linux-kernel - [PATCH v2 1/2] vfs: block chmod of symlinks

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Message-ID: <20200916002253.GP3265@brightrain.aerifal.cx>
Date:   Tue, 15 Sep 2020 20:22:54 -0400
From:   Rich Felker <dalias@...c.org>
To:     linux-api@...r.kernel.org
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/2] vfs: block chmod of symlinks

It was discovered while implementing userspace emulation of fchmodat
AT_SYMLINK_NOFOLLOW (using O_PATH and procfs magic symlinks; otherwise
it's not possible to target symlinks with chmod operations) that some
filesystems erroneously allow access mode of symlinks to be changed,
but return failure with EOPNOTSUPP (see glibc issue #14578 and commit
a492b1e5ef). This inconsistency is non-conforming and wrong, and the
consensus seems to be that it was unintentional to allow link modes to
be changed in the first place.

Signed-off-by: Rich Felker <dalias@...c.org>
---
 fs/open.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/open.c b/fs/open.c
index 9af548fb841b..cdb7964aaa6e 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -570,6 +570,12 @@ int chmod_common(const struct path *path, umode_t mode)
 	struct iattr newattrs;
 	int error;

+	/* Block chmod from getting to fs layer. Ideally the fs would either
+	 * allow it or fail with EOPNOTSUPP, but some are buggy and return
+	 * an error but change the mode, which is non-conforming and wrong. */
+	if (S_ISLNK(inode->i_mode))
+		return -EOPNOTSUPP;
+
 	error = mnt_want_write(path->mnt);
 	if (error)
 		return error;
-- 
2.21.0