x86/hvm: Fixes to hvmemul_insn_fetch()

Force insn_off to a single byte, as offset can wrap around or truncate with
respect to sh_ctxt->insn_buf_eip under a number of normal circumstances.

Furthermore, don't use an ASSERT() for bounds checking the write into
hvmemul_ctxt->insn_buf[].

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
x86/hvm: Fix boundary check in hvmemul_insn_fetch()

c/s 0943a03037 added some extra protection for overflowing the emulation
instruction cache, but Coverity points out that boundary condition is off by
one when memcpy()'ing out of the buffer.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
x86/HVM: fix boundary check in hvmemul_insn_fetch() (again)

Commit 5a992b670b ("x86/hvm: Fix boundary check in
hvmemul_insn_fetch()") went a little too far in its correction to
commit 0943a03037 ("x86/hvm: Fixes to hvmemul_insn_fetch()"): Keep the
start offset check, but restore the original end offset one.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
master commit: 0943a03037418e6e40cdd420f2472bbf9afae7a2
master date: 2017-07-19 10:25:18 +0100
master commit: 5a992b670bff697c40b513c9e037598ba35ca7d4
master date: 2017-07-27 11:39:57 +0100
master commit: 58e8986267d976b00c60e0089baa2e5f66f16d3e
master date: 2017-08-10 12:37:24 +0200

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index 40fa4f03452291786917e0ebc0e395e0b987d399..098f0972a21e429bfaba49dc5397a96bbec29f7e 100644 (file)
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -844,7 +844,8 @@ int hvmemul_insn_fetch(
 {
     struct hvm_emulate_ctxt *hvmemul_ctxt =
         container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
-    unsigned int insn_off = offset - hvmemul_ctxt->insn_buf_eip;
+    /* Careful, as offset can wrap or truncate WRT insn_buf_eip. */
+    uint8_t insn_off = offset - hvmemul_ctxt->insn_buf_eip;
 
     /*
      * Fall back if requested bytes are not in the prefetch cache.
@@ -858,7 +859,17 @@ int hvmemul_insn_fetch(
 
         if ( rc == X86EMUL_OKAY && bytes )
         {
-            ASSERT(insn_off + bytes <= sizeof(hvmemul_ctxt->insn_buf));
+            /*
+             * Will we overflow insn_buf[]?  This shouldn't be able to happen,
+             * which means something went wrong with instruction decoding...
+             */
+            if ( insn_off >= sizeof(hvmemul_ctxt->insn_buf) ||
+                 insn_off + bytes > sizeof(hvmemul_ctxt->insn_buf) )
+            {
+                ASSERT_UNREACHABLE();
+                return X86EMUL_UNHANDLEABLE;
+            }
+
             memcpy(&hvmemul_ctxt->insn_buf[insn_off], p_data, bytes);
             hvmemul_ctxt->insn_buf_bytes = insn_off + bytes;
         }