[PATCH v2 5/6] x86: guard against straight-line speculation past RET

[Jan Beulich <jbeulich@suse.com>]

Under certain conditions CPUs can speculate into the instruction stream
past a RET instruction. Guard against this just like 3b7dab93f240
("x86/spec-ctrl: Protect against CALL/JMP straight-line speculation")
did - by inserting an "INT $3" insn. It's merely the mechanics of how to
achieve this that differ: A set of macros gets introduced to post-
process RET insns issued by the compiler (or living in assembly files).

Unfortunately for clang this requires further features their built-in
assembler doesn't support: We need to be able to override insn mnemonics
produced by the compiler (which may be impossible, if internally
assembly mnemonics never get generated), and we want to use \(text)
escaping / quoting in the auxiliary macro.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
TBD: Should this depend on CONFIG_SPECULATIVE_HARDEN_BRANCH?
TBD: Would be nice to avoid the additions in .init.text, but a query to
     the binutils folks regarding the ability to identify the section
     stuff is in (by Peter Zijlstra over a year ago:
     https://sourceware.org/pipermail/binutils/2019-July/107528.html)
     has been left without helpful replies.
---
v2: Fix build with newer clang. Use int3 mnemonic. Also override retq.

--- a/xen/Makefile
+++ b/xen/Makefile
@@ -145,7 +145,15 @@ t2 = $(call as-insn,$(CC) -I$(BASEDIR)/i
 # https://bugs.llvm.org/show_bug.cgi?id=36110
 t3 = $(call as-insn,$(CC),".macro FOO;.endm"$(close); asm volatile $(open)".macro FOO;.endm",-no-integrated-as)
 
-CLANG_FLAGS += $(call or,$(t1),$(t2),$(t3))
+# Check whether \(text) escaping in macro bodies is supported.
+t4 = $(call as-insn,$(CC),".macro m ret:req; \\(ret) $$\\ret; .endm; m 8",,-no-integrated-as)
+
+# Check whether macros can override insn mnemonics in inline assembly.
+t5 = $(call as-insn,$(CC),".macro ret; .error; .endm; .macro retq; .error; .endm",-no-integrated-as)
+
+acc1 := $(call or,$(t1),$(t2),$(t3),$(t4))
+
+CLANG_FLAGS += $(call or,$(acc1),$(t5))
 endif
 
 CLANG_FLAGS += -Werror=unknown-warning-option
--- a/xen/include/asm-x86/asm-defns.h
+++ b/xen/include/asm-x86/asm-defns.h
@@ -50,3 +50,22 @@
 .macro INDIRECT_JMP arg:req
     INDIRECT_BRANCH jmp \arg
 .endm
+
+/*
+ * To guard against speculation past RET, insert a breakpoint insn
+ * immediately after them.
+ */
+.macro ret operand:vararg
+    ret$ \operand
+.endm
+.macro retq operand:vararg
+    ret$ \operand
+.endm
+.macro ret$ operand:vararg
+    .purgem ret
+    ret \operand
+    int3
+    .macro ret operand:vararg
+        ret$ \\(operand)
+    .endm
+.endm