[PATCH v2] x86: Prevent uninitialized stack byte read in apply_alternatives()

[Mateusz Jurczyk <mjurczyk@google.com>]

In the current form of the code, if a->replacementlen is 0, the reference
to *insnbuf for comparison touches potentially garbage memory. While it
doesn't affect the execution flow due to the subsequent a->replacementlen
comparison, it is (rightly) detected as use of uninitialized memory by a
runtime instrumentation currently under my development, and could be
detected as such by other tools in the future, too (e.g. KMSAN).

Fix the "false-positive" by reordering the conditions to first check the
replacement instruction length before referencing specific opcode bytes.

Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
---
Changes in v2:
  - Add an explaining comment, as per Borislav Petkov's request.

 arch/x86/kernel/alternative.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c5b8f760473c..32e14d137416 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -409,8 +409,13 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		memcpy(insnbuf, replacement, a->replacementlen);
 		insnbuf_sz = a->replacementlen;
 
-		/* 0xe8 is a relative jump; fix the offset. */
-		if (*insnbuf == 0xe8 && a->replacementlen == 5) {
+		/*
+		 * 0xe8 is a relative jump; fix the offset.
+		 *
+		 * Instruction length is checked before the opcode to avoid
+		 * accessing uninitialized bytes for zero-length replacements.
+		 */
+		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
 			*(s32 *)(insnbuf + 1) += replacement - instr;
 			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
 				*(s32 *)(insnbuf + 1),
-- 
2.13.0.219.gdb65acc882-goog