[Xen-changelog] [xen master] x86: clear EFLAGS.NT in SYSENTER entry path

[patchbot@xxxxxxx]

commit fdac9515607b757c044e7ef0d61b1453ef999b08
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Apr 18 16:00:35 2013 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 18 16:00:35 2013 +0200

    x86: clear EFLAGS.NT in SYSENTER entry path
    
    ... as it causes problems if we happen to exit back via IRET: In the
    course of trying to handle the fault, the hypervisor creates a stack
    frame by hand, and uses PUSHFQ to set the respective EFLAGS field, but
    expects to be able to IRET through that stack frame to the second
    portion of the fixup code (which causes a #GP due to the stored EFLAGS
    having NT set).
    
    And even if this worked (e.g if we cleared NT in that path), it would
    then (through the fail safe callback) cause a #GP in the guest with the
    SYSENTER handler's first instruction as the source, which in turn would
    allow guest user mode code to crash the guest kernel.
    
    Inject a #GP on the fake (NULL) address of the SYSENTER instruction
    instead, just like in the case where the guest kernel didn't register
    a corresponding entry point.
    
    This is CVE-2013-1917 / XSA-44.
    
    Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/x86_64/entry.S |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 03e352b..5beeccb 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -282,7 +282,14 @@ sysenter_eflags_saved:
         cmpb  $0,VCPU_sysenter_disables_events(%rbx)
         movq  VCPU_sysenter_addr(%rbx),%rax
         setne %cl
+        testl $X86_EFLAGS_NT,UREGS_eflags(%rsp)
         leaq  VCPU_trap_bounce(%rbx),%rdx
+UNLIKELY_START(nz, sysenter_nt_set)
+        pushfq
+        andl  $~X86_EFLAGS_NT,(%rsp)
+        popfq
+        xorl  %eax,%eax
+UNLIKELY_END(sysenter_nt_set)
         testq %rax,%rax
         leal  (,%rcx,TBF_INTERRUPT),%ecx
 UNLIKELY_START(z, sysenter_gpf)
--
generated by git-patchbot for /home/xen/git/xen.git#master

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