[Xen-changelog] [xen stable-4.8] x86: don't latch wrong (stale) GS base addresses

[patchbot@xxxxxxx]

commit 651d839afa0f2f8dea1379a7207c22232c75ca63
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Nov 16 11:51:46 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Nov 16 11:51:46 2017 +0100

    x86: don't latch wrong (stale) GS base addresses
    
    load_segments() writes selector registers before doing any of the base
    address updates. Any of these selector loads can cause a page fault in
    case it references the LDT, and the LDT page accessed was only recently
    installed. Therefore the call tree map_ldt_shadow_page() ->
    guest_get_eff_kern_l1e() -> toggle_guest_mode() would in such a case
    wrongly latch the outgoing vCPU's GS.base into the incoming vCPU's
    recorded state.
    
    Split page table toggling from GS handling - neither
    guest_get_eff_kern_l1e() nor guest_io_okay() need more than the page
    tables being the kernel ones for the memory access they want to do.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: a711f6f24a7157ae70d1cc32e61b98f23dc0c584
    master date: 2017-10-27 13:49:10 +0100
---
 xen/arch/x86/mm.c            |  2 +-
 xen/arch/x86/traps.c         |  2 +-
 xen/arch/x86/x86_64/traps.c  | 11 ++++++++++-
 xen/include/asm-x86/domain.h |  2 ++
 4 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 494a757..3f05663 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -582,7 +582,7 @@ static inline void guest_get_eff_kern_l1e(struct vcpu *v, 
unsigned long addr,
                                           void *eff_l1e)
 {
     bool_t user_mode = !(v->arch.flags & TF_kernel_mode);
-#define TOGGLE_MODE() if ( user_mode ) toggle_guest_mode(v)
+#define TOGGLE_MODE() if ( user_mode ) toggle_guest_pt(v)
 
     TOGGLE_MODE();
     guest_get_eff_l1e(addr, eff_l1e);
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 2d48f2c..249932a 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2056,7 +2056,7 @@ static int guest_io_okay(
 {
     /* If in user mode, switch to kernel mode just to read I/O bitmap. */
     int user_mode = !(v->arch.flags & TF_kernel_mode);
-#define TOGGLE_MODE() if ( user_mode ) toggle_guest_mode(v)
+#define TOGGLE_MODE() if ( user_mode ) toggle_guest_pt(v)
 
     if ( iopl_ok(v, regs) )
         return 1;
diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index 02f4717..a9b0282 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -278,8 +278,17 @@ void toggle_guest_mode(struct vcpu *v)
         else
             v->arch.pv_vcpu.gs_base_user = __rdgsbase();
     }
-    v->arch.flags ^= TF_kernel_mode;
     asm volatile ( "swapgs" );
+
+    toggle_guest_pt(v);
+}
+
+void toggle_guest_pt(struct vcpu *v)
+{
+    if ( is_pv_32bit_vcpu(v) )
+        return;
+
+    v->arch.flags ^= TF_kernel_mode;
     update_cr3(v);
     /* Don't flush user global mappings from the TLB. Don't tick TLB clock. */
     asm volatile ( "mov %0, %%cr3" : : "r" (v->arch.cr3) : "memory" );
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 60bb8c9..82dca93 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -76,6 +76,8 @@ void mapcache_override_current(struct vcpu *);
 
 /* x86/64: toggle guest between kernel and user modes. */
 void toggle_guest_mode(struct vcpu *);
+/* x86/64: toggle guest page tables between kernel and user modes. */
+void toggle_guest_pt(struct vcpu *);
 
 /*
  * Initialise a hypercall-transfer page. The given pointer must be mapped
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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