Re: [PATCH] dma-buf: fix use-after-free in dmabuffs_dname

From: Charan Teja Kalla
Date: Tue May 12 2020 - 01:13:59 EST


Thank you Greg for the comments.

On 5/6/2020 2:30 PM, Greg KH wrote:
On Wed, May 06, 2020 at 02:00:10PM +0530, Charan Teja Kalla wrote:
Thank you Greg for the reply.

On 5/5/2020 3:38 PM, Greg KH wrote:
On Tue, Apr 28, 2020 at 01:24:02PM +0530, Charan Teja Reddy wrote:
The following race occurs while accessing the dmabuf object exported as
file:
P1 P2
dma_buf_release() dmabuffs_dname()
[say lsof reading /proc/<P1 pid>/fd/<num>]

read dmabuf stored in dentry->fsdata
Free the dmabuf object
Start accessing the dmabuf structure

In the above description, the dmabuf object freed in P1 is being
accessed from P2 which is resulting into the use-after-free. Below is
the dump stack reported.

Call Trace:
kasan_report+0x12/0x20
__asan_report_load8_noabort+0x14/0x20
dmabuffs_dname+0x4f4/0x560
tomoyo_realpath_from_path+0x165/0x660
tomoyo_get_realpath
tomoyo_check_open_permission+0x2a3/0x3e0
tomoyo_file_open
tomoyo_file_open+0xa9/0xd0
security_file_open+0x71/0x300
do_dentry_open+0x37a/0x1380
vfs_open+0xa0/0xd0
path_openat+0x12ee/0x3490
do_filp_open+0x192/0x260
do_sys_openat2+0x5eb/0x7e0
do_sys_open+0xf2/0x180

Fixes: bb2bb90 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
Nit, please read the documentation for how to do a Fixes: line properly,
you need more digits:
Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")

Will update the patch


Reported-by:syzbot+3643a18836bce555bff6@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Charan Teja Reddy<charante@xxxxxxxxxxxxxx>
Also, any reason you didn't include the other mailing lists that
get_maintainer.pl said to?

Really sorry for not sending to complete list. Added now.


And finally, no cc: stable in the s-o-b area for an issue that needs to
be backported to older kernels?

Will update the patch.


---
drivers/dma-buf/dma-buf.c | 25 +++++++++++++++++++++++--
include/linux/dma-buf.h | 1 +
2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 570c923..069d8f78 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -25,6 +25,7 @@
#include <linux/mm.h>
#include <linux/mount.h>
#include <linux/pseudo_fs.h>
+#include <linux/dcache.h>
#include <uapi/linux/dma-buf.h>
#include <uapi/linux/magic.h>
@@ -38,18 +39,34 @@ struct dma_buf_list {
static struct dma_buf_list db_list;
+static void dmabuf_dent_put(struct dma_buf *dmabuf)
+{
+ if (atomic_dec_and_test(&dmabuf->dent_count)) {
+ kfree(dmabuf->name);
+ kfree(dmabuf);
+ }
Why not just use a kref instead of an open-coded atomic value?

Kref approach looks cleaner. will update the patch accordingly.


+}
+
static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
{
struct dma_buf *dmabuf;
char name[DMA_BUF_NAME_LEN];
size_t ret = 0;
+ spin_lock(&dentry->d_lock);
dmabuf = dentry->d_fsdata;
+ if (!dmabuf || !atomic_add_unless(&dmabuf->dent_count, 1, 0)) {
+ spin_unlock(&dentry->d_lock);
+ goto out;
How can dmabuf not be valid here?

And isn't there already a usecount for the buffer?

dmabuf exported as file simply relies on that file's refcount, thus fput()
releases the dmabuf.

We are storing the dmabuf in the dentry->d_fsdata but there is no binding
between the dentry and the dmabuf. So, flow will be like

1) P1 calls fput(dmabuf_fd)

2) P2 trying to access the file information of P1.
ÂÂÂ Eg: lsof command trying to list out the dmabuf_fd information using
/proc/<P1 pid>/fd/dmabuf_fd

3) P1 calls the file->f_op->release(dmabuf_fd_file)(ends up in calling
dma_buf_release()),ÂÂ thus frees up the dmabuf buffer.

4) P2 access the dmabuf stored in the dentry->d_fsdata which was freed in
step 3.

So we need to have some refcount mechanism to avoid the use-after-free in
step 4.
Ok, but watch out, now you have 2 different reference counts for the
same structure. Keeping them coordinated is almost always an impossible
task so you need to only rely on one. If you can't use the file api,
just drop all of the reference counting logic in there and only use the
kref one.

I feel that changing the refcount logic now to dma-buf objects involve changes in

the core dma-buf framework. NO? Instead, how about passing the user passed name directly

in the ->d_fsdata inplace of dmabuf object? Because we just need user passed name in the

dmabuffs_dname(). With this we can avoid the need for extra refcount on dmabuf.

Posted patch-V2: https://lkml.org/lkml/2020/5/8/158



good luck!

greg k-h

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