On Wed, 10 Jun 2020 at 14:57, Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Wed, Jun 10, 2020 at 10:33 AM Sumit Semwal <sumit.semwal@xxxxxxxxxx> wrote: > > > > Charan Teja reported a 'use-after-free' in dmabuffs_dname [1], which > > happens if the dma_buf_release() is called while the userspace is > > accessing the dma_buf pseudo fs's dmabuffs_dname() in another process, > > and dma_buf_release() releases the dmabuf object when the last reference > > to the struct file goes away. > > > > I discussed with Arnd Bergmann, and he suggested that rather than tying > > the dma_buf_release() to the file_operations' release(), we can tie it to > > the dentry_operations' d_release(), which will be called when the last ref > > to the dentry is removed. > > > > The path exercised by __fput() calls f_op->release() first, and then calls > > dput, which eventually calls d_op->d_release(). > > > > In the 'normal' case, when no userspace access is happening via dma_buf > > pseudo fs, there should be exactly one fd, file, dentry and inode, so > > closing the fd will kill of everything right away. > > > > In the presented case, the dentry's d_release() will be called only when > > the dentry's last ref is released. > > > > Therefore, lets move dma_buf_release() from fops->release() to > > d_ops->d_release(). > > > > Many thanks to Arnd for his FS insights :) > > > > [1]: https://lore.kernel.org/patchwork/patch/1238278/ > > > > Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls") > > Reported-by: syzbot+3643a18836bce555bff6@xxxxxxxxxxxxxxxxxxxxxxxxx > > Cc: <stable@xxxxxxxxxxxxxxx> [5.3+] > > Cc: Arnd Bergmann <arnd@xxxxxxxx> > > Reported-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx> > > Signed-off-by: Sumit Semwal <sumit.semwal@xxxxxxxxxx> > > The patch looks correct to me. > > Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx> > > Obviously this should still be verified against the original report if possible. Thanks, Arnd! > > > drivers/dma-buf/dma-buf.c | 13 +++++++------ > > 1 file changed, 7 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c > > index 01ce125f8e8d..92ba4b6ef3e7 100644 > > --- a/drivers/dma-buf/dma-buf.c > > +++ b/drivers/dma-buf/dma-buf.c > > @@ -54,8 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen) > > dentry->d_name.name, ret > 0 ? name : ""); > > } > > > > +static void dma_buf_release(struct dentry *dentry); > > + > > static const struct dentry_operations dma_buf_dentry_ops = { > > .d_dname = dmabuffs_dname, > > + .d_release = dma_buf_release, > > }; > > I'd suggest rearranging the file to avoid the forward declaration, even > if it makes it a little harder to review the change, the resulting code > will remain organized more logically. Got it, will update it in v2. > > > static struct vfsmount *dma_buf_mnt; > > @@ -77,14 +80,14 @@ static struct file_system_type dma_buf_fs_type = { > > .kill_sb = kill_anon_super, > > }; > > > > -static int dma_buf_release(struct inode *inode, struct file *file) > > +static void dma_buf_release(struct dentry *dentry) > > { > > struct dma_buf *dmabuf; > > > > - if (!is_dma_buf_file(file)) > > - return -EINVAL; > > + if (dentry->d_op != &dma_buf_dentry_ops) > > + return; > > I think the check here is redundant and it's clearer without it. Ok, will remove. > > Arnd Best, Sumit.
