linux-kernel - [tip:x86/pti] objtool: Fix segfault in ignore_unreachable

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Date:   Tue, 13 Feb 2018 03:29:46 -0800
From:   tip-bot for Josh Poimboeuf <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     brgerst@...il.com, tglx@...utronix.de, bp@...en8.de,
        torvalds@...ux-foundation.org, jpoimboe@...hat.com,
        luto@...nel.org, arjan@...ux.intel.com, fengguang.wu@...el.com,
        peterz@...radead.org, mingo@...nel.org, hpa@...or.com,
        dvlasenk@...hat.com, linux-kernel@...r.kernel.org
Subject: [tip:x86/pti] objtool: Fix segfault in ignore_unreachable_insn()

Commit-ID:  59d13cd6789e8feb4615bb999d46bbd439a9a396
Gitweb:     https://git.kernel.org/tip/59d13cd6789e8feb4615bb999d46bbd439a9a396
Author:     Josh Poimboeuf <jpoimboe@...hat.com>
AuthorDate: Thu, 8 Feb 2018 17:09:25 -0600
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 13 Feb 2018 11:46:59 +0100

objtool: Fix segfault in ignore_unreachable_insn()

Peter Zijlstra's patch for converting WARN() to use UD2 triggered a
bunch of false "unreachable instruction" warnings, which then triggered
a seg fault in ignore_unreachable_insn().

The seg fault happened when it tried to dereference a NULL 'insn->func'
pointer.  Thanks to static_cpu_has(), some functions can jump to a
non-function area in the .altinstr_aux section.  That breaks
ignore_unreachable_insn()'s assumption that it's always inside the
original function.

Make sure ignore_unreachable_insn() only follows jumps within the
current function.

Reported-by: Borislav Petkov <bp@...en8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: kbuild test robot <fengguang.wu@...el.com>
Link: http://lkml.kernel.org/r/bace77a60d5af9b45eddb8f8fb9c776c8de657ef.1518130694.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 tools/objtool/check.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 2e458eb..c7fb5c2 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1935,13 +1935,19 @@ static bool ignore_unreachable_insn(struct instruction *insn)
 		if (is_kasan_insn(insn) || is_ubsan_insn(insn))
 			return true;
 
-		if (insn->type == INSN_JUMP_UNCONDITIONAL && insn->jump_dest) {
-			insn = insn->jump_dest;
-			continue;
+		if (insn->type == INSN_JUMP_UNCONDITIONAL) {
+			if (insn->jump_dest &&
+			    insn->jump_dest->func == insn->func) {
+				insn = insn->jump_dest;
+				continue;
+			}
+
+			break;
 		}
 
 		if (insn->offset + insn->len >= insn->func->offset + insn->func->len)
 			break;
+
 		insn = list_next_entry(insn, list);
 	}