Re: SPARC version of arch_validate_prot() looks broken (UAF read)

[Khalid Aziz]

On 9/28/20 6:14 AM, Jann Horn wrote:
> From what I can tell from looking at the code:
> 
> SPARC's arch_validate_prot() looks up the VMA and peeks at it; that's
> not permitted though. do_mprotect_pkey() calls arch_validate_prot()
> before taking the mmap lock, so we can hit use-after-free reads if
> someone concurrently deletes a VMA we're looking at.

That makes sense. It will be a good idea to encapsulate vma access
inside sparc_validate_prot() between mmap_read_lock() and
mmap_read_unlock().

> 
> Additionally, arch_validate_prot() currently only accepts the start
> address as a parameter, but the SPARC code probably should be checking
> the entire given range, which might consist of multiple VMAs?
> 
> I'm not sure what the best fix is here; it kinda seems like what SPARC
> really wants is a separate hook that is called from inside the loop in
> do_mprotect_pkey() that iterates over the VMAs? So maybe commit
> 9035cf9a97e4 ("mm: Add address parameter to arch_validate_prot()")
> should be reverted, and a separate hook should be created?
> 
> (Luckily the ordering of the vmacache operations works out suIch that
> AFAICS, despite calling find_vma() without holding the mmap_sem, we
> can never end up establishing a vmacache entry with a dangling pointer
> that might be considered valid on a subsequent call. So this should be
> limited to a rather boring UAF data read, and not be exploitable for a
> UAF write or UAF function pointer read.)
> 

I think arch_validate_prot() is still the right hook to validate the
protection bits. sparc_validate_prot() can iterate over VMAs with read
lock. This will, of course, require range as well to be passed to
arch_validate_prot().

Thanks,
Khalid