Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754958Ab3JVTNU (ORCPT ); Tue, 22 Oct 2013 15:13:20 -0400 Received: from static.92.5.9.176.clients.your-server.de ([176.9.5.92]:55688 "EHLO hallynmail2" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753987Ab3JVTNS (ORCPT ); Tue, 22 Oct 2013 15:13:18 -0400 Date: Tue, 22 Oct 2013 19:13:16 +0000 From: "Serge E. Hallyn" To: "Eric W. Biederman" Cc: Miklos Szeredi , Andy Lutomirski , "Serge E. Hallyn" , Al Viro , Linux-Fsdevel , Kernel Mailing List , Rob Landley , Linus Torvalds , Matthias Schniedermeyer , Linux Containers Subject: Re: [REVIEW][PATCH 4/4] vfs: Lazily remove mounts on unlinked files and directories. v2 Message-ID: <20131022191316.GD10600@mail.hallyn.com> References: <20131008161135.GK14242@tucsk.piliscsaba.szeredi.hu> <87li23trll.fsf@tw-ebiederman.twitter.com> <87vc15mjuw.fsf@xmission.com> <87iox38fkv.fsf@xmission.com> <87d2nb8dxy.fsf@xmission.com> <87iowyxpci.fsf_-_@xmission.com> <87vc0ywan7.fsf_-_@xmission.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87vc0ywan7.fsf_-_@xmission.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 12304 Lines: 372 Quoting Eric W. Biederman (ebiederm@xmission.com): > > With the introduction of mount namespaces and bind mounts it became > possible to access files and directories that on some paths are mount > points but are not mount points on other paths. It is very confusing > when rm -rf somedir returns -EBUSY simply because somedir is mounted > somewhere else. With the addition of user namespaces allowing > unprivileged mounts this condition has gone from annoying to allowing > a DOS attack on other users in the system. > > The possibility for mischief is removed by updating the vfs to support > rename, unlink and rmdir on a dentry that is a mountpoint and by > lazily unmounting mountpoints on deleted dentries. > > In particular this change allows rename, unlink and rmdir system calls > on a dentry without a mountpoint in the current mount namespace to > succeed, and it allows rename, unlink, and rmdir performed on a > distributed filesystem to update the vfs cache even if when there is a > mount in some namespace on the original dentry. > > There are two common patterns of maintaining mounts: Mounts on trusted > paths with the parent directory of the mount point and all ancestory > directories up to / owned by root and modifiable only by root > (i.e. /media/xxx, /dev, /dev/pts, /proc, /sys, /sys/fs/cgroup/{cpu, > cpuacct, ...}, /usr, /usr/local). Mounts on unprivileged directories > maintained by fusermount. > > In the case of mounts in trusted directories owned by root and > modifiable only by root the current parent directory permissions are > sufficient to ensure a mount point on a trusted path is not removed > or renamed by anyone other than root, even if there is a context > where the there are no mount points to prevent this. > > In the case of mounts in directories owned by less privileged users > races with users modifying the path of a mount point are already a > danger. fusermount already uses a combination of chdir, > /proc//fd/NNN, and UMOUNT_NOFOLLOW to prevent these races. The > removable of global rename, unlink, and rmdir protection really adds > nothing new to consider only a widening of the attack window, and > fusermount is already safe against unprivileged users modifying the > directory simultaneously. > > In principle for perfect userspace programs returning -EBUSY for > unlink, rmdir, and rename of dentires that have mounts in the local > namespace is actually unnecessary. Unfortunately not all userspace > programs are perfect so retaining -EBUSY for unlink, rmdir and rename > of dentries that have mounts in the current mount namespace plays an > important role of maintaining consistency with historical behavior and > making imperfect userspace applications hard to exploit. > > v2: Remove spurious old_dentry. > > Signed-off-by: "Eric W. Biederman" Acked-by: Serge Hallyn > --- > fs/afs/dir.c | 3 +- > fs/dcache.c | 80 ++++++++++++++++++++---------------------------- > fs/fuse/dir.c | 3 +- > fs/gfs2/dentry.c | 4 +-- > fs/namei.c | 30 ++++++------------ > fs/nfs/dir.c | 5 +-- > fs/sysfs/dir.c | 9 +----- > include/linux/dcache.h | 3 +- > 8 files changed, 50 