On Wed, Jul 12, 2023 at 06:21:51PM +0200, Greg KH wrote:
On Wed, Jul 12, 2023 at 11:56:35AM +0200, Christian Brauner wrote:
Changing the mode of symlinks is meaningless as the vfs doesn't take the mode of a symlink into account during path lookup permission checking.
However, the vfs doesn't block mode changes on symlinks. This however, has lead to an untenable mess roughly classifiable into the following two categories:
(1) Filesystems that don't implement a i_op->setattr() for symlinks.
Such filesystems may or may not know that without i_op->setattr() defined, notify_change() falls back to simple_setattr() causing the inode's mode in the inode cache to be changed. That's a generic issue as this will affect all non-size changing inode attributes including ownership changes. Example: afs(2) Filesystems that fail with EOPNOTSUPP but change the mode of the symlink nonetheless.
Some filesystems will happily update the mode of a symlink but still return EOPNOTSUPP. This is the biggest source of confusion for userspace. The EOPNOTSUPP in this case comes from POSIX ACLs. Specifically it comes from filesystems that call posix_acl_chmod(), e.g., btrfs via if (!err && attr->ia_valid & ATTR_MODE) err = posix_acl_chmod(idmap, dentry, inode->i_mode); Filesystems including btrfs don't implement i_op->set_acl() so posix_acl_chmod() will report EOPNOTSUPP. When posix_acl_chmod() is called, most filesystems will have finished updating the inode. Perversely, this has the consequences that this behavior may depend on two kconfig options and mount options: * CONFIG_POSIX_ACL={y,n} * CONFIG_${FSTYPE}_POSIX_ACL={y,n} * Opt_acl, Opt_noacl Example: btrfs, ext4, xfsThe only way to change the mode on a symlink currently involves abusing an O_PATH file descriptor in the following manner:
fd = openat(-1, "/path/to/link", O_CLOEXEC | O_PATH | O_NOFOLLOW); char path[PATH_MAX]; snprintf(path, sizeof(path), "/proc/self/fd/%d", fd); chmod(path, 0000);But for most major filesystems with POSIX ACL support such as btrfs, ext4, ceph, tmpfs, xfs and others this will fail with EOPNOTSUPP with the mode still updated due to the aforementioned posix_acl_chmod() nonsense.
So, given that for all major filesystems this would fail with EOPNOTSUPP and that both glibc (cf. [1]) and musl (cf. [2]) outright block mode changes on symlinks we should just try and block mode changes on symlinks directly in the vfs and have a clean break with this nonsense.
If this causes any regressions, we do the next best thing and fix up all filesystems that do return EOPNOTSUPP with the mode updated to not call posix_acl_chmod() on symlinks.
But as usual, let's try the clean cut solution first. It's a simple patch that can be easily reverted. Not marking this for backport as I'll do that manually if we're reasonably sure that this works and there are no strong objections.
We could block this in chmod_common() but it's more appropriate to do it notify_change() as it will also mean that we catch filesystems that change symlink permissions explicitly or accidently.
Similar proposals were floated in the past as in [3] and [4] and again recently in [5]. There's also a couple of bugs about this inconsistency as in [6] and [7].
Link: https://sourceware.org/git/?p=glibc.git%3Ba=blob%3Bf=sysdeps/unix/sysv/linux... [1] Link: https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c [2] Link: https://lore.kernel.org/all/20200911065733.GA31579@infradead.org [3] Link: https://sourceware.org/legacy-ml/libc-alpha/2020-02/msg00518.html [4] Link: https://lore.kernel.org/lkml/87lefmbppo.fsf@oldenburg.str.redhat.com [5] Link: https://sourceware.org/legacy-ml/libc-alpha/2020-02/msg00467.html [6] Link: https://sourceware.org/bugzilla/show_bug.cgi?id=14578#c17 [7] Cc: stable@vger.kernel.org # no backport before v6.6-rc2 is tagged
How far back should this go?
If this holds up without regressions than all LTSes. That's what Amir and Leah did for some other work. I can add that to the comment for clarity.