xen: x86_32: do not enable iterrupts when returning from exception in interrupt context

If vmalloc page_fault happens inside of interrupt handler with interrupts
disabled then on exit path from exception handler when there is no pending
interrupts, the following code (arch/x86/xen/xen-asm_32.S:112):

	cmpw $0x0001, XEN_vcpu_info_pending(%eax)
	sete XEN_vcpu_info_mask(%eax)

will enable interrupts even if they has been previously disabled according to
eflags from the bounce frame (arch/x86/xen/xen-asm_32.S:99)

	testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp)
	setz XEN_vcpu_info_mask(%eax)

Solution is in setting XEN_vcpu_info_mask only when it should be set
according to
	cmpw $0x0001, XEN_vcpu_info_pending(%eax)
but not clearing it if there isn't any pending events.

Reproducer for bug is attached to RHBZ 707552

CC: stable@kernel.org
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S
index 22a2093b5862..b040b0e518ca 100644
--- a/arch/x86/xen/xen-asm_32.S
+++ b/arch/x86/xen/xen-asm_32.S
@@ -113,11 +113,13 @@ xen_iret_start_crit:
 
         /*
          * If there's something pending, mask events again so we can
-         * jump back into xen_hypervisor_callback
+         * jump back into xen_hypervisor_callback. Otherwise do not
+         * touch XEN_vcpu_info_mask.
          */
-        sete XEN_vcpu_info_mask(%eax)
+        jne 1f
+        movb $1, XEN_vcpu_info_mask(%eax)
 
-        popl %eax
+1:      popl %eax
 
         /*
          * From this point on the registers are restored and the stack