insertions(+), 87 deletions(-) > > diff --git a/fs/afs/dir.c b/fs/afs/dir.c > index 646337dc5201..7fb69d45f1b9 100644 > --- a/fs/afs/dir.c > +++ b/fs/afs/dir.c > @@ -686,8 +686,7 @@ not_found: > > out_bad: > /* don't unhash if we have submounts */ > - if (check_submounts_and_drop(dentry) != 0) > - goto out_skip; > + shrink_submounts_and_drop(dentry); > > _debug("dropping dentry %s/%s", > parent->d_name.name, dentry->d_name.name); > diff --git a/fs/dcache.c b/fs/dcache.c > index 41000305d716..1e9bf96b0132 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -1373,7 +1373,7 @@ int d_set_mounted(struct dentry *dentry) > int ret = -ENOENT; > write_seqlock(&rename_lock); > for (p = dentry->d_parent; !IS_ROOT(p); p = p->d_parent) { > - /* Need exclusion wrt. check_submounts_and_drop() */ > + /* Need exclusion wrt. shrink_submounts_and_drop() */ > spin_lock(&p->d_lock); > if (unlikely(d_unhashed(p))) { > spin_unlock(&p->d_lock); > @@ -1478,70 +1478,56 @@ void shrink_dcache_parent(struct dentry *parent) > } > EXPORT_SYMBOL(shrink_dcache_parent); > > -static enum d_walk_ret check_and_collect(void *_data, struct dentry *dentry) > +struct detach_data { > + struct dentry *found; > +}; > +static enum d_walk_ret do_detach_submounts(void *ptr, struct dentry *dentry) > { > - struct select_data *data = _data; > - > - if (d_mountpoint(dentry)) { > - data->found = -EBUSY; > - return D_WALK_QUIT; > - } > - > - return select_collect(_data, dentry); > -} > + struct detach_data *data = ptr; > > -static void check_and_drop(void *_data) > -{ > - struct select_data *data = _data; > + if (d_mountpoint(dentry)) > + data->found = dentry; > > - if (d_mountpoint(data->start)) > - data->found = -EBUSY; > - if (!data->found) > - __d_drop(data->start); > + return data->found ? D_WALK_QUIT : D_WALK_CONTINUE; > } > > /** > - * check_submounts_and_drop - prune dcache, check for submounts and drop > + * detach_submounts - check for submounts and detach them. > * > - * All done as a single atomic operation relative to has_unlinked_ancestor(). > - * Returns 0 if successfully unhashed @parent. If there were submounts then > - * return -EBUSY. > + * @dentry: dentry to find mount points under. > * > - * @dentry: dentry to prune and drop > + * If dentry or any of it's children is a mount point detach those mounts. > */ > -int check_submounts_and_drop(struct dentry *dentry) > +void detach_submounts(struct dentry *dentry) > { > - int ret = 0; > - > - /* Negative dentries can be dropped without further checks */ > - if (!dentry->d_inode) { > - d_drop(dentry); > - goto out; > - } > - > + struct detach_data data; > for (;;) { > - struct select_data data; > - > - INIT_LIST_HEAD(&data.dispose); > - data.start = dentry; > - data.found = 0; > + data.found = NULL; > + d_walk(dentry, &data, do_detach_submounts, NULL); > > - d_walk(dentry, &data, check_and_collect, check_and_drop); > - ret = data.found; > - > - if (!list_empty(&data.dispose)) > - shrink_dentry_list(&data.dispose); > - > - if (ret <= 0) > + if (!data.found) > break; > > + detach_mounts(data.found); > cond_resched(); > } > + detach_mounts(dentry); > +} > > -out: > - return ret; > +/** > + * shrink_submounts_and_drop - detach submounts, prune dcache, and drop > + * > + * All done as a single atomic operation reletaive to d_set_mounted(). > + * > + * @dentry: dentry to detach, prune and drop > + */ > +void shrink_submounts_and_drop(struct dentry *dentry) > +{ > + d_drop(dentry); > + detach_submounts(dentry); > + shrink_dcache_parent(dentry); > } > -EXPORT_SYMBOL(check_submounts_and_drop); > +EXPORT_SYMBOL(shrink_submounts_and_drop); > > /** > * __d_alloc - allocate a dcache entry > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c > index 62b43b577bfc..b1cd7b79a325 100644 > --- a/fs/fuse/dir.c > +++ b/fs/fuse/dir.c > @@ -259,8 +259,7 @@ out: > > invalid: > ret = 0; > - if (check_submounts_and_drop(entry) != 0) > - ret = 1; > + shrink_submounts_and_drop(entry); > goto out; > } > > diff --git a/fs/gfs2/dentry.c b/fs/gfs2/dentry.c > index d3a5d4e29ba5..2ecc2b873829 100644 > --- a/fs/gfs2/dentry.c > +++ b/fs/gfs2/dentry.c > @@ -93,9 +93,7 @@ invalid_gunlock: > if (!had_lock) > gfs2_glock_dq_uninit(&d_gh); > invalid: > - if (check_submounts_and_drop(dentry) != 0) > - goto