Miklos Szeredi <mszeredi@xxxxxxxxxx> writes: > On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu@xxxxxxxxxx> wrote: >> From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> >> >> Allow users with CAP_SYS_CHOWN over the superblock of a filesystem to >> chown files. Ordinarily the capable_wrt_inode_uidgid check is >> sufficient to allow access to files but when the underlying filesystem >> has uids or gids that don't map to the current user namespace it is >> not enough, so the chown permission checks need to be extended to >> allow this case. >> >> Calling chown on filesystem nodes whose uid or gid don't map is >> necessary if those nodes are going to be modified as writing back >> inodes which contain uids or gids that don't map is likely to cause >> filesystem corruption of the uid or gid fields. > > How can the filesystem be corrupted if chown is denied? > > It is not clear to me what the purpose of this patch is or what the > exact usecase this is fixing. It isn't a fix and we can delay this one and similar patches that enable things until we are certain all of the necessary restrictions are in place. This is not essential for safely getting fully unprivileged mounting of fuse to work. The overall strategy has been to handle as many of the generic concerns at the vfs level as possible to separate filesystem concerns and generic concerns. In this case the generic concern is what happens when the uid is read from the filesystem and it gets mapped to INVALID_UID and then the inode for that file is written back. That is a trap for the unwary filesystem implementation and not a case that I think anyone will actually care about. It is just not useful to mount a filesystem and to not map some of it's ids. So the generic vfs code just denies writes to files like show with uid of INVALID_UID or gid of INVALID_GID. Just to ensure that problems don't show up. This patch gets through those defenses. Eric
