[Xen-devel] [PATCH v2] xen: x86_32: do not enable iterrupts when returning from exception in interrupt context

[Igor Mammedov <imammedo@xxxxxxxxxx>]

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

Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
---
 arch/x86/xen/xen-asm_32.S |    8 +++++---
 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 22a2093..b040b0e 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
-- 
1.7.1


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