linux-kernel - [PATCH v2] procfs: block chmod on /proc/thread-self/comm

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Date:   Fri, 14 Jul 2023 00:09:58 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <brauner@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Chinner <dchinner@...hat.com>,
        xu xin <cgel.zte@...il.com>, Al Viro <viro@...iv.linux.org.uk>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Zhihao Cheng <chengzhihao1@...wei.com>,
        Stefan Roesch <shr@...kernel.io>,
        Janis Danisevskis <jdanis@...gle.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Thomas Weißschuh <thomas@...ch.de>,
        Aleksa Sarai <cyphar@...har.com>, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [PATCH v2] procfs: block chmod on /proc/thread-self/comm

Due to an oversight in commit 1b3044e39a89 ("procfs: fix pthread
cross-thread naming if !PR_DUMPABLE") in switching from REG to NOD,
chmod operations on /proc/thread-self/comm were no longer blocked as
they are on almost all other procfs files.

A very similar situation with /proc/self/environ was used to as a root
exploit a long time ago, but procfs has SB_I_NOEXEC so this is simply a
correctness issue.

Ref: https://lwn.net/Articles/191954/
Ref: 6d76fa58b050 ("Don't allow chmod() on the /proc/<pid>/ files")
Fixes: 1b3044e39a89 ("procfs: fix pthread cross-thread naming if !PR_DUMPABLE")
Cc: stable@...r.kernel.org # v4.7+
Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
v2: removed nolibc selftests as per review
v1: <https://lore.kernel.org/linux-fsdevel/20230713121907.9693-1-cyphar@cyphar.com/>
---
 fs/proc/base.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 05452c3b9872..7394229816f3 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3583,7 +3583,8 @@ static int proc_tid_comm_permission(struct mnt_idmap *idmap,
 }
 
 static const struct inode_operations proc_tid_comm_inode_operations = {
-		.permission = proc_tid_comm_permission,
+		.setattr	= proc_setattr,
+		.permission	= proc_tid_comm_permission,
 };
 
 /*
-- 
2.41.0