[PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes

[David Laight]

From: Christian Brauner
> Sent: 12 June 2020 19:28
...
> > > 	if (size < 32)
> > > 		return -EINVAL;
> > > 	if (size > PAGE_SIZE)
> > > 		return -E2BIG;
> >
> > (Tanget: what was the reason for copy_struct_from_user() not including
> > the min/max check? I have a memory of Al objecting to having an
> > "internal" limit?)
> 
> Al didn't want the PAGE_SIZE limit in there because there's nothing
> inherently wrong with copying insane amounts of memory.

The problem is really allowing a user process to allocate
unbounded blocks of memory, not the copy itself.

The limit for IOW() etc is 16k - not a problem.
If a 32bit size is set to just under 4GB so you really want
to allocate 4GB of memory then find the request is garbage.
Seems like a nice DoS attack.
A 64bit size can be worse.

Potentially the limit should be in memdup_user() itself.
And possibly an extra parameter giving a per-call lower? limit.

	David

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