x86/segment: Bounds check accesses to emulation ctxt->seg_reg[]

HVM HAP codepaths have space for all segment registers in the seg_reg[]
cache (with x86_seg_none still risking an array overrun), while the shadow
codepaths only have space for the user segments.

Range check the input segment of *_get_seg_reg() against the size of the array
used to cache the results, to avoid overruns in the case that the callers
don't filter their input suitably.

Subsume the is_x86_user_segment(seg) checks from the shadow code, which were
an incomplete attempt at range checking, and are now superceeded.  Make
hvm_get_seg_reg() static, as it is not used outside of shadow/common.c

No functional change, but far easier to reason that no overflow is possible.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/x86: Fix build with clang following c/s 4fa0105

https://travis-ci.org/xen-project/xen/jobs/158494027#L2344

Clang complains:

  emulate.c:2016:14: error: comparison of unsigned enum expression < 0
  is always false [-Werror,-Wtautological-compare]
      if ( seg < 0 || seg >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
           ~~~ ^ ~

Clang is wrong to raise a warning like this.  The signed-ness of an enum is
implementation defined in C, and robust code must not assume the choices made
by the compiler.

In this case, dropping the < 0 check creates a latent bug which would result
in an array underflow when compiled with a compiler which chooses a signed
enum.

Work around the bug by explicitly pulling seg into an unsigned integer, and
only perform the upper bounds check.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
master commit: 4fa0105d95
master date: 2016-09-08 16:39:46 +0100
master commit: 4c47c47938
master date: 2016-09-09 15:31:01 +0100

xen/arch/x86/hvm/emulate.c

@@ -421,6 +421,8 @@ static int hvmemul_virtual_to_linear(
                            ? 1 : 4096);
 
     reg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
+    if ( IS_ERR(reg) )
+        return -PTR_ERR(reg);
 
     if ( (hvmemul_ctxt->ctxt.regs->eflags & X86_EFLAGS_DF) && (*reps > 1) )
     {
@@ -1000,6 +1002,10 @@ static int hvmemul_read_segment(
     struct hvm_emulate_ctxt *hvmemul_ctxt =
         container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
     struct segment_register *sreg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
+
+    if ( IS_ERR(sreg) )
+         return -PTR_ERR(sreg);
+
     memcpy(reg, sreg, sizeof(struct segment_register));
     return X86EMUL_OKAY;
 }
@@ -1013,6 +1019,9 @@ static int hvmemul_write_segment(
         container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
     struct segment_register *sreg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
 
+    if ( IS_ERR(sreg) )
+         return -PTR_ERR(sreg);
+
     memcpy(sreg, reg, sizeof(struct segment_register));
     __set_bit(seg, &hvmemul_ctxt->seg_reg_dirty);
 
@@ -1477,13 +1486,22 @@ void hvm_emulate_writeback(
     }
 }
 
+/*
+ * Callers which pass a known in-range x86_segment can rely on the return
+ * pointer being valid.  Other callers must explicitly check for errors.
+ */
 struct segment_register *hvmemul_get_seg_reg(
     enum x86_segment seg,
     struct hvm_emulate_ctxt *hvmemul_ctxt)
 {
-    if ( !__test_and_set_bit(seg, &hvmemul_ctxt->seg_reg_accessed) )
-        hvm_get_segment_register(current, seg, &hvmemul_ctxt->seg_reg[seg]);
-    return &hvmemul_ctxt->seg_reg[seg];
+    unsigned int idx = seg;
+
+    if ( idx >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
+        return ERR_PTR(-X86EMUL_UNHANDLEABLE);
+
+    if ( !__test_and_set_bit(idx, &hvmemul_ctxt->seg_reg_accessed) )
+        hvm_get_segment_register(current, idx, &hvmemul_ctxt->seg_reg[idx]);
+    return &hvmemul_ctxt->seg_reg[idx];
 }
 
 static const char *guest_x86_mode_to_str(int mode)

xen/arch/x86/mm/shadow/common.c

@@ -120,12 +120,22 @@ __initcall(shadow_audit_key_init);
 /* x86 emulator support for the shadow code
  */
 
+/*
+ * Callers which pass a known in-range x86_segment can rely on the return
+ * pointer being valid.  Other callers must explicitly check for errors.
+ */
 struct segment_register *hvm_get_seg_reg(
     enum x86_segment seg, struct sh_emulate_ctxt *sh_ctxt)
 {
-    struct segment_register *seg_reg = &sh_ctxt->seg_reg[seg];
-    if ( !__test_and_set_bit(seg, &sh_ctxt->valid_seg_regs) )
-        hvm_get_segment_register(current, seg, seg_reg);
+    unsigned int idx = seg;
+    struct segment_register *seg_reg;
+
+    if ( idx >= ARRAY_SIZE(sh_ctxt->seg_reg) )
+        return ERR_PTR(-X86EMUL_UNHANDLEABLE);
+
+    seg_reg = &sh_ctxt->seg_reg[idx];
+    if ( !__test_and_set_bit(idx, &sh_ctxt->valid_seg_regs) )
+        hvm_get_segment_register(current, idx, seg_reg);
     return seg_reg;
 }
 
@@ -140,14 +150,9 @@ static int hvm_translate_linear_addr(
     struct segment_register *reg;
     int okay;
 
-    /*
-     * Can arrive here with non-user segments.  However, no such cirucmstance
-     * is part of a legitimate pagetable update, so fail the emulation.
-     */
-    if ( !is_x86_user_segment(seg) )
-        return X86EMUL_UNHANDLEABLE;
-
     reg = hvm_get_seg_reg(seg, sh_ctxt);
+    if ( IS_ERR(reg) )
+        return -PTR_ERR(reg);
 
     okay = hvm_virtual_to_linear_addr(
         seg, reg, offset, bytes, access_type, sh_ctxt->ctxt.addr_size, paddr);
@@ -249,9 +254,6 @@ hvm_emulate_write(enum x86_segment seg,
     unsigned long addr;
     int rc;
 
-    if ( !is_x86_user_segment(seg) )
-        return X86EMUL_UNHANDLEABLE;
-
     /* How many emulations could we save if we unshadowed on stack writes? */
     if ( seg == x86_seg_ss )
         perfc_incr(shadow_fault_emulate_stack);
@@ -279,9 +281,6 @@ hvm_emulate_cmpxchg(enum x86_segment seg,
     unsigned long addr, old[2], new[2];
     int rc;
 
-    if ( !is_x86_user_segment(seg) )
-        return X86EMUL_UNHANDLEABLE;
-
     rc = hvm_translate_linear_addr(
         seg, offset, bytes, hvm_access_write, sh_ctxt, &addr);
     if ( rc )

xen/include/asm-x86/hvm/emulate.h

@@ -13,6 +13,7 @@
 #define __ASM_X86_HVM_EMULATE_H__
 
 #include <xen/config.h>
+#include <xen/err.h>
 #include <asm/hvm/hvm.h>
 #include <asm/x86_emulate.h>