diff options
authorAndrew Cooper <andrew.cooper3@citrix.com>2013-01-16 12:00:55 +0000
committerKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>2013-01-16 16:17:42 -0500
commit9174adbee4a9a49d0139f5d71969852b36720809 (patch)
parentd55bf532d72b3cfdfe84e696ace995067324c96c (diff)
xen: Fix stack corruption in xen_failsafe_callback for 32bit PVOPS guests.
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> Cc: stable@vger.kernel.org Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
1 files changed, 0 insertions, 1 deletions
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 88b725aa1d5..cf8639b4dcf 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1084,7 +1084,6 @@ ENTRY(xen_failsafe_callback)
lea 16(%esp),%esp
jz 5f
- addl $16,%esp
jmp iret_exc
5: pushl_cfi $-1 /* orig_ax = -1 => not a system call */