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@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 0e47f92b072548800223f9a21ea051a017173915
master date: 2016-12-21 16:46:13 +0100

diff --git a/xen/arch/x86/x86_64/compat/entry.S b/xen/arch/x86/x86_64/compat/entry.S
index 94f5b8d6721e53647cefc443ed166c1c803dac8a..f3372510b3f18102e193b998ccd66cd31f4057f2 100644 (file)
--- 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 4bb48e78e86ba415b840cf9208b702e774898a6a..76530f3baf212114843299db94dc561cf44827c8 100644 (file)
--- 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