On Fri, Jul 09, 2021 at 08:34:41AM -0700, Casey Schaufler wrote: > 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? tmpfs does not support user.* xattr at all on any kind of files. overlayfs works fine. I updated my test too. nfs seems to have some issues. - I can set user.foo xattr on symlink and query it back using xattr name. getfattr -h -n user.foo foo-link.txt But when I try to dump all xattrs on this file, user.foo is being filtered out it looks like. Not sure why. - I can't set "user.foo" xattr on a device node on nfs and I get "Permission denied". I am assuming nfs server is returning this. I am using knfsd with following in /etc/exports. /mnt/test/nfs-server 127.0.0.1(insecure,no_root_squash,rw,async) Copying Bruce. He might have an idea. Thanks Vivek > > > > > 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 > >>> >
