linux-kernel - Re: [RFC] vivi, videobuf_to_vmalloc() and related breakage

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Message-Id: <200710151759.19839.nickpiggin@yahoo.com.au>
Date:	Mon, 15 Oct 2007 17:59:19 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Al Viro <viro@....linux.org.uk>
Cc:	mchehab@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] vivi, videobuf_to_vmalloc() and related breakage

On Monday 15 October 2007 12:01, Al Viro wrote:
> 	AFAICS, videobuf-vmalloc use of mem->vma and mem->vmalloc is
> bogus.
>
> You obtain the latter with vmalloc_user(); so far, so good.  Then you have
>         retval=remap_vmalloc_range(vma, mem->vmalloc,0);
> where vma is given to you by mmap(); again, fine - we get the memory
> pointed to be mem->vmalloc() mapped at vma->vm_start.
>
> Now we get the trouble: things like
>
> static void vivi_fillbuff(struct vivi_dev *dev,struct vivi_buffer *buf)
> {
> 	...
> 	void *vbuf=videobuf_to_vmalloc (&buf->vb);
> 	...
> 	copy_to_user(vbuf + ..., ..., ...)
>
> get vbuf equal to ->vmalloc of buf->vp.priv and that is _not_ a userland
> address.  Giving it to copy_to_user() is not going to do anything good.
> On some targets it'll fail, on some - write to unrelated user memory.
> What is going on there?  If that's an attempt to copy into that buffer
> allocated by vmalloc_user(), why are we doing copy_to_user() at all?

Right you are. remap_vmalloc_range doesn't turn the passed vmalloc
area into user memory (it creates a completely new mapping).

Presumably it either wants to copy_to_user to that new mapping, or
memcpy to ->vmalloc? Would the former be an attempt to avoid some
virtual aliasing issues?

> But there's more; we have made a copy of vma (kmalloc+memcpy), stored it in
> mem->vma and later we cheerfully do remap_vmalloc_range(mem->vma,....).
> And kfree that mem->vma immediately afterwards.  What the hell?  It might
> not break now, but that seems to be playing very fast and loose with the
> warranties provided by VM.

I don't know why one would be finding remap_vmalloc_range to fail
it mmap time but not later? Should just do it at mmap time and if
that is failing, then work out why (or ask linux-mm for help).
Actually there is probably a window where we can get subsequent
anonymous pages faulted into the empty vma there if we haven't
remapped it, then the subsequent attempt to remap will hit the
BUG_ON in remap_pte_range.

(that's aside from the big conceptual problem with passing in an
"invented" vma... don't do that! (: )
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