Re: [PATCH] binfmt_elf: Take the mmap lock when walking the VMA list

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 31, 2022 at 10:03:31AM -0600, Eric W. Biederman wrote:
> "Matthew Wilcox (Oracle)" <willy@infradead.org> writes:
> 
> > I'm not sure if the VMA list can change under us, but dump_vma_snapshot()
> > is very careful to take the mmap_lock in write mode.  We only need to
> > take it in read mode here as we do not care if the size of the stack
> > VMA changes underneath us.
> >
> > If it can be changed underneath us, this is a potential use-after-free
> > for a multithreaded process which is dumping core.
> 
> The problem is not multi-threaded process so much as processes that
> share their mm.

I don't understand the difference.  I appreciate that another process can
get read access to an mm through, eg, /proc, but how can another process
(that isn't a thread of this process) modify the VMAs?

> I think rather than take a lock we should be using the snapshot captured
> with dump_vma_snapshot.  Otherwise we have the very real chance that the
> two get out of sync.  Which would result in a non-sense core file.
> 
> Probably that means that dump_vma_snapshot needs to call get_file on
> vma->vm_file store it in core_vma_metadata.
> 
> Do you think you can fix it something like that?

Uhh .. that seems like it needs a lot more understanding of binfmt_elf
than I currently possess.  I'd rather spend my time working on folios
than learning much more about binfmt_elf.  I was just trying to fix an
assertion failure with the maple tree patches (we now assert that you're
holding a lock when walking the list of VMAs).