Re: [PATCH] dma-buf: Fix memory leak in dma_buf_set_name

[Bharath Vedartham]

On Fri, Aug 16, 2019 at 05:14:24PM -0300, Ezequiel Garcia wrote:
> Hi Bharath,
> 
> Thanks for taking the time to try to fix this report.
> 
> However, this doesn't look right.
> 
> On Fri, 2019-08-16 at 23:30 +0530, Bharath Vedartham wrote:
> > 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: syzbot+b2098bc44728a4efb3e9@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Bharath Vedartham <linux.bhar@xxxxxxxxx>
> > ---
> >  drivers/dma-buf/dma-buf.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index f45bfb2..9798f6d 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -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

Hi Ezequiel,

Thank you for taking the time to review this.

I made a mistake here and thank you for notifying me of it.

Thank you for your comments, I ll keep them in mind before sending
patches to the kernel :)

Thank you
Bharath