linux-kernel - [PATCH RESEND v17 01/13] namei: only return -ECHILD from follow_dotdot

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Message-Id: <20191120050631.12816-2-cyphar@cyphar.com>
Date:   Wed, 20 Nov 2019 16:06:19 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Jonathan Corbet <corbet@....net>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        dev@...ncontainers.org, containers@...ts.linux-foundation.org,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-alpha@...r.kernel.org, linux-api@...r.kernel.org,
        libc-alpha@...rceware.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
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        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: [PATCH RESEND v17 01/13] namei: only return -ECHILD from follow_dotdot_rcu()

It's over-zealous to return hard errors under RCU-walk here, given that
a REF-walk will be triggered for all other cases handling ".." under
RCU.

The original purpose of this check was to ensure that if a rename occurs
such that a directory is moved outside of the bind-mount which the
resolution started in, it would be detected and blocked to avoid being
able to mess with paths outside of the bind-mount. However, triggering a
new REF-walk is just as effective a solution.

Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Fixes: 397d425dc26d ("vfs: Test for and handle paths that are unreachable from their mnt_root")
Suggested-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 fs/namei.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 671c3c1a3425..5a47d9c09581 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1359,7 +1359,7 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 			nd->path.dentry = parent;
 			nd->seq = seq;
 			if (unlikely(!path_connected(&nd->path)))
-				return -ENOENT;
+				return -ECHILD;
 			break;
 		} else {
 			struct mount *mnt = real_mount(nd->path.mnt);
-- 
2.24.0