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 45f59ed8865318bb0356954bad067f329677ce9e)

diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 933036ea34674048c72450cfca68d734842cbe54..e9147e24c7efede13ff75d80e2ad7e68aa703976 100644 (file)
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -97,6 +97,8 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
     BUILD_BUG_ON(STUB_BUF_SIZE / 2 < MAX(9, /* Default emul stub */
                                          5 + IOEMUL_QUIRK_STUB_BYTES));
 
+    asm volatile ( "lfence" ::: "memory" ); /* SCSB */
+
     /* Handy function-typed pointer to the stub. */
     return (void *)stub_va;
 }

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index 09de6ee15d0eaf8568c7a211f93ebc7a8f5f8ee2..c7c55fa5722bc6b9e2ed8ce3c45fb1420072d4cd 100644 (file)
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1093,6 +1093,7 @@ 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 */ \
+    asm volatile ( "lfence" ::: "memory" ); /* SCSB */                  \
     asm volatile ( pre "\n\tINDIRECT_CALL %[stub]\n\t" post "\n"        \
                    ".Lret%=:\n\t"                                       \
                    ".pushsection .fixup,\"ax\"\n"                       \