Re: [functionfs] mainline UAF (was Re: [PATCH v3 36/50] functionfs: switch to simple_remove_by_name())

Christian Brauner
2025-11-14T11:42:48Z

On Fri, Nov 14, 2025 at 07:46:14AM +0000, Al Viro wrote:
> On Thu, Nov 13, 2025 at 04:20:08PM -0500, Greg Kroah-Hartman wrote:
>
> > Sorry for the delay. Yes, we should be grabing the mutex in there, good
> > catch. There's been more issues pointed out with the gadget code in the
> > past year or so as more people are starting to actually use it and
> > stress it more. So if you have a patch for this, I'll gladly take it :)
>
> How about the following?
>
> commit 330837c8101578438f64cfaec3fb85521d668e56
> Author: Al Viro <viro@zeniv.linux.org.uk>
> Date: Fri Nov 14 02:18:22 2025 -0500
>
> functionfs: fix the open/removal races
>
> ffs_epfile_open() can race with removal, ending up with file->private_data

Very apt prefix though.
(Like Paul would say: "Sorry, couldn't resist.")

> pointing to freed object.
>
> There is a total count of opened files on functionfs (both ep0 and
> dynamic ones) and when it hits zero, dynamic files get removed.
> Unfortunately, that removal can happen while another thread is
> in ffs_epfile_open(), but has not incremented the count yet.
> In that case open will succeed, leaving us with UAF on any subsequent
> read() or write().
>
> The root cause is that ffs->opened is misused; atomic_dec_and_test() vs.
> atomic_add_return() is not a good idea, when object remains visible all
> along.
>
> To untangle that
> * serialize openers on ffs->mutex (both for ep0 and for dynamic files)
> * have dynamic ones use atomic_inc_not_zero() and fail if we had
> zero ->opened; in that case the file we are opening is doomed.
> * have the inodes of dynamic files marked on removal (from the
> callback of simple_recursive_removal()) - clear ->i_private there.
> * have open of dynamic ones verify they hadn't been already removed,
> along with checking that state is FFS_ACTIVE.
>
> Fix another abuse of ->opened, while we are at it - it starts equal to 0,
> is incremented on opens and decremented on ->release()... *and* decremented
> (always from 0 to -1) in ->kill_sb(). Handling that case has no business
> in ffs_data_closed() (or to ->opened); just have ffs_kill_sb() do what
> ffs_data_closed() would in case of decrement to negative rather than
> calling ffs_data_closed() there.
>
> And don't bother with bumping ffs->ref when opening a file - superblock
> already holds the reference and it won't go away while there are any opened
> files on the filesystem.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
>
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index 47cfbe41fdff..ed7fa869ea77 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -640,13 +640,22 @@ static ssize_t ffs_ep0_read(struct file *file, char __user *buf,
>
> static int ffs_ep0_open(struct inode *inode, struct file *file)
> {
> - struct ffs_data *ffs = inode->i_private;
> + struct ffs_data *ffs = inode->i_sb->s_fs_info;
> + int ret;
>
> - if (ffs->state == FFS_CLOSING)
> - return -EBUSY;
> + /* Acquire mutex */
> + ret = ffs_mutex_lock(&ffs->mutex, file->f_flags & O_NONBLOCK);
> + if (ret < 0)
> + return ret;
>
> - file->private_data = ffs;
> ffs_data_opened(ffs);
> + if (ffs->state == FFS_CLOSING) {
> + ffs_data_closed(ffs);
> + mutex_unlock(&ffs->mutex);
> + return -EBUSY;
> + }
> + mutex_unlock(&ffs->mutex);
> + file->private_data = ffs;
>
> return stream_open(inode, file);
> }
> @@ -1193,14 +1202,33 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
> static int
> ffs_epfile_open(struct inode *inode, struct file *file)
> {
> - struct ffs_epfile *epfile = inode->i_private;
> + struct ffs_data *ffs = inode->i_sb->s_fs_info;
