dma-buf: Fix memory leak in dma_buf_set_name

This patch fixes a memory leak bug reported by syzbot. Link to the bug is given at [1].

A local variable name is used to hold the copied user buffer string using strndup_user. strndup_user allocates memory using kmalloc_track_caller in memdup_user. This kmalloc allocation needs to be followed by a kfree.

This patch has been tested by a compile test.

[1] https://syzkaller.appspot.com/bug?id=ce692a3aa13e00e335e090be7846c6eb60ddff7a

Reported-by [email protected]

Signed-off-by Bharath Vedartham <[email protected]>

---
 drivers/dma-buf/dma-buf.c | 1 +
 1 file changed, 1 insertion(+)

drivers/dma-buf/dma-buf.c +0 -0

index f45bfb2..9798f6d 100644

Hi Bharath,

Thanks for taking the time to try to fix this report.

However, this doesn't look right.

Quoted 23 lines · Bharath Vedartham

This patch fixes a memory leak bug reported by syzbot. Link to the bug is given at [1].

A local variable name is used to hold the copied user buffer string using strndup_user. strndup_user allocates memory using kmalloc_track_caller in memdup_user. This kmalloc allocation needs to be followed by a kfree.

This patch has been tested by a compile test.

[1] https://syzkaller.appspot.com/bug?id=ce692a3aa13e00e335e090be7846c6eb60ddff7a

Reported-by [email protected]

Signed-off-by Bharath Vedartham <[email protected]>

---
 drivers/dma-buf/dma-buf.c | 1 +
 1 file changed, 1 insertion(+)

drivers/dma-buf/dma-buf.c +0 -0

index f45bfb2..9798f6d 100644

drivers/dma-buf/dma-buf.c static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf) · line 346

Quoted 5 lines

drivers/dma-buf/dma-buf.c +1 -0

@@ -342,6 +342,7 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
 	}
 	kfree(dmabuf->name);
 	dmabuf->name = name;
+	kfree(name);
 

Just by looking at this, you can deduce something is not right. You are assigning "name" to dmabuf->name, but then releasing "name"!

So now, dmabuf->name has free memory, which will lead to user-after-free issues.

Note also, that this function doesn't look leaky since the previous "name" is freed, before setting a new one.

Maybe the syzbot report is some kind of false positive?

Also, I _strongly_ suggest that in the future you don't compile-test only these kind of not trivial fixes. Since you are touching a crucial part of the kernel here, you should really be testing properly.

Specially since syzbot produces a reproducer.

Consider compile test as something you do when your changes are only cosmetic, and you are completely and absolutely sure things will be OK.

Thanks.
Ezequiel