kernel/git/stable/linux-stable.git/fs/namespace.c, branch linux-4.10.y

fs: Better permission checking for submounts

2017-03-15T02:20:10+00:00

commit 93faccbbfa958a9668d3ab4e30f38dd205cee8d8 upstream. To support unprivileged users mounting filesystems two permission checks have to be performed: a test to see if the user allowed to create a mount in the mount namespace, and a test to see if the user is allowed to access the specified filesystem. The automount case is special in that mounting the original filesystem grants permission to mount the sub-filesystems, to any user who happens to stumble across the their mountpoint and satisfies the ordinary filesystem permission checks. Attempting to handle the automount case by using override_creds almost works. It preserves the idea that permission to mount the original filesystem is permission to mount the sub-filesystem. Unfortunately using override_creds messes up the filesystems ordinary permission checks. Solve this by being explicit that a mount is a submount by introducing vfs_submount, and using it where appropriate. vfs_submount uses a new mount internal mount flags MS_SUBMOUNT, to let sget and friends know that a mount is a submount so they can take appropriate action. sget and sget_userns are modified to not perform any permission checks on submounts. follow_automount is modified to stop using override_creds as that has proven problemantic. do_mount is modified to always remove the new MS_SUBMOUNT flag so that we know userspace will never by able to specify it. autofs4 is modified to stop using current_real_cred that was put in there to handle the previous version of submount permission checking. cifs is modified to pass the mountpoint all of the way down to vfs_submount. debugfs is modified to pass the mountpoint all of the way down to trace_automount by adding a new parameter. To make this change easier a new typedef debugfs_automount_t is introduced to capture the type of the debugfs automount function. Fixes: 069d5ac9ae0d ("autofs: Fix automounts by using current_real_cred()->uid") Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems creds") Reviewed-by: Trond Myklebust Reviewed-by: Seth Forshee Signed-off-by: "Eric W. Biederman" Signed-off-by: Greg Kroah-Hartman

mnt: Tuck mounts under others instead of creating shadow/side mounts.

2017-03-15T02:20:09+00:00

commit 1064f874abc0d05eeed8993815f584d847b72486 upstream. Ever since mount propagation was introduced in cases where a mount in propagated to parent mount mountpoint pair that is already in use the code has placed the new mount behind the old mount in the mount hash table. This implementation detail is problematic as it allows creating arbitrary length mount hash chains. Furthermore it invalidates the constraint maintained elsewhere in the mount code that a parent mount and a mountpoint pair will have exactly one mount upon them. Making it hard to deal with and to talk about this special case in the mount code. Modify mount propagation to notice when there is already a mount at the parent mount and mountpoint where a new mount is propagating to and place that preexisting mount on top of the new mount. Modify unmount propagation to notice when a mount that is being unmounted has another mount on top of it (and no other children), and to replace the unmounted mount with the mount on top of it. Move the MNT_UMUONT test from __lookup_mnt_last into __propagate_umount as that is the only call of __lookup_mnt_last where MNT_UMOUNT may be set on any mount visible in the mount hash table. These modifications allow: - __lookup_mnt_last to be removed. - attach_shadows to be renamed __attach_mnt and its shadow handling to be removed. - commit_tree to be simplified - copy_tree to be simplified The result is an easier to understand tree of mounts that does not allow creation of arbitrary length hash chains in the mount hash table. The result is also a very slight userspace visible difference in semantics. The following two cases now behave identically, where before order mattered: case 1: (explicit user action) B is a slave of A mount something on A/a , it will propagate to B/a and than mount something on B/a case 2: (tucked mount) B is a slave of A mount something on B/a and than mount something on A/a Histroically umount A/a would fail in case 1 and succeed in case 2. Now umount A/a succeeds in both configurations. This very small change in semantics appears if anything to be a bug fix to me and my survey of userspace leads me to believe that no programs will notice or care of this subtle semantic change. v2: Updated to mnt_change_mountpoint to not call dput or mntput and instead to decrement the counts directly. It is guaranteed that there will be other references when mnt_change_mountpoint is called so this is safe. v3: Moved put_mountpoint under mount_lock in attach_recursive_mnt As the locking in fs/namespace.c changed between v2 and v3. v4: Reworked the logic in propagate_mount_busy and __propagate_umount that detects when a mount completely covers another mount. v5: Removed unnecessary tests whose result is alwasy true in find_topper and attach_recursive_mnt. v6: Document the user space visible semantic difference. Fixes: b90fa9ae8f51 ("[PATCH] shared mount handling: bind and rbind") Tested-by: Andrei Vagin Signed-off-by: "Eric W. Biederman" Signed-off-by: Greg Kroah-Hartman

mnt: Protect the mountpoint hashtable with mount_lock

2017-01-10T00:34:43+00:00

Protecting the mountpoint hashtable with namespace_sem was sufficient until a call to umount_mnt was added to mntput_no_expire. At which point it became possible for multiple calls of put_mountpoint on the same hash chain to happen on the same time. Kristen Johansen reported: > This can cause a panic when simultaneous callers of put_mountpoint > attempt to free the same mountpoint. This occurs because some callers > hold the mount_hash_lock, while others hold the namespace lock. Some > even hold both. > > In this submitter's case, the panic manifested itself as a GP fault in > put_mountpoint() when it called hlist_del() and attempted to dereference > a m_hash.pprev that had been poisioned by another thread. Al Viro observed that the simple fix is to switch from using the namespace_sem to the mount_lock to protect the mountpoint hash table. I have taken Al's suggested patch moved put_mountpoint in pivot_root (instead of taking mount_lock an additional time), and have replaced new_mountpoint with get_mountpoint a function that does the hash table lookup and addition under the mount_lock. The introduction of get_mounptoint ensures that only the mount_lock is needed to manipulate the mountpoint hashtable. d_set_mounted is modified to only set DCACHE_MOUNTED if it is not already set. This allows get_mountpoint to use the setting of DCACHE_MOUNTED to ensure adding a struct mountpoint for a dentry happens exactly once. Cc: stable@vger.kernel.org Fixes: ce07d891a089 ("mnt: Honor MNT_LOCKED when detaching mounts") Reported-by: Krister Johansen Suggested-by: Al Viro Acked-by: Al Viro Signed-off-by: "Eric W. Biederman"

Merge branch 'work.namespace' into for-linus

2016-12-23T04:04:31+00:00

Merge branch 'work.autofs' into for-linus

2016-12-16T21:34:52+00:00

Signed-off-by: Al Viro

reorganize do_make_slave()

2016-12-16T21:30:49+00:00

Make sure that clone_mnt() never returns a mount with MNT_SHARED in flags, but without a valid ->mnt_group_id. That allows to demystify do_make_slave() quite a bit, among other things. Signed-off-by: Al Viro

clone_private_mount() doesn't need to touch namespace_sem

2016-12-16T21:30:49+00:00

not for CL_PRIVATE clone_mnt() Signed-off-by: Al Viro

remove a bogus claim about namespace_sem being held by callers of mnt_alloc_id()

2016-12-16T21:30:48+00:00

Hadn't been true for quite a while Signed-off-by: Al Viro

namespace.c: constify struct path passed to a bunch of primitives

2016-12-06T00:03:12+00:00

Signed-off-by: Al Viro

fs: Constify path_is_under()'s arguments

2016-12-05T23:55:47+00:00

The function path_is_under() doesn't modify the paths pointed by its arguments but only browse them. Constifying this pointers make a cleaner interface to be used by (future) code which may only have access to const struct path pointers (e.g. LSM hooks). Signed-off-by: Mickaël Salaün Cc: Alexander Viro Signed-off-by: Al Viro