4.19-stable review patch. If anyone has any objections, please let me know.
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From: Christian Brauner christian@brauner.io
commit 94f82008ce30e2624537d240d64ce718255e0b80 upstream.
This reverts commit 55956b59df336f6738da916dbb520b6e37df9fbd.
commit 55956b59df33 ("vfs: Allow userns root to call mknod on owned filesystems.") enabled mknod() in user namespaces for userns root if CAP_MKNOD is available. However, these device nodes are useless since any filesystem mounted from a non-initial user namespace will set the SB_I_NODEV flag on the filesystem. Now, when a device node s created in a non-initial user namespace a call to open() on said device node will fail due to:
bool may_open_dev(const struct path *path) { return !(path->mnt->mnt_flags & MNT_NODEV) && !(path->mnt->mnt_sb->s_iflags & SB_I_NODEV); }
The problem with this is that as of the aforementioned commit mknod() creates partially functional device nodes in non-initial user namespaces. In particular, it has the consequence that as of the aforementioned commit open() will be more privileged with respect to device nodes than mknod(). Before it was the other way around. Specifically, if mknod() succeeded then it was transparent for any userspace application that a fatal error must have occured when open() failed.
All of this breaks multiple userspace workloads and a widespread assumption about how to handle mknod(). Basically, all container runtimes and systemd live by the slogan "ask for forgiveness not permission" when running user namespace workloads. For mknod() the assumption is that if the syscall succeeds the device nodes are useable irrespective of whether it succeeds in a non-initial user namespace or not. This logic was chosen explicitly to allow for the glorious day when mknod() will actually be able to create fully functional device nodes in user namespaces. A specific problem people are already running into when running 4.18 rc kernels are failing systemd services. For any distro that is run in a container systemd services started with the PrivateDevices= property set will fail to start since the device nodes in question cannot be opened (cf. the arguments in [1]).
Full disclosure, Seth made the very sound argument that it is already possible to end up with partially functional device nodes. Any filesystem mounted with MS_NODEV set will allow mknod() to succeed but will not allow open() to succeed. The difference to the case here is that the MS_NODEV case is transparent to userspace since it is an explicitly set mount option while the SB_I_NODEV case is an implicit property enforced by the kernel and hence opaque to userspace.
[1]: https://github.com/systemd/systemd/pull/9483
Signed-off-by: Christian Brauner christian@brauner.io Cc: "Eric W. Biederman" ebiederm@xmission.com Cc: Seth Forshee seth.forshee@canonical.com Cc: Serge Hallyn serge@hallyn.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/namei.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
--- a/fs/namei.c +++ b/fs/namei.c @@ -3701,8 +3701,7 @@ int vfs_mknod(struct inode *dir, struct if (error) return error;
- if ((S_ISCHR(mode) || S_ISBLK(mode)) && - !ns_capable(dentry->d_sb->s_user_ns, CAP_MKNOD)) + if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD)) return -EPERM;
if (!dir->i_op->mknod)