CVE-2013-0228 xen: xen_iret() invalid %ds local DoS (rhbz 910848 906309)

kernel.spec

@@ -62,7 +62,7 @@ Summary: The Linux kernel
 # For non-released -rc kernels, this will be appended after the rcX and
 # gitX tags, so a 3 here would become part of release "0.rcX.gitX.3"
 #
-%global baserelease 1
+%global baserelease 2
 %global fedora_build %{baserelease}
 
 # base_sublevel is the kernel version we're starting with and patching
@@ -745,6 +745,9 @@ Patch21247: ath9k_rx_dma_stop_check.patch
 #rhbz 910126
 Patch21249: pstore-Create-a-convenient-mount-point-for-pstore.patch
 
+#rhbz 906309 910848 CVE-2013-0228
+Patch21260: xen-dont-assume-ds-is-usable-in-xen_iret-for-32-bit-PVOPS.patch
+
 Patch22000: weird-root-dentry-name-debug.patch
 
 #selinux ptrace child permissions
@@ -1455,6 +1458,9 @@ ApplyPatch ath9k_rx_dma_stop_check.patch
 #rhbz 910126
 ApplyPatch pstore-Create-a-convenient-mount-point-for-pstore.patch
 
+#rhbz 906309 910848 CVE-2013-0228
+ApplyPatch xen-dont-assume-ds-is-usable-in-xen_iret-for-32-bit-PVOPS.patch
+
 # END OF PATCH APPLICATIONS
 
 %endif
@@ -2310,6 +2316,9 @@ fi
 #                 ||----w |
 #                 ||     ||
 %changelog
+* Thu Feb 14 2013 Josh Boyer <jwboyer@redhat.com>
+- CVE-2013-0228 xen: xen_iret() invalid %ds local DoS (rhbz 910848 906309)
+
 * Wed Feb 13 2013 Peter Robinson <pbrobinson@fedoraproject.org>
 - Disable tegra30
 

xen-dont-assume-ds-is-usable-in-xen_iret-for-32-bit-PVOPS.patch

@@ -0,0 +1,131 @@
+From 13d2b4d11d69a92574a55bfd985cfb0ca77aebdc Mon Sep 17 00:00:00 2001
+From: Jan Beulich <JBeulich@suse.com>
+Date: Thu, 24 Jan 2013 13:11:10 +0000
+Subject: [PATCH] x86/xen: don't assume %ds is usable in xen_iret for 32-bit
+ PVOPS.
+
+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@redhat.com>
+Reported-by: Petr Matousek <pmatouse@redhat.com>
+Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
+Signed-off-by: Jan Beulich <jbeulich@suse.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+---
+ arch/x86/xen/xen-asm_32.S |   14 +++++++-------
+ 1 files changed, 7 insertions(+), 7 deletions(-)
+
+diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S
+index f9643fc..33ca6e4 100644
+--- a/arch/x86/xen/xen-asm_32.S
++++ b/arch/x86/xen/xen-asm_32.S
+@@ -89,11 +89,11 @@ ENTRY(xen_iret)
+ 	 */
+ #ifdef CONFIG_SMP
+ 	GET_THREAD_INFO(%eax)
+-	movl TI_cpu(%eax), %eax
+-	movl __per_cpu_offset(,%eax,4), %eax
+-	mov xen_vcpu(%eax), %eax
++	movl %ss:TI_cpu(%eax), %eax
++	movl %ss:__per_cpu_offset(,%eax,4), %eax
++	mov %ss:xen_vcpu(%eax), %eax
+ #else
+-	movl xen_vcpu, %eax
++	movl %ss:xen_vcpu, %eax
+ #endif
+ 
+ 	/* check IF state we're restoring */
+@@ -106,11 +106,11 @@ 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
+@@ -118,7 +118,7 @@ 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
+ 
+-- 
+1.7.7.6
+