On 7/9/2021 8:27 AM, Vivek Goyal wrote: > On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote: >> On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: >>> Currently user.* xattr are not allowed on symlink and special files. >>> >>> man xattr and recent discussion suggested that primary reason for this >>> restriction is how file permissions for symlinks and special files >>> are little different from regular files and directories. >>> >>> For symlinks, they are world readable/writable and if user xattr were >>> to be permitted, it will allow unpriviliged users to dump a huge amount >>> of user.* xattrs on symlinks without any control. >>> >>> For special files, permissions typically control capability to read/write >>> from devices (and not necessarily from filesystem). So if a user can >>> write to device (/dev/null), does not necessarily mean it should be allowed >>> to write large number of user.* xattrs on the filesystem device node is >>> residing in. >>> >>> This patch proposes to relax the restrictions a bit and allow file owner >>> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs >>> on symlink and special files. >>> >>> virtiofs daemon has a need to store user.* xatrrs on all the files >>> (including symlinks and special files), and currently that fails. This >>> patch should help. >>> >>> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@xxxxxxxxxx/ >>> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx> >>> --- >> Seems reasonable and useful. >> Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx> >> >> One question, do all filesystem supporting xattrs deal with setting them >> on symlinks/device files correctly? > Wrote a simple bash script to do setfattr/getfattr user.foo xattr on > symlink and device node on ext4, xfs and btrfs and it works fine. How about nfs, tmpfs, overlayfs and/or some of the other less conventional filesystems? > > https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh > > I probably can add some more filesystems to test. > > Thanks > Vivek > >>> fs/xattr.c | 10 ++++++---- >>> 1 file changed, 6 insertions(+), 4 deletions(-) >>> >>> diff --git a/fs/xattr.c b/fs/xattr.c >>> index 5c8c5175b385..2f1855c8b620 100644 >>> --- a/fs/xattr.c >>> +++ b/fs/xattr.c >>> @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, >>> } >>> >>> /* >>> - * In the user.* namespace, only regular files and directories can have >>> - * extended attributes. For sticky directories, only the owner and >>> - * privileged users can write attributes. >>> + * In the user.* namespace, for symlinks and special files, only >>> + * the owner and priviliged users can read/write attributes. >>> + * For sticky directories, only the owner and privileged users can >>> + * write attributes. >>> */ >>> if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { >>> - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) >>> + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && >>> + !inode_owner_or_capable(mnt_userns, inode)) >>> return (mask & MAY_WRITE) ? -EPERM : -ENODATA; >>> if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && >>> (mask & MAY_WRITE) && >>> -- >>> 2.25.4 >>>
