Please drop this from -stable -- it's part of a larger refactoring that shouldn't be backported without explicit effort/testing. -Kees On Mon, Nov 06, 2023 at 06:14:20PM -0500, Sasha Levin wrote: > From: Christian Brauner <christian.brauner@xxxxxxxxxx> > > [ Upstream commit 1c5976ef0f7ad76319df748ccb99a4c7ba2ba464 ] > > Currently, registering a new binary type pins the binfmt_misc > filesystem. Specifically, this means that as long as there is at least > one binary type registered the binfmt_misc filesystem survives all > umounts, i.e. the superblock is not destroyed. Meaning that a umount > followed by another mount will end up with the same superblock and the > same binary type handlers. This is a behavior we tend to discourage for > any new filesystems (apart from a few special filesystems such as e.g. > configfs or debugfs). A umount operation without the filesystem being > pinned - by e.g. someone holding a file descriptor to an open file - > should usually result in the destruction of the superblock and all > associated resources. This makes introspection easier and leads to > clearly defined, simple and clean semantics. An administrator can rely > on the fact that a umount will guarantee a clean slate making it > possible to reinitialize a filesystem. Right now all binary types would > need to be explicitly deleted before that can happen. > > This allows us to remove the heavy-handed calls to simple_pin_fs() and > simple_release_fs() when creating and deleting binary types. This in > turn allows us to replace the current brittle pinning mechanism abusing > dget() which has caused a range of bugs judging from prior fixes in [2] > and [3]. The additional dget() in load_misc_binary() pins the dentry but > only does so for the sake to prevent ->evict_inode() from freeing the > node when a user removes the binary type and kill_node() is run. Which > would mean ->interpreter and ->interp_file would be freed causing a UAF. > > This isn't really nicely documented nor is it very clean because it > relies on simple_pin_fs() pinning the filesystem as long as at least one > binary type exists. Otherwise it would cause load_misc_binary() to hold > on to a dentry belonging to a superblock that has been shutdown. > Replace that implicit pinning with a clean and simple per-node refcount > and get rid of the ugly dget() pinning. A similar mechanism exists for > e.g. binderfs (cf. [4]). All the cleanup work can now be done in > ->evict_inode(). > > In a follow-up patch we will make it possible to use binfmt_misc in > sandboxes. We will use the cleaner semantics where a umount for the > filesystem will cause the superblock and all resources to be > deallocated. In preparation for this apply the same semantics to the > initial binfmt_misc mount. Note, that this is a user-visible change and > as such a uapi change but one that we can reasonably risk. We've > discussed this in earlier versions of this patchset (cf. [1]). > > The main user and provider of binfmt_misc is systemd. Systemd provides > binfmt_misc via autofs since it is configurable as a kernel module and > is used by a few exotic packages and users. As such a binfmt_misc mount > is triggered when /proc/sys/fs/binfmt_misc is accessed and is only > provided on demand. Other autofs on demand filesystems include EFI ESP > which systemd umounts if the mountpoint stays idle for a certain amount > of time. This doesn't apply to the binfmt_misc autofs mount which isn't > touched once it is mounted meaning this change can't accidently wipe > binary type handlers without someone having explicitly unmounted > binfmt_misc. After speaking to systemd folks they don't expect this > change to affect them. > > In line with our general policy, if we see a regression for systemd or > other users with this change we will switch back to the old behavior for > the initial binfmt_misc mount and have binary types pin the filesystem > again. But while we touch this code let's take the chance and let's > improve on the status quo. > > [1]: