linux-kernel - [tip: x86/bugs] objtool: Fix return thunk patching in retpolines

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <169713303673.3135.1345736146578099050.tip-bot2@tip-bot2>
Date:   Thu, 12 Oct 2023 17:50:36 -0000
From:   "tip-bot2 for Josh Poimboeuf" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     David Kaplan <david.kaplan@....com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/bugs] objtool: Fix return thunk patching in retpolines

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID:     eadbe6606a85610c63e6cfaa9257dc7e3bbb901c
Gitweb:        https://git.kernel.org/tip/eadbe6606a85610c63e6cfaa9257dc7e3bbb901c
Author:        Josh Poimboeuf <jpoimboe@...nel.org>
AuthorDate:    Wed, 11 Oct 2023 19:47:37 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Thu, 12 Oct 2023 19:43:51 +02:00

objtool: Fix return thunk patching in retpolines

With CONFIG_RETHUNK enabled, the compiler replaces every RET with a tail
call to a return thunk ('JMP __x86_return_thunk').  Objtool annotates
all such return sites so they can be patched during boot by
apply_returns().

The implementation of __x86_return_thunk() is just a bare RET.  It's
only meant to be used temporarily until apply_returns() patches all
return sites with either a JMP to another return thunk or an actual RET.

The following commit:

  e92626af3234 ("x86/retpoline: Remove .text..__x86.return_thunk section")

broke objtool's detection of return sites in retpolines.  Since
retpolines and return thunks are now in the same section, the compiler
no longer uses relocations for the intra-section jumps between the
retpolines and the return thunk, causing objtool to overlook them.

As a result, none of the retpolines' return sites get patched.  Each one
stays at 'JMP __x86_return_thunk', effectively a bare RET.

Fix it by teaching objtool to detect when a non-relocated jump target is
a return thunk (or retpoline).

Fixes: e92626af3234 ("x86/retpoline: Remove .text..__x86.return_thunk section")
Reported-by: David Kaplan <david.kaplan@....com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20231012024737.eg5phclogp67ik6x@treble
---
 tools/objtool/check.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e308d1b..e94756e 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1611,6 +1611,22 @@ static int add_jump_destinations(struct objtool_file *file)
 		}

 		/*
+		 * An intra-TU jump in retpoline.o might not have a relocation
+		 * for its jump dest, in which case the above
+		 * add_{retpoline,return}_call() didn't happen.
+		 */
+		if (jump_dest->sym && jump_dest->offset == jump_dest->sym->offset) {
+			if (jump_dest->sym->retpoline_thunk) {
+				add_retpoline_call(file, insn);
+				continue;
+			}
+			if (jump_dest->sym->return_thunk) {
+				add_return_call(file, insn, true);
+				continue;
+			}
+		}
+
+		/*
 		 * Cross-function jump.
 		 */
 		if (insn_func(insn) && insn_func(jump_dest) &&