diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c5b8f760473c32f090c430693c5b2e8a61cc196f..32e14d13741670efa680c3c9e9facd1ffc0415a5 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),