On Mon, Jan 04, 2021 at 07:51:27PM +0000, Al Viro wrote: > On Wed, Dec 23, 2020 at 02:26:04AM -0800, Sargun Dhillon wrote: > > ksys_umount was refactored to into split into another function > > (path_umount) to enable sharing code. This changed the order that flags and > > permissions are validated in, and made it so that user_path_at was called > > before validating flags and capabilities. > > > > Unfortunately, libfuse2[1] and libmount[2] rely on the old flag validation > > behaviour to determine whether or not the kernel supports UMOUNT_NOFOLLOW. > > The other path that this validation is being checked on is > > init_umount->path_umount->can_umount. That's all internal to the kernel. > > > > [1]: https://github.com/libfuse/libfuse/blob/9bfbeb576c5901b62a171d35510f0d1a922020b7/util/fusermount.c#L403 > > [2]: https://github.com/karelzak/util-linux/blob/7ed579523b556b1270f28dbdb7ee07dee310f157/libmount/src/context_umount.c#L813 > > Sorry, I don't like that solution. If nothing else, it turns path_umount() into > a landmine for the future. Yes, we have a regression, yes, we need to do something > about it, but that's not a good way to do that. > > FWIW, I would rather separate the check of flags validity from can_umount() > and lift _that_ into ksys_umount(), with "path_umount() relies upon the > flags being minimally sane" comment slapped at path_umount() definition. > The rest of can_umount() really shouldn't be taken out of there. I mean something like the following; unlike your variant, may_mount() is left where it is. commit a0a6df9afcaf439a6b4c88a3b522e3d05fdef46f Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Date: Mon Jan 4 15:25:34 2021 -0500 umount(2): move the flag validity checks first Unfortunately, there's userland code that used to rely upon these checks being done before anything else to check for UMOUNT_NOFOLLOW support. That broke in 41525f56e256 ("fs: refactor ksys_umount"). Separate those from the rest of checks and move them to ksys_umount(); unlike everything else in there, this can be sanely done there. Reported-by: Sargun Dhillon <sargun@xxxxxxxxx> Fixes: 41525f56e256 ("fs: refactor ksys_umount") Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> diff --git a/fs/namespace.c b/fs/namespace.c index d2db7dfe232b..9d33909d0f9e 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1713,8 +1713,6 @@ static int can_umount(const struct path *path, int flags) { struct mount *mnt = real_mount(path->mnt); - if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW)) - return -EINVAL; if (!may_mount()) return -EPERM; if (path->dentry != path->mnt->mnt_root) @@ -1728,6 +1726,7 @@ static int can_umount(const struct path *path, int flags) return 0; } +// caller is responsible for flags being sane int path_umount(struct path *path, int flags) { struct mount *mnt = real_mount(path->mnt); @@ -1749,6 +1748,10 @@ static int ksys_umount(char __user *name, int flags) struct path path; int ret; + // basic validity checks done first + if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW)) + return -EINVAL; + if (!(flags & UMOUNT_NOFOLLOW)) lookup_flags |= LOOKUP_FOLLOW; ret = user_path_at(AT_FDCWD, name, lookup_flags, &path);
