Subject: Re: [PATCH 0/2] Don’t leave executable TLB entries to freed pages

From: Will Deacon <[email-masked]>
Date: Wed, 28 Nov 2018 09:57:35 +0000

On Tue, Nov 27, 2018 at 05:21:08PM -0800, Nadav Amit wrote:
> > On Nov 27, 2018, at 5:06 PM, Nadav Amit <[email-masked]> wrote:
> > 
> >> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <[email-masked]> wrote:
> >> 
> >> Sometimes when memory is freed via the module subsystem, an executable
> >> permissioned TLB entry can remain to a freed page. If the page is re-used to
> >> back an address that will receive data from userspace, it can result in user
> >> data being mapped as executable in the kernel. The root of this behavior is
> >> vfree lazily flushing the TLB, but not lazily freeing the underlying pages. 
> >> 
> >> There are sort of three categories of this which show up across modules, bpf,
> >> kprobes and ftrace:
> >> 
> >> 1. When executable memory is touched and then immediatly freed
> >> 
> >>  This shows up in a couple error conditions in the module loader and BPF JIT
> >>  compiler.
> > 
> > Interesting!
> > 
> > Note that this may cause conflict with "x86: avoid W^X being broken during
> > modules loading”, which I recently submitted.
> 
> I actually have not looked on the vmalloc() code too much recent, but it
> seems … strange:
> 
>   void vm_unmap_aliases(void)
>   {       
> 
>   ...
>   	mutex_lock(&vmap_purge_lock);
>   	purge_fragmented_blocks_allcpus();
>   	if (!__purge_vmap_area_lazy(start, end) && flush)
>   		flush_tlb_kernel_range(start, end);
>   	mutex_unlock(&vmap_purge_lock);
>   }
> 
> Since __purge_vmap_area_lazy() releases the memory, it seems there is a time
> window between the release of the region and the TLB flush, in which the
> area can be allocated for another purpose. This can result in a
> (theoretical) correctness issue. No?

If __purge_vmap_area_lazy() returns false, then it hasn't freed the memory,
so we only invalidate the TLB if 'flush' is true in that case. If
__purge_vmap_area_lazy() returns true instead, then it takes care of the TLB
invalidation before the freeing.

Will