[PATCH v5 01/17] x86/alternative: Support alt_replace_call() with instructions after call

[Juergen Gross]

Today alt_replace_call() requires the initial indirect call not to be
followed by any further instructions, including padding NOPs. In case
any replacement is longer than 6 bytes, a subsequent replacement of
the indirect call with a direct one will result in a crash.

Fix that by crashing only if the original instruction is less than
6 bytes long or not a known indirect call.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/kernel/alternative.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 91b1cdd16569..21b344761ee2 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -525,6 +525,7 @@ noinstr void BUG_func(void)
 }
 EXPORT_SYMBOL(BUG_func);
 
+#define CALL_RIP_INSTRLEN	6
 #define CALL_RIP_REL_OPCODE	0xff
 #define CALL_RIP_REL_MODRM	0x15
 
@@ -542,7 +543,7 @@ static unsigned int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr
 		BUG();
 	}
 
-	if (a->instrlen != 6 ||
+	if (a->instrlen < CALL_RIP_INSTRLEN ||
 	    instr[0] != CALL_RIP_REL_OPCODE ||
 	    instr[1] != CALL_RIP_REL_MODRM) {
 		pr_err("ALT_FLAG_DIRECT_CALL set for unrecognized indirect call\n");
@@ -554,7 +555,7 @@ static unsigned int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr
 #ifdef CONFIG_X86_64
 	/* ff 15 00 00 00 00   call   *0x0(%rip) */
 	/* target address is stored at "next instruction + disp". */
-	target = *(void **)(instr + a->instrlen + disp);
+	target = *(void **)(instr + CALL_RIP_INSTRLEN + disp);
 #else
 	/* ff 15 00 00 00 00   call   *0x0 */
 	/* target address is stored at disp. */
-- 
2.55.0