x86/PV: address odd UB in I/O emulation

Compilers are certainly right in detecting UB here, given that fully
parenthesized (to express precedence) the original offending expression
was (((stub_va + p) - ctxt->io_emul_stub) + 5), which in fact exhibits
two overflows in pointer calculations. We really want to calculate
(p - ctxt->io_emul_stub) first, which is guaranteed to not overflow.

The issue was observed with clang 9 on 4.13.

The oddities are
- the issue was detected on APPEND_CALL(save_guest_gprs), despite the
  earlier similar APPEND_CALL(load_guest_gprs),
- merely casting the original offending expression to long was reported
  to also help.

While at it also avoid converting guaranteed (with our current address
space layout) negative values to unsigned long (which has implementation
defined behavior): Have stub_va be of pointer type. And since it's on an
immediately adjacent line, also constify this_stubs.

Fixes: d89e5e65f305 ("x86/ioemul: Rewrite stub generation to be shadow stack compatible")
Reported-by: Franklin Shen <2284696125@qq.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index d0df5bc616c0bf4cabcf7d5aaa7bddc69069cc47..7f4279a0517b9b5906deadb002aec1f5612238af 100644 (file)
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -90,8 +90,8 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
         0xc3,       /* ret       */
     };
 
-    struct stubs *this_stubs = &this_cpu(stubs);
-    unsigned long stub_va = this_stubs->addr + STUB_BUF_SIZE / 2;
+    const struct stubs *this_stubs = &this_cpu(stubs);
+    const void *stub_va = (void *)this_stubs->addr + STUB_BUF_SIZE / 2;
     unsigned int quirk_bytes = 0;
     char *p;
 
@@ -99,7 +99,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
 #define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p += sizeof(b); })
 #define APPEND_CALL(f)                                                  \
     ({                                                                  \
-        long disp = (long)(f) - (stub_va + p - ctxt->io_emul_stub + 5); \
+        long disp = (void *)(f) - (stub_va + (p - ctxt->io_emul_stub) + 5); \
         BUG_ON((int32_t)disp != disp);                                  \
         *p++ = 0xe8;                                                    \
         *(int32_t *)p = disp; p += 4;                                   \
@@ -107,7 +107,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
 
     if ( !ctxt->io_emul_stub )
         ctxt->io_emul_stub =
-            map_domain_page(_mfn(this_stubs->mfn)) + (stub_va & ~PAGE_MASK);
+            map_domain_page(_mfn(this_stubs->mfn)) + PAGE_OFFSET(stub_va);
 
     p = ctxt->io_emul_stub;
 
@@ -142,7 +142,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
     block_speculation(); /* SCSB */
 
     /* Handy function-typed pointer to the stub. */
-    return (void *)stub_va;
+    return stub_va;
 
 #undef APPEND_CALL
 #undef APPEND_BUFF