valid; > - > + shrink_submounts_and_drop(dentry); > dput(parent); > return 0; > > diff --git a/fs/namei.c b/fs/namei.c > index df7bd6e9c7b6..a12c1d31d4c8 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -3574,10 +3574,6 @@ int vfs_rmdir(struct inode *dir, struct dentry *dentry) > dget(dentry); > mutex_lock(&dentry->d_inode->i_mutex); > > - error = -EBUSY; > - if (d_mountpoint(dentry)) > - goto out; > - > error = security_inode_rmdir(dir, dentry); > if (error) > goto out; > @@ -3589,6 +3585,7 @@ int vfs_rmdir(struct inode *dir, struct dentry *dentry) > > dentry->d_inode->i_flags |= S_DEAD; > dont_mount(dentry); > + detach_mounts(dentry); > > out: > mutex_unlock(&dentry->d_inode->i_mutex); > @@ -3674,14 +3671,12 @@ int vfs_unlink(struct inode *dir, struct dentry *dentry) > return -EPERM; > > mutex_lock(&dentry->d_inode->i_mutex); > - if (d_mountpoint(dentry)) > - error = -EBUSY; > - else { > - error = security_inode_unlink(dir, dentry); > + error = security_inode_unlink(dir, dentry); > + if (!error) { > + error = dir->i_op->unlink(dir, dentry); > if (!error) { > - error = dir->i_op->unlink(dir, dentry); > - if (!error) > - dont_mount(dentry); > + dont_mount(dentry); > + detach_mounts(dentry); > } > } > mutex_unlock(&dentry->d_inode->i_mutex); > @@ -4008,10 +4003,6 @@ static int vfs_rename_dir(struct inode *old_dir, struct dentry *old_dentry, > if (target) > mutex_lock(&target->i_mutex); > > - error = -EBUSY; > - if (d_mountpoint(old_dentry) || d_mountpoint(new_dentry)) > - goto out; > - > error = -EMLINK; > if (max_links && !target && new_dir != old_dir && > new_dir->i_nlink >= max_links) > @@ -4026,6 +4017,7 @@ static int vfs_rename_dir(struct inode *old_dir, struct dentry *old_dentry, > if (target) { > target->i_flags |= S_DEAD; > dont_mount(new_dentry); > + detach_mounts(new_dentry); > } > out: > if (target) > @@ -4051,16 +4043,14 @@ static int vfs_rename_other(struct inode *old_dir, struct dentry *old_dentry, > if (target) > mutex_lock(&target->i_mutex); > > - error = -EBUSY; > - if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry)) > - goto out; > - > error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry); > if (error) > goto out; > > - if (target) > + if (target) { > dont_mount(new_dentry); > + detach_mounts(new_dentry); > + } > if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE)) > d_move(old_dentry, new_dentry); > out: > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c > index 854a8f05a610..e8e35acd8850 100644 > --- a/fs/nfs/dir.c > +++ b/fs/nfs/dir.c > @@ -1142,10 +1142,7 @@ out_zap_parent: > if (dentry->d_flags & DCACHE_DISCONNECTED) > goto out_valid; > } > - /* If we have submounts, don't unhash ! */ > - if (check_submounts_and_drop(dentry) != 0) > - goto out_valid; > - > + shrink_submounts_and_drop(dentry); > dput(parent); > dfprintk(LOOKUPCACHE, "NFS: %s(%s/%s) is invalid\n", > __func__, dentry->d_parent->d_name.name, > diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c > index 4d83cedb9fcb..477c66d4e2a8 100644 > --- a/fs/sysfs/dir.c > +++ b/fs/sysfs/dir.c > @@ -327,7 +327,6 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, unsigned int flags) > } > > mutex_unlock(&sysfs_mutex); > -out_valid: > return 1; > out_bad: > /* Remove the dentry from the dcache hashes. > @@ -341,13 +340,7 @@ out_bad: > * to the dcache hashes. > */ > mutex_unlock(&sysfs_mutex); > - > - /* If we have submounts we must allow the vfs caches > - * to lie about the state of the filesystem to prevent > - * leaks and other nasty things. > - */ > - if (check_submounts_and_drop(dentry) != 0) > - goto out_valid; > + shrink_submounts_and_drop(dentry); > > return 0; > } > diff --git a/include/linux/dcache.h b/include/linux/dcache.h > index 59066e0b4ff1..17948b49f3d5 100644 > --- a/include/linux/dcache.h > +++ b/include/linux/dcache.h > @@ -254,7 +254,8 @@ extern void d_prune_aliases(struct inode *); > > /* test whether we have any submounts in a subdir tree */ > extern int have_submounts(struct dentry *); > -extern int check_submounts_and_drop(struct dentry *); > +extern void detach_submounts(struct dentry *dentry); > +extern void shrink_submounts_and_drop(struct dentry *); > > /* > * This adds the entry to the hash queues. > -- > 1.7.5.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/