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). Signed-off-by: Jan Beulich <jbeul...@suse.com> Acked-by: Roger Pau Monné <roger....@citrix.com> --- 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. --- v4: Drop left-over checking of clang for \(text) handling. v3: Use .byte 0xc[23] instead of the nested macros. v2: Fix build with newer clang. Use int3 mnemonic. Also override retq. --- a/xen/Makefile +++ b/xen/Makefile @@ -145,7 +145,10 @@ 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 macros can override insn mnemonics in inline assembly. +t4 = $(call as-insn,$(CC),".macro ret; .error; .endm; .macro retq; .error; .endm",-no-integrated-as) + +CLANG_FLAGS += $(call or,$(t1),$(t2),$(t3),$(t4)) endif CLANG_FLAGS += -Werror=unknown-warning-option --- a/xen/include/asm-x86/asm-defns.h +++ b/xen/include/asm-x86/asm-defns.h @@ -44,3 +44,19 @@ .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 + retq \operand +.endm +.macro retq operand:vararg + .ifb \operand + .byte 0xc3 + .else + .byte 0xc2 + .word \operand + .endif +.endm