[PATCH 195/270] xen/x86: don't corrupt %eip when returning from a signal handler

Herton Ronaldo Krzesinski herton.krzesinski at canonical.com
Mon Nov 26 16:58:05 UTC 2012

3.5.7u1 -stable review patch.  If anyone has any objections, please let me know.


From: David Vrabel <david.vrabel at citrix.com>

commit a349e23d1cf746f8bdc603dcc61fae9ee4a695f6 upstream.

In 32 bit guests, if a userspace process has %eax == -ERESTARTSYS
(-512) or -ERESTARTNOINTR (-513) when it is interrupted by an event
/and/ the process has a pending signal then %eip (and %eax) are
corrupted when returning to the main process after handling the
signal.  The application may then crash with SIGSEGV or a SIGILL or it
may have subtly incorrect behaviour (depending on what instruction it
returned to).

The occurs because handle_signal() is incorrectly thinking that there
is a system call that needs to restarted so it adjusts %eip and %eax
to re-execute the system call instruction (even though user space had
not done a system call).

(-516) then handle_signal() only corrupted %eax (by setting it to
-EINTR).  This may cause the application to crash or have incorrect

handle_signal() assumes that regs->orig_ax >= 0 means a system call so
any kernel entry point that is not for a system call must push a
negative value for orig_ax.  For example, for physical interrupts on
bare metal the inverse of the vector is pushed and page_fault() sets
regs->orig_ax to -1, overwriting the hardware provided error code.

xen_hypervisor_callback() was incorrectly pushing 0 for orig_ax
instead of -1.

Classic Xen kernels pushed %eax which works as %eax cannot be both
non-negative and -RESTARTSYS (etc.), but using -1 is consistent with
other non-system call entry points and avoids some of the tests in

There were similar bugs in xen_failsafe_callback() of both 32 and
64-bit guests. If the fault was corrected and the normal return path
was used then 0 was incorrectly pushed as the value for orig_ax.

Signed-off-by: David Vrabel <david.vrabel at citrix.com>
Acked-by: Jan Beulich <JBeulich at suse.com>
Acked-by: Ian Campbell <ian.campbell at citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
 arch/x86/kernel/entry_32.S |    8 +++++---
 arch/x86/kernel/entry_64.S |    2 +-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 623f288..8f8e8ee 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1016,7 +1016,7 @@ ENTRY(xen_sysenter_target)
-	pushl_cfi $0
+	pushl_cfi $-1 /* orig_ax = -1 => not a system call */
@@ -1058,14 +1058,16 @@ ENTRY(xen_failsafe_callback)
 2:	mov 8(%esp),%es
 3:	mov 12(%esp),%fs
 4:	mov 16(%esp),%gs
+	/* EAX == 0 => Category 1 (Bad segment)
+	   EAX != 0 => Category 2 (Bad IRET) */
 	testl %eax,%eax
 	popl_cfi %eax
 	lea 16(%esp),%esp
 	jz 5f
 	addl $16,%esp
-	jmp iret_exc		# EAX != 0 => Category 2 (Bad IRET)
-5:	pushl_cfi $0		# EAX == 0 => Category 1 (Bad segment)
+	jmp iret_exc
+5:	pushl_cfi $-1 /* orig_ax = -1 => not a system call */
 	jmp ret_from_exception
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 7d65133..e1ed683 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1381,7 +1381,7 @@ ENTRY(xen_failsafe_callback)
 	addq $0x30,%rsp
-	pushq_cfi $0
+	pushq_cfi $-1 /* orig_ax = -1 => not a system call */
 	jmp error_exit

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