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From 777ebe30e81ab284f9b78392875fe884a593df35 Mon Sep 17 00:00:00 2001
From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Wed, 15 Jun 2016 18:32:14 +0100
Subject: [PATCH] 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>
---
v2:
 * Include CLAC on the exit paths from compat_create_bounce_frame which occur
   from faults attempting to load %fs
 * Reposition ASM_STAC to avoid breaking the macro-op fusion of test/jz
---
 xen/arch/x86/x86_64/compat/entry.S | 3 +++
 xen/arch/x86/x86_64/entry.S        | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/xen/arch/x86/x86_64/compat/entry.S b/xen/arch/x86/x86_64/compat/entry.S
index 0e3db7c..1eaf4bb 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -350,6 +350,7 @@ 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. */
@@ -403,6 +404,7 @@ 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|\
@@ -448,6 +450,7 @@ 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 6e27508..0c2e63a 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -462,9 +462,11 @@ 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
-- 
2.1.4