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

[Eric Biggers <ebiggers3@gmail.com>]

On Tue, Apr 10, 2018 at 10:58:22AM +0300, Kirill A. Shutemov wrote:
> On Mon, Apr 09, 2018 at 01:36:35PM -0700, Eric Biggers wrote:
> > On Mon, Apr 09, 2018 at 01:12:32PM -0700, Davidlohr Bueso wrote:
> > > 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
> > 
> > In the remap_file_pages() case, a reference is taken to the ->vm_file, then the
> > segment is unmapped.  If that brings ->shm_nattch to 0, then the underlying shm
> > segment and ID can be removed, which (currently) causes the real shm file to be
> > freed.  But, the outer file still exists and will have ->mmap() called on it.
> > That's why the outer file needs to hold a reference to the real shm file.
> 
> Okay, fair enough. Logic in SysV IPC implementation is often hard to follow.
> Could you include the description in the commit message?
> 
> And feel free to use my
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> 

I'll send v2 to update the commit message and add a comment.

Thanks,

Eric