linux-kernel - [PATCH 10/13] x86/alternative: Support indirect call replacement

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Date:   Wed,  4 Oct 2017 10:58:31 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        live-patching@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>,
        Chris Wright <chrisw@...s-sol.org>,
        Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        virtualization@...ts.linux-foundation.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Subject: [PATCH 10/13] x86/alternative: Support indirect call replacement

Add alternative patching support for replacing an instruction with an
indirect call.  This will be needed for the paravirt alternatives.

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/kernel/alternative.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d3382e91..81c577c7deba 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -410,20 +410,28 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		insnbuf_sz = a->replacementlen;
 
 		/*
-		 * 0xe8 is a relative jump; fix the offset.
-		 *
-		 * Instruction length is checked before the opcode to avoid
-		 * accessing uninitialized bytes for zero-length replacements.
+		 * Fix the address offsets for call and jump instructions which
+		 * use PC-relative addressing.
 		 */
 		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
+			/* direct call */
 			*(s32 *)(insnbuf + 1) += replacement - instr;
-			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
+			DPRINTK("Fix direct CALL offset: 0x%x, CALL 0x%lx",
 				*(s32 *)(insnbuf + 1),
 				(unsigned long)instr + *(s32 *)(insnbuf + 1) + 5);
-		}
 
-		if (a->replacementlen && is_jmp(replacement[0]))
+		} else if (a->replacementlen == 6 && *insnbuf == 0xff &&
+			   *(insnbuf+1) == 0x15) {
+			/* indirect call */
+			*(s32 *)(insnbuf + 2) += replacement - instr;
+			DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx",
+				*(s32 *)(insnbuf + 2),
+				(unsigned long)instr + *(s32 *)(insnbuf + 2) + 6);
+
+		} else if (a->replacementlen && is_jmp(replacement[0])) {
+			/* direct jump */
 			recompute_jump(a, instr, replacement, insnbuf);
+		}
 
 		if (a->instrlen > a->replacementlen) {
 			add_nops(insnbuf + a->replacementlen,
-- 
2.13.6