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? > 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 >
