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> --- 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, }; 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; - dmabuf = file->private_data; + dmabuf = dentry->d_fsdata; BUG_ON(dmabuf->vmapping_counter); @@ -110,7 +113,6 @@ static int dma_buf_release(struct inode *inode, struct file *file) module_put(dmabuf->owner); kfree(dmabuf->name); kfree(dmabuf); - return 0; } static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma) @@ -412,7 +414,6 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file) } static const struct file_operations dma_buf_fops = { - .release = dma_buf_release, .mmap = dma_buf_mmap_internal, .llseek = dma_buf_llseek, .poll = dma_buf_poll, -- 2.27.0
