Re: [PATCH] ipc/shm: fix use-after-free of shm file via remap_file_pages()

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 09 Apr 2018, Eric Biggers wrote:

>It's necessary because if we don't hold a reference to sfd->file, then it can be
>a stale pointer when we compare it in __shm_open().  In particular, if the new
>struct file happened to be allocated at the same address as the old one, then
>'sfd->file == shp->shm_file' so the mmap would be allowed.  But, it will be a
>different shm segment than was intended.  The caller may not even have
>permissions to map it normally, yet it would be done anyway.
>
>In the end it's just broken to have a pointer to something that can be freed out
>from under you...

So this is actually handled by shm_nattch, serialized by the ipc perm->lock.
shm_destroy() is called when 0, which in turn does the fput(shm_file). Note
that shm_file is given a count of 1 when a new segment is created (deep in
get_empty_filp()). So I don't think the pointer is going anywhere, or am I missing
something?

Thanks,
Davidlohr