[Xen-changelog] [xen stable-4.4] x86: force EFLAGS.IF on when exiting to PV guests

[patchbot@xxxxxxx]

commit 394ddc2de62cbcaf9d28cc7373fde175e6ba3a5d
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Dec 21 17:48:43 2016 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Dec 21 17:48:43 2016 +0100

    x86: force EFLAGS.IF on when exiting to PV guests
    
    Guest kernels modifying instructions in the process of being emulated
    for another of their vCPU-s may effect EFLAGS.IF to be cleared upon
    next exiting to guest context, by converting the being emulated
    instruction to CLI (at the right point in time). Prevent any such bad
    effects by always forcing EFLAGS.IF on. And to cover hypothetical other
    similar issues, also force EFLAGS.{IOPL,NT,VM} to zero.
    
    This is CVE-2016-10024 / XSA-202.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 0e47f92b072548800223f9a21ea051a017173915
    master date: 2016-12-21 16:46:13 +0100
---
 xen/arch/x86/x86_64/compat/entry.S |  4 ++++
 xen/arch/x86/x86_64/entry.S        | 15 ++++++++-------
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/x86_64/compat/entry.S 
b/xen/arch/x86/x86_64/compat/entry.S
index 94f5b8d..f337251 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -173,6 +173,10 @@ compat_bad_hypercall:
 /* %rbx: struct vcpu, interrupts disabled */
 ENTRY(compat_restore_all_guest)
         ASSERT_INTERRUPTS_DISABLED
+        mov   $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),%r11d
+        and   UREGS_eflags(%rsp),%r11d
+        or    $X86_EFLAGS_IF,%r11
+        mov   %r11d,UREGS_eflags(%rsp)
         RESTORE_ALL adj=8 compat=1
 .Lft0:  iretq
 
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 4bb48e7..76530f3 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -41,28 +41,29 @@ restore_all_guest:
         testw $TRAP_syscall,4(%rsp)
         jz    iret_exit_to_guest
 
+        movq  24(%rsp),%r11           # RFLAGS
+        andq  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),%r11
+        orq   $X86_EFLAGS_IF,%r11
+
         /* Don't use SYSRET path if the return address is not canonical. */
         movq  8(%rsp),%rcx
         sarq  $47,%rcx
         incl  %ecx
         cmpl  $1,%ecx
-        ja    .Lforce_iret
+        movq  8(%rsp),%rcx            # RIP
+        ja    iret_exit_to_guest
 
         cmpw  $FLAT_USER_CS32,16(%rsp)# CS
-        movq  8(%rsp),%rcx            # RIP
-        movq  24(%rsp),%r11           # RFLAGS
         movq  32(%rsp),%rsp           # RSP
         je    1f
         sysretq
 1:      sysretl
 
-.Lforce_iret:
-        /* Mimic SYSRET behavior. */
-        movq  8(%rsp),%rcx            # RIP
-        movq  24(%rsp),%r11           # RFLAGS
         ALIGN
 /* No special register assumptions. */
 iret_exit_to_guest:
+        andl  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),24(%rsp)
+        orl   $X86_EFLAGS_IF,24(%rsp)
         addq  $8,%rsp
 .Lft0:  iretq
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.4

_______________________________________________
Xen-changelog mailing list
Xen-changelog@xxxxxxxxxxxxx
https://lists.xenproject.org/xen-changelog