From: Radoslaw Burny rburny@google.com
commit 5ec27ec735ba0477d48c80561cc5e856f0c5dfaf upstream.
Normally, the inode's i_uid/i_gid are translated relative to s_user_ns, but this is not a correct behavior for proc. Since sysctl permission check in test_perm is done against GLOBAL_ROOT_[UG]ID, it makes more sense to use these values in u_[ug]id of proc inodes. In other words: although uid/gid in the inode is not read during test_perm, the inode logically belongs to the root of the namespace. I have confirmed this with Eric Biederman at LPC and in this thread: https://lore.kernel.org/lkml/87k1kzjdff.fsf@xmission.com
Consequences ============
Since the i_[ug]id values of proc nodes are not used for permissions checks, this change usually makes no functional difference. However, it causes an issue in a setup where:
* a namespace container is created without root user in container - hence the i_[ug]id of proc nodes are set to INVALID_[UG]ID
* container creator tries to configure it by writing /proc/sys files, e.g. writing /proc/sys/kernel/shmmax to configure shared memory limit
Kernel does not allow to open an inode for writing if its i_[ug]id are invalid, making it impossible to write shmmax and thus - configure the container.
Using a container with no root mapping is apparently rare, but we do use this configuration at Google. Also, we use a generic tool to configure the container limits, and the inability to write any of them causes a failure.
History =======
The invalid uids/gids in inodes first appeared due to 81754357770e (fs: Update i_[ug]id_(read|write) to translate relative to s_user_ns). However, AFAIK, this did not immediately cause any issues. The inability to write to these "invalid" inodes was only caused by a later commit 0bd23d09b874 (vfs: Don't modify inodes with a uid or gid unknown to the vfs).
Tested: Used a repro program that creates a user namespace without any mapping and stat'ed /proc/$PID/root/proc/sys/kernel/shmmax from outside. Before the change, it shows the overflow uid, with the change it's 0. The overflow uid indicates that the uid in the inode is not correct and thus it is not possible to open the file for writing.
Link: http://lkml.kernel.org/r/20190708115130.250149-1-rburny@google.com Fixes: 0bd23d09b874 ("vfs: Don't modify inodes with a uid or gid unknown to the vfs") Signed-off-by: Radoslaw Burny rburny@google.com Acked-by: Luis Chamberlain mcgrof@kernel.org Cc: Kees Cook keescook@chromium.org Cc: "Eric W . Biederman" ebiederm@xmission.com Cc: Seth Forshee seth.forshee@canonical.com Cc: John Sperbeck jsperbeck@google.com Cc: Alexey Dobriyan adobriyan@gmail.com Cc: stable@vger.kernel.org [4.8+] Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/proc/proc_sysctl.c | 4 ++++ 1 file changed, 4 insertions(+)
--- a/fs/proc/proc_sysctl.c +++ b/fs/proc/proc_sysctl.c @@ -498,6 +498,10 @@ static struct inode *proc_sys_make_inode
if (root->set_ownership) root->set_ownership(head, table, &inode->i_uid, &inode->i_gid); + else { + inode->i_uid = GLOBAL_ROOT_UID; + inode->i_gid = GLOBAL_ROOT_GID; + }
return inode; }