[Xen-changelog] [xen stable-4.6] x86: force EFLAGS.IF on when exiting to PV guests

[patchbot@xxxxxxx]

commit c7f06e4eedcfd17024d48849bc8b047b813da0f7
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Dec 21 17:45:05 2016 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Dec 21 17:45:05 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 1eaf4bb..9f8053e 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -174,6 +174,8 @@ 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
 .Lcr4_orig:
         .skip .Lcr4_alt_end - .Lcr4_alt, 0x90
 .Lcr4_orig_end:
@@ -209,6 +211,8 @@ 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
 
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 0c2e63a..ba5925b 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
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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