https://lore.kernel.org/r/20191216091220.465626-2-laurent@xxxxxxxxx > [2]: commit 43a4f2619038 ("exec: binfmt_misc: fix race between load_misc_binary() and kill_node()" > [3]: commit 83f918274e4b ("exec: binfmt_misc: shift filp_close(interp_file) from kill_node() to bm_evict_inode()") > [4]: commit f0fe2c0f050d ("binder: prevent UAF for binderfs devices II") > > Link: https://lore.kernel.org/r/20211028103114.2849140-1-brauner@xxxxxxxxxx (v1) > Cc: Sargun Dhillon <sargun@xxxxxxxxx> > Cc: Serge Hallyn <serge@xxxxxxxxxx> > Cc: Jann Horn <jannh@xxxxxxxxxx> > Cc: Henning Schild <henning.schild@xxxxxxxxxxx> > Cc: Andrei Vagin <avagin@xxxxxxxxx> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > Cc: Laurent Vivier <laurent@xxxxxxxxx> > Cc: linux-fsdevel@xxxxxxxxxxxxxxx > Acked-by: Serge Hallyn <serge@xxxxxxxxxx> > Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > /* v2 */ > - Christian Brauner <christian.brauner@xxxxxxxxxx>: > - Add more comments that explain what's going on. > - Rename functions while changing them to better reflect what they are > doing to make the code easier to understand. > - In the first version when a specific binary type handler was removed > either through a write to the entry's file or all binary type > handlers were removed by a write to the binfmt_misc mount's status > file all cleanup work happened during inode eviction. > That includes removal of the relevant entries from entry list. While > that works fine I disliked that model after thinking about it for a > bit. Because it means that there was a window were someone has > already removed a or all binary handlers but they could still be > safely reached from load_misc_binary() when it has managed to take > the read_lock() on the entries list while inode eviction was already > happening. Again, that perfectly benign but it's cleaner to remove > the binary handler from the list immediately meaning that ones the > write to then entry's file or the binfmt_misc status file returns > the binary type cannot be executed anymore. That gives stronger > guarantees to the user. > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> > --- > fs/binfmt_misc.c | 216 ++++++++++++++++++++++++++++++++++++----------- > 1 file changed, 168 insertions(+), 48 deletions(-) > > diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c > index e0108d17b085c..cf5ed5cd4102d 100644 > --- a/fs/binfmt_misc.c > +++ b/fs/binfmt_misc.c > @@ -60,12 +60,11 @@ typedef struct { > char *name; > struct dentry *dentry; > struct file *interp_file; > + refcount_t users; /* sync removal with load_misc_binary() */ > } Node; > > static DEFINE_RWLOCK(entries_lock); > static struct file_system_type bm_fs_type; > -static struct vfsmount *bm_mnt; > -static int entry_count; > > /* > * Max length of the register string. Determined by: > @@ -82,19 +81,23 @@ static int entry_count; > */ > #define MAX_REGISTER_LENGTH 1920 > > -/* > - * Check if we support the binfmt > - * if we do, return the node, else NULL > - * locking is done in load_misc_binary > +/** > + * search_binfmt_handler - search for a binary handler for @bprm > + * @misc: handle to binfmt_misc instance > + * @bprm: binary for which we are looking for a handler > + * > + * Search for a binary type handler for @bprm in the list of registered binary > + * type handlers. > + * > + * Return: binary type list entry on success, NULL on failure > */ > -static Node *check_file(struct linux_binprm *bprm) > +static Node *search_binfmt_handler(struct linux_binprm *bprm) > { > char *p = strrchr(bprm->interp, '.'); > - struct list_head *l; > + Node *e; > > /* Walk all the registered handlers. */ > - list_for_each(l, &entries) { > - Node *e = list_entry(l, Node, list); > + list_for_each_entry(e, &entries, list) { > char *s; > int j; > > @@ -123,9 +126,49 @@ static Node *check_file(struct linux_binprm *bprm) > if (j == e->size) > return e; > } > + > return NULL; > } > > +/** > + * get_binfmt_handler - try to find a binary type handler > + * @misc: handle to binfmt_misc instance > + * @bprm: binary for which we are looking for a handler > + * > + * Try