Date: Wed, 16 Jan 2013 12:00:55 +0000 From: Andrew Cooper <andrew.cooper3@...rix.com> To: <linux-kernel@...r.kernel.org> CC: Andrew Cooper <andrew.cooper3@...rix.com>, Frediano Ziglio <frediano.ziglio@...rix.com>, <stable@...r.kernel.org>, <oss-security@...ts.openwall.com>, Konrad Rzeszutek Wilk <konrad@...nel.org>, <xen-devel@...ts.xen.org>, <security@....org> Subject: [PATCH] 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@...rix.com> Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com> Cc: stable@...r.kernel.org --- Cc: oss-security@...ts.openwall.com Cc: Konrad Rzeszutek Wilk <konrad@...nel.org> Cc: xen-devel@...ts.xen.org Cc: security@....org --- arch/x86/kernel/entry_32.S | 1 - 1 files changed, 0 insertions(+), 1 deletions(-) diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S index ff84d54..6ed91d9 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 -- 126.96.36.199
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