3 8b 44 24 18 81 4c 24 38 00 02 00 00 8d 64 24 30 e9 03 00 00 00
> 8d 76 00 f7 44 24 08 00 00 02 80 75 33 50 b8 00 e0 ff ff 21 e0 <8b> 40
> 10 8b 04 85 a0 f6 ab c0 8b 80 0c b0 b3 c0 f6 44 24 0d 02
> EIP: [<c0407462>] xen_iret+0x12/0x2b SS:ESP 0069:eb8d1fe0
> general protection fault: 0000 [#2]
> ---[ end trace ab0d29a492dcd330 ]---
> Kernel panic - not syncing: Fatal exception
> Pid: 1250, comm: r Tainted: G      D    ---------------
> 2.6.32-356.el6.i686 #1
> Call Trace:
>  [<c08476df>] ? panic+0x6e/0x122
>  [<c084b63c>] ? oops_end+0xbc/0xd0
>  [<c084b260>] ? do_general_protection+0x0/0x210
>  [<c084a9b7>] ? error_code+0x73/
> -------------
> 
> Petr says: "
>  I've analysed the bug and I think that xen_iret() cannot cope with
>  mangled DS, in this case zeroed out (null selector/descriptor) by either
>  xen_failsafe_callback() or RESTORE_REGS because the corresponding LDT
>  entry was invalidated by the reproducer. "
> 
> Jan took a look at the preliminary patch and came up a fix that solves
> this problem:
> 
> "This code gets called after all registers other than those handled by
> IRET got already restored, hence a null selector in %ds or a non-null
> one that got loaded from a code or read-only data descriptor would
> cause a kernel mode fault (with the potential of crashing the kernel
> as a whole, if panic_on_oops is set)."
> 
> The way to fix this is to realize that the we can only relay on the
> registers that IRET restores. The two that are guaranteed are the
> %cs and %ss as they are always fixed GDT selectors. Also they are
> inaccessible from user mode - so they cannot be altered. This is
> the approach taken in this patch.
> 
> Another alternative option suggested by Jan would be to relay on
> the subtle realization that using the %ebp or %esp relative references uses
> the %ss segment.  In which case we could switch from using %eax to %ebp and
> would not need the %ss over-rides. That would also require one extra
> instruction to compensate for the one place where the register is used
> as scaled index. However Andrew pointed out that is too subtle and if
> further work was to be done in this code-path it could escape folks attention
> and lead to accidents.
> 
> Reviewed-by: Petr Matousek <pmatouse@xxxxxxxxxx>
> Reported-by: Petr Matousek <pmatouse@xxxxxxxxxx>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> [dannf: backported to Debian's 2.6.32]
> Signed-off-by: Willy Tarreau <w@xxxxxx>
> ---
>  arch/x86/xen/xen-asm_32.S | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S
> index 9a95a9c..d05bd11 100644
> --- a/arch/x86/xen/xen-asm_32.S
> +++ b/arch/x86/xen/xen-asm_32.S
> @@ -88,11 +88,11 @@ ENTRY(xen_iret)
>  	 */
>  #ifdef CONFIG_SMP
>  	GET_THREAD_INFO(%eax)
> -	movl TI_cpu(%eax), %eax
> -	movl __per_cpu_offset(,%eax,4), %eax
> -	mov per_cpu__xen_vcpu(%eax), %eax
> +	movl %ss:TI_cpu(%eax), %eax
> +	movl %ss:__per_cpu_offset(,%eax,4), %eax
> +	mov %ss:per_cpu__xen_vcpu(%eax), %eax
>  #else
> -	movl per_cpu__xen_vcpu, %eax
> +	movl %ss:per_cpu__xen_vcpu, %eax
>  #endif
>  
>  	/* check IF state we're restoring */
> @@ -105,11 +105,11 @@ ENTRY(xen_iret)
>  	 * resuming the code, so we don't have to be worried about
>  	 * being preempted to another CPU.
>  	 */
> -	setz XEN_vcpu_info_mask(%eax)
> +	setz %ss:XEN_vcpu_info_mask(%eax)
>  xen_iret_start_crit:
>  
>  	/* check for unmasked and pending */
> -	cmpw $0x0001, XEN_vcpu_info_pending(%eax)
> +	cmpw $0x0001, %ss:XEN_vcpu_info_pending(%eax)
>  
>  	/*
>  	 * If there's something pending, mask events again so we can
> @@ -117,7 +117,7 @@ xen_iret_start_crit:
>  	 * touch XEN_vcpu_info_mask.
>  	 */
>  	jne 1f
> -	movb $1, XEN_vcpu_info_mask(%eax)
> +	movb $1, %ss:XEN_vcpu_info_mask(%eax)
>  
>  1:	popl %eax
>  

-- 
Ben Hutchings
Theory and practice are closer in theory than in practice.
                                - John Levine, moderator of comp.compilers

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