to find a binfmt handler for the binary type. If one is found take a > + * reference to protect against removal via bm_{entry,status}_write(). > + * > + * Return: binary type list entry on success, NULL on failure > + */ > +static Node *get_binfmt_handler(struct linux_binprm *bprm) > +{ > + Node *e; > + > + read_lock(&entries_lock); > + e = search_binfmt_handler(bprm); > + if (e) > + refcount_inc(&e->users); > + read_unlock(&entries_lock); > + return e; > +} > + > +/** > + * put_binfmt_handler - put binary handler node > + * @e: node to put > + * > + * Free node syncing with load_misc_binary() and defer final free to > + * load_misc_binary() in case it is using the binary type handler we were > + * requested to remove. > + */ > +static void put_binfmt_handler(Node *e) > +{ > + if (refcount_dec_and_test(&e->users)) { > + if (e->flags & MISC_FMT_OPEN_FILE) > + filp_close(e->interp_file, NULL); > + kfree(e); > + } > +} > + > /* > * the loader itself > */ > @@ -139,12 +182,7 @@ static int load_misc_binary(struct linux_binprm *bprm) > if (!enabled) > return retval; > > - /* to keep locking time low, we copy the interpreter string */ > - read_lock(&entries_lock); > - fmt = check_file(bprm); > - if (fmt) > - dget(fmt->dentry); > - read_unlock(&entries_lock); > + fmt = get_binfmt_handler(bprm); > if (!fmt) > return retval; > > @@ -198,7 +236,16 @@ static int load_misc_binary(struct linux_binprm *bprm) > > retval = 0; > ret: > - dput(fmt->dentry); > + > + /* > + * If we actually put the node here all concurrent calls to > + * load_misc_binary() will have finished. We also know > + * that for the refcount to be zero ->evict_inode() must have removed > + * the node to be deleted from the list. All that is left for us is to > + * close and free. > + */ > + put_binfmt_handler(fmt); > + > return retval; > } > > @@ -552,30 +599,90 @@ static struct inode *bm_get_inode(struct super_block *sb, int mode) > return inode; > } > > +/** > + * bm_evict_inode - cleanup data associated with @inode > + * @inode: inode to which the data is attached > + * > + * Cleanup the binary type handler data associated with @inode if a binary type > + * entry is removed or the filesystem is unmounted and the super block is > + * shutdown. > + * > + * If the ->evict call was not caused by a super block shutdown but by a write > + * to remove the entry or all entries via bm_{entry,status}_write() the entry > + * will have already been removed from the list. We keep the list_empty() check > + * to make that explicit. > +*/ > static void bm_evict_inode(struct inode *inode) > { > Node *e = inode->i_private; > > - if (e && e->flags & MISC_FMT_OPEN_FILE) > - filp_close(e->interp_file, NULL); > - > clear_inode(inode); > - kfree(e); > + > + if (e) { > + write_lock(&entries_lock); > + if (!list_empty(&e->list)) > + list_del_init(&e->list); > + write_unlock(&entries_lock); > + put_binfmt_handler(e); > + } > } > > -static void kill_node(Node *e) > +/** > + * unlink_binfmt_dentry - remove the dentry for the binary type handler > + * @dentry: dentry associated with the binary type handler > + * > + * Do the actual filesystem work to remove a dentry for a registered binary > + * type handler. Since binfmt_misc only allows simple files to be created > + * directly under the root dentry of the filesystem we ensure that we are > + * indeed passed a dentry directly beneath the root dentry, that the inode > + * associated with the root dentry is locked, and that it is a regular file we > + * are asked to remove. > + */ > +static void unlink_binfmt_dentry(struct dentry *dentry) > { > - struct dentry *dentry; > + struct dentry *parent = dentry->d_parent; > + struct inode *inode, *parent_inode; > + > + /* All entries are immediate descendants of the root dentry. */ > + if (WARN_ON_ONCE(dentry->d_sb->s_root != parent)) > + return; > > + /* We only expect to be called on regular files. */ > + inode = d_inode(dentry); > + if (WARN_ON_ONCE(!S_ISREG(inode->i_mode))) > + return; > + > + /* The parent inode must be locked. */ > + parent_inode = d_inode(parent); > + if (WARN_ON_ONCE(!inode_is_locked(parent_inode))) > + return; > + > + if (simple_positive(dentry)) { > + dget(dentry); > + simple_unlink(parent_inode, dentry); > + d_delete(dentry); > + dput(dentry); > + } > +} > + > +/** > + * remove_binfmt_handler - remove a binary type handler > + * @misc: handle to binfmt_misc instance > + * @e: binary type handler to remove > + * > + * Remove a binary type handler from the list of binary type handlers and > + * remove its associated dentry. This is called from > + * binfmt_{entry,status}_write(). In the future, we might want to think about > + * adding a proper ->unlink() method to binfmt_misc instead of forcing caller's > + * to use writes to files in order to delete binary type handlers. But it has > + * worked for so long that it's not a pressing issue. > + */ > +static void remove_binfmt_handler(Node *e) > +{ > write_lock(&entries_lock); > list_del_init(&e->list); > write_unlock(&entries_lock); > - > - dentry = e->dentry; > - drop_nlink(d_inode(dentry)); > - d_drop(dentry); > - dput(dentry); > - simple_release_fs(&bm_mnt, &entry_count); > + unlink_binfmt_dentry(e->dentry); > } > > /* /<entry> */ > @@ -602,8 +709,8 @@ bm_entry_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos) > static ssize_t bm_entry_write(struct file *file, const char __user *buffer, > size_t count, loff_t *ppos) > { > - struct dentry *root; > - Node *e = file_inode(file)->i_private; > + struct inode *inode = file_inode(file); > + Node *e = inode->i_private; > int res = parse_command(buffer, count); > > switch (res) { > @@ -617,13 +724,22 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer, > break; > case 3: > /* Delete this handler. */ > - root = file_inode(file)->i_sb->s_root; > - inode_lock(d_inode(root)); > + inode = d_inode(inode->i_sb->s_root); > + inode_lock(inode); > > + /* > + * In order to add new element or remove elements from the list > + * via bm_{entry,register,status}_write() inode_lock() on the > + * root inode must be held. > + * The lock is exclusive ensuring that the list can't be > + * modified. Only load_misc_binary() can access but does so > + * read-only. So we only need to take the write lock when we > + * actually remove the entry from the list. > + */ > if (!list_empty(&e->list)) > - kill_node(e); > + remove_binfmt_handler(e); > > - inode_unlock(d_inode(root)); > + inode_unlock(inode); > break; > default: > return res; > @@ -682,13 +798,7 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer, > if (!inode) > goto out2; > > - err = simple_pin_fs(&bm_fs_type, &bm_mnt, &entry_count); > - if (err) { > - iput(inode); > - inode = NULL; > - goto out2; > - } > - > + refcount_set(&e->users, 1); > e->dentry = dget(dentry); > inode->i_private = e; > inode->i_fop = &bm_entry_operations; > @@ -732,7 +842,8 @@ static ssize_t bm_status_write(struct file *file, const char __user *buffer, > size_t count, loff_t *ppos) > { > int res = parse_command(buffer, count); > - struct dentry *root; > + Node *e, *next; > + struct inode *inode; > > switch (res) { > case 1: > @@ -745,13 +856,22 @@ static ssize_t bm_status_write(struct file *file, const char __user *buffer, > break; > case 3: > /* Delete all handlers. */ > - root = file_inode(file)->i_sb->s_root; > - inode_lock(d_inode(root)); > + inode = d_inode(file_inode(file)->i_sb->s_root); > + inode_lock(inode); > > - while (!list_empty(&entries)) > - kill_node(list_first_entry(&entries, Node, list)); > + /* > + * In order to add new element or remove elements from the list > + * via bm_{entry,register,status}_write() inode_lock() on the > + * root inode must be held. > + * The lock is exclusive ensuring that the list can't be > + * modified. Only load_misc_binary() can access but does so > + * read-only. So we only need to take the write lock when we > + * actually remove the entry from the list. > + */ > + list_for_each_entry_safe(e, next, &entries, list) > + remove_binfmt_handler(e); > > - inode_unlock(d_inode(root)); > + inode_unlock(inode); > break; > default: > return res; > -- > 2.42.0 > -- Kees Cook