> + struct ffs_epfile *epfile;
> + int ret;
>
> - if (WARN_ON(epfile->ffs->state != FFS_ACTIVE))
> + /* Acquire mutex */
> + ret = ffs_mutex_lock(&ffs->mutex, file->f_flags & O_NONBLOCK);
> + if (ret < 0)
> + return ret;
> +
> + if (!atomic_inc_not_zero(&ffs->opened)) {
> + mutex_unlock(&ffs->mutex);
> return -ENODEV;
> + }
> + /*
> + * we want the state to be FFS_ACTIVE; FFS_ACTIVE alone is
> + * not enough, though - we might have been through FFS_CLOSING
> + * and back to FFS_ACTIVE, with our file already removed.
> + */
> + epfile = smp_load_acquire(&inode->i_private);
> + if (unlikely(ffs->state != FFS_ACTIVE || !epfile)) {
> + mutex_unlock(&ffs->mutex);
> + ffs_data_closed(ffs);
> + return -ENODEV;
> + }
> + mutex_unlock(&ffs->mutex);
>
> file->private_data = epfile;
> - ffs_data_opened(epfile->ffs);
> -
> return stream_open(inode, file);
> }
>
> @@ -1332,7 +1360,7 @@ static void ffs_dmabuf_put(struct dma_buf_attachment *attach)
> static int
> ffs_epfile_release(struct inode *inode, struct file *file)
> {
> - struct ffs_epfile *epfile = inode->i_private;
> + struct ffs_epfile *epfile = file->private_data;
> struct ffs_dmabuf_priv *priv, *tmp;
> struct ffs_data *ffs = epfile->ffs;
>
> @@ -2071,12 +2099,18 @@ static int ffs_fs_init_fs_context(struct fs_context *fc)
> return 0;
> }
>
> +static void ffs_data_reset(struct ffs_data *ffs);
> +
> static void
> ffs_fs_kill_sb(struct super_block *sb)
> {
> kill_litter_super(sb);
> - if (sb->s_fs_info)
> - ffs_data_closed(sb->s_fs_info);
> + if (sb->s_fs_info) {
> + struct ffs_data *ffs = sb->s_fs_info;
> + ffs->state = FFS_CLOSING;
> + ffs_data_reset(ffs);
> + ffs_data_put(ffs);
> + }
> }
>
> static struct file_system_type ffs_fs_type = {
> @@ -2114,7 +2148,6 @@ static void functionfs_cleanup(void)
> /* ffs_data and ffs_function construction and destruction code **************/
>
> static void ffs_data_clear(struct ffs_data *ffs);
> -static void ffs_data_reset(struct ffs_data *ffs);
>
> static void ffs_data_get(struct ffs_data *ffs)
> {
> @@ -2123,7 +2156,6 @@ static void ffs_data_get(struct ffs_data *ffs)
>
> static void ffs_data_opened(struct ffs_data *ffs)
> {
> - refcount_inc(&ffs->ref);
> if (atomic_add_return(1, &ffs->opened) == 1 &&
> ffs->state == FFS_DEACTIVATED) {
> ffs->state = FFS_CLOSING;
> @@ -2148,11 +2180,11 @@ static void ffs_data_put(struct ffs_data *ffs)
>
> static void ffs_data_closed(struct ffs_data *ffs)
> {
> - struct ffs_epfile *epfiles;
> - unsigned long flags;
> -
> if (atomic_dec_and_test(&ffs->opened)) {
> if (ffs->no_disconnect) {
> + struct ffs_epfile *epfiles;
> + unsigned long flags;
> +
> ffs->state = FFS_DEACTIVATED;
> spin_lock_irqsave(&ffs->eps_lock, flags);
> epfiles = ffs->epfiles;
> @@ -2171,12 +2203,6 @@ static void ffs_data_closed(struct ffs_data *ffs)
> ffs_data_reset(ffs);
> }
> }
> - if (atomic_read(&ffs->opened) < 0) {
> - ffs->state = FFS_CLOSING;
> - ffs_data_reset(ffs);
> - }
> -
> - ffs_data_put(ffs);
> }
>
> static struct ffs_data *ffs_data_new(const char *dev_name)
> @@ -2352,6 +2378,11 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
> return 0;
> }
>
> +static void clear_one(struct dentry *dentry)
> +{
> + smp_store_release(&dentry->d_inode->i_private, NULL);
> +}
> +
> static void ffs_epfiles_destroy(struct ffs_epfile *epfiles, unsigned count)
> {
> struct ffs_epfile *epfile = epfiles;
> @@ -2359,7 +2390,7 @@ static void ffs_epfiles_destroy(struct ffs_epfile *epfiles, unsigned count)
> for (; count; --count, ++epfile) {
> BUG_ON(mutex_is_locked(&epfile->mutex));
> if (epfile->dentry) {
> - simple_recursive_removal(epfile->dentry, NULL);
> + simple_recursive_removal(epfile->dentry, clear_one);
> epfile->dentry = NULL;
> }
> }