Re: [PATCH] mm: Don't warn if memdup_user fails

[Pekka Enberg]

On Wed, 11 Jan 2012, Andrew Morton wrote:
> > diff --git a/mm/util.c b/mm/util.c
> > index 136ac4f..88bb4d4 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -91,7 +91,7 @@ void *memdup_user(const void __user *src, size_t len)
> >  	 * cause pagefault, which makes it pointless to use GFP_NOFS
> >  	 * or GFP_ATOMIC.
> >  	 */
> > -	p = kmalloc_track_caller(len, GFP_KERNEL);
> > +	p = kmalloc_track_caller(len, GFP_KERNEL | __GFP_NOWARN);
> >  	if (!p)
> >  		return ERR_PTR(-ENOMEM);
>
> There's nothing particularly special about memdup_user(): there are
> many ways in which userspace can trigger GFP_KERNEL allocations.
>
> The problem here (one which your patch carefully covers up) is that
> ecryptfs_miscdev_write() is passing an unchecked userspace-provided
> `count' direct into kmalloc().  This is a bit problematic for other
> reasons: it gives userspace a way to trigger heavy reclaim activity and
> perhaps even to trigger the oom-killer.
>
> A better fix here would be to validate the incoming arg before using
> it.  Preferably by running ecryptfs_parse_packet_length() before taking
> a copy of the data.  That would require adding a small copy_from_user()
> to peek at the message header.

Yup, right you are. I didn't think about the reclaim and oom issue. We 
should add a big fat warning on top of memdup_user() to tell users to 
check 'len' for sanity themselves. I think they're now fooled into 
thinking memdup_user() automagically does the right thing.

			Pekka
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