[Xen-changelog] [xen stable-4.8] x86: force EFLAGS.IF on when exiting to PV guests

[patchbot@xxxxxxx]

commit 7628c7efb4afca8cb2264bc3ab59830e481b37a8
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Dec 21 17:40:37 2016 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Dec 21 17:40:37 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 3bb6b61..474ffbc 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -109,6 +109,8 @@ compat_process_trap:
 /* %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
 .Lcr4_orig:
         .skip .Lcr4_alt_end - .Lcr4_alt, 0x90
 .Lcr4_orig_end:
@@ -144,6 +146,8 @@ ENTRY(compat_restore_all_guest)
                              (.Lcr4_orig_end - .Lcr4_orig), \
                              (.Lcr4_alt_end - .Lcr4_alt)
         .popsection
+        or    $X86_EFLAGS_IF,%r11
+        mov   %r11d,UREGS_eflags(%rsp)
         RESTORE_ALL adj=8 compat=1
 .Lft0:  iretq
         _ASM_PRE_EXTABLE(.Lft0, handle_exception)
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 66aefaa..85f1a4b 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -40,28 +40,29 @@ 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
         _ASM_PRE_EXTABLE(.Lft0, handle_exception)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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