Hello,

If there are no objections to this, I will plan to merge it soon.

Daniel, Chris, Chenbo?

On Thu, 11 Jun 2020 at 17:14, Sumit Semwal sumit.semwal@linaro.org 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 :)

Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls") Reported-by: syzbot+3643a18836bce555bff6@syzkaller.appspotmail.com Cc: stable@vger.kernel.org [5.3+] Cc: Arnd Bergmann arnd@arndb.de Reported-by: Charan Teja Reddy charante@codeaurora.org Reviewed-by: Arnd Bergmann arnd@arndb.de Signed-off-by: Sumit Semwal sumit.semwal@linaro.org

---

v2: per Arnd: Moved dma_buf_release() above to avoid forward declaration; removed dentry_ops check.

---

drivers/dma-buf/dma-buf.c | 54 ++++++++++++++++++--------------------- 1 file changed, 25 insertions(+), 29 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 01ce125f8e8d..412629601ad3 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -54,37 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen) dentry->d_name.name, ret > 0 ? name : ""); }

-static const struct dentry_operations dma_buf_dentry_ops = {

•   .d_dname = dmabuffs_dname,
  

-};

-static struct vfsmount *dma_buf_mnt;

-static int dma_buf_fs_init_context(struct fs_context *fc) -{

•   struct pseudo_fs_context *ctx;
  

•   ctx = init_pseudo(fc, DMA_BUF_MAGIC);
  

•   if (!ctx)
  

•           return -ENOMEM;
  

•   ctx->dops = &dma_buf_dentry_ops;
  

•   return 0;
  

-}

-static struct file_system_type dma_buf_fs_type = {

•   .name = "dmabuf",
  

•   .init_fs_context = dma_buf_fs_init_context,
  

•   .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;
  

•   dmabuf = file->private_data;
  

•   dmabuf = dentry->d_fsdata;
  
    BUG_ON(dmabuf->vmapping_counter);
  
  

@@ -110,9 +84,32 @@ static int dma_buf_release(struct inode *inode, struct file *file) module_put(dmabuf->owner); kfree(dmabuf->name); kfree(dmabuf); +}

+static const struct dentry_operations dma_buf_dentry_ops = {

•   .d_dname = dmabuffs_dname,
  

•   .d_release = dma_buf_release,
  

+};

+static struct vfsmount *dma_buf_mnt;

+static int dma_buf_fs_init_context(struct fs_context *fc) +{

•   struct pseudo_fs_context *ctx;
  

•   ctx = init_pseudo(fc, DMA_BUF_MAGIC);
  

•   if (!ctx)
  

•           return -ENOMEM;
  

•   ctx->dops = &dma_buf_dentry_ops;
    return 0;
  

}

+static struct file_system_type dma_buf_fs_type = {

•   .name = "dmabuf",
  

•   .init_fs_context = dma_buf_fs_init_context,
  

•   .kill_sb = kill_anon_super,
  

+};

static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma) { struct dma_buf *dmabuf; @@ -412,7 +409,6 @@ 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

Best, Sumit.