x86/spec-ctrl: Protect against Speculative Code Store Bypass

Modern x86 processors have far-better-than-architecturally-guaranteed self
modifying code detection.  Typically, when a write hits an instruction in
flight, a Machine Clear occurs to flush stale content in the frontend and
backend.

For self modifying code, before a write which hits an instruction in flight
retires, the frontend can speculatively decode and execute the old instruction
stream.  Speculation of this form can suffer from type confusion in registers,
and potentially leak data.

Furthermore, updates are typically byte-wise, rather than atomic.  Depending
on timing, speculation can race ahead multiple times between individual
writes, and execute the transiently-malformed instruction stream.

Xen has stubs which are used in certain cases for emulation purposes.  Inhibit
speculation between updating the stub and executing it.

This is XSA-375 / CVE-2021-0089.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 45f59ed886)

xen/arch/x86/pv/emul-priv-op.c

@@ -138,6 +138,8 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
     /* Runtime confirmation that we haven't clobbered an adjacent stub. */
     BUG_ON(STUB_BUF_SIZE / 2 < (p - ctxt->io_emul_stub));
 
+    block_speculation(); /* SCSB */
+
     /* Handy function-typed pointer to the stub. */
     return (void *)stub_va;
 

xen/arch/x86/x86_emulate/x86_emulate.c

@@ -1256,6 +1256,7 @@ static inline int mkec(uint8_t e, int32_t ec, ...)
 # define invoke_stub(pre, post, constraints...) do {                    \
     stub_exn.info = (union stub_exception_token) { .raw = ~0 };         \
     stub_exn.line = __LINE__; /* Utility outweighs livepatching cost */ \
+    block_speculation(); /* SCSB */                                     \
     asm volatile ( pre "\n\tINDIRECT_CALL %[stub]\n\t" post "\n"        \
                    ".Lret%=:\n\t"                                       \
                    ".pushsection .fixup,\"ax\"\n"                       \