x86/entry: Avoid SMAP violation in compat_create_bounce_frame()

A 32bit guest kernel might be running on user mappings.
compat_create_bounce_frame() must whitelist its guest accesses to avoid
risking a SMAP violation.

For both variants of create_bounce_frame(), re-blacklist user accesses if
execution exits via an exception table redirection.

This is XSA-183 / CVE-2016-6259

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/x86_64/compat/entry.S b/xen/arch/x86/x86_64/compat/entry.S
index 94bc3a3c75c1be7f73229fb5ab0066fb647eb412..f69813ddba9986e317b7255efd025249ff2aa403 100644 (file)
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -337,6 +337,7 @@ ENTRY(compat_int80_direct_trap)
 compat_create_bounce_frame:
         ASSERT_INTERRUPTS_ENABLED
         mov   %fs,%edi
+        ASM_STAC
         testb $2,UREGS_cs+8(%rsp)
         jz    1f
         /* Push new frame at registered guest-OS stack base. */
@@ -390,6 +391,7 @@ UNLIKELY_START(nz, compat_bounce_failsafe)
         movl  %ds,%eax
 .Lft12: movl  %eax,%fs:0*4(%rsi)        # DS
 UNLIKELY_END(compat_bounce_failsafe)
+        ASM_CLAC
         /* Rewrite our stack frame and return to guest-OS mode. */
         /* IA32 Ref. Vol. 3: TF, VM, RF and NT flags are cleared on trap. */
         andl  $~(X86_EFLAGS_VM|X86_EFLAGS_RF|\
@@ -435,6 +437,7 @@ compat_crash_page_fault_4:
         addl  $4,%esi
 compat_crash_page_fault:
 .Lft14: mov   %edi,%fs
+        ASM_CLAC
         movl  %esi,%edi
         call  show_page_walk
         jmp   dom_crash_sync_extable

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 91d20d4caa1a1262ee84cd115994006f5f8b5b4a..c6c0014638898a947e8fced2dff0fd15eabe0c90 100644 (file)
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -464,9 +464,11 @@ domain_crash_page_fault_16:
 domain_crash_page_fault_8:
         addq  $8,%rsi
 domain_crash_page_fault:
+        ASM_CLAC
         movq  %rsi,%rdi
         call  show_page_walk
 ENTRY(dom_crash_sync_extable)
+        ASM_CLAC
         # Get out of the guest-save area of the stack.
         GET_STACK_BASE(%rax)
         leaq  STACK_CPUINFO_FIELD(guest_cpu_user_regs)(%rax),%rsp