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/arch/x86/hvm/emulate.c

@@ -535,6 +535,8 @@ static int hvmemul_virtual_to_linear(
     *reps = min_t(unsigned long, *reps, max_reps);
 
     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) )
     {
@@ -1430,6 +1432,10 @@ 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;
 }
@@ -1443,6 +1449,9 @@ 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);
 
@@ -1995,10 +2004,17 @@ 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 ( seg < 0 || seg >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
+        return ERR_PTR(-X86EMUL_UNHANDLEABLE);
+
     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];

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

@@ -123,10 +123,19 @@ __initcall(shadow_audit_key_init);
 /* x86 emulator support for the shadow code
  */
 
-struct segment_register *hvm_get_seg_reg(
+/*
+ * Callers which pass a known in-range x86_segment can rely on the return
+ * pointer being valid.  Other callers must explicitly check for errors.
+ */
+static 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];
+    struct segment_register *seg_reg;
+
+    if ( seg < 0 || seg >= ARRAY_SIZE(sh_ctxt->seg_reg) )
+        return ERR_PTR(-X86EMUL_UNHANDLEABLE);
+
+    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);
     return seg_reg;
@@ -143,14 +152,9 @@ static int hvm_translate_linear_addr(
     const 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);
@@ -253,9 +257,6 @@ 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);
@@ -283,7 +284,7 @@ hvm_emulate_cmpxchg(enum x86_segment seg,
     unsigned long addr, old, new;
     int rc;
 
-    if ( !is_x86_user_segment(seg) || bytes > sizeof(long) )
+    if ( bytes > sizeof(long) )
         return X86EMUL_UNHANDLEABLE;
 
     rc = hvm_translate_linear_addr(

xen/arch/x86/mm/shadow/private.h

@@ -740,8 +740,6 @@ const struct x86_emulate_ops *shadow_init_emulation(
     struct sh_emulate_ctxt *sh_ctxt, struct cpu_user_regs *regs);
 void shadow_continue_emulation(
     struct sh_emulate_ctxt *sh_ctxt, struct cpu_user_regs *regs);
-struct segment_register *hvm_get_seg_reg(
-    enum x86_segment seg, struct sh_emulate_ctxt *sh_ctxt);
 
 #if (SHADOW_OPTIMIZATIONS & SHOPT_VIRTUAL_TLB)
 /**************************************************************************/

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>