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[Xen-devel] [PATCHv3] xen/x86: don't corrupt %eip when returning from a signal handler

From: David Vrabel <david.vrabel@xxxxxxxxxx>

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@xxxxxxxxxx>
Acked-by: Jan Beulich <JBeulich@xxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
  - fixup the right push in the 32-bit xen_failsafe_callback().
 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 2c63407..985f7b3 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1042,7 +1042,7 @@ ENTRY(xen_sysenter_target)
-       pushl_cfi $0
+       pushl_cfi $-1 /* orig_ax = -1 => not a system call */
@@ -1084,14 +1084,16 @@ 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 cdc790c..430b1fc 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1451,7 +1451,7 @@ ENTRY(xen_failsafe_callback)
        CFI_RESTORE r11
        addq $0x30,%rsp
-       pushq_cfi $0
+       pushq_cfi $-1 /* orig_ax = -1 => not a system call */
        jmp error_exit

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