On Tue, Apr 14, 2020 at 9:58 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Fri, Apr 10, 2020 at 10:26 PM Andrii Nakryiko <andriin@xxxxxx> wrote:
> > VM_MAYWRITE flag during initial memory mapping determines if already mmap()'ed
> > pages can be later remapped as writable ones through mprotect() call. To
> > prevent user application to rewrite contents of memory-mapped as read-only and
> > subsequently frozen BPF map, remove VM_MAYWRITE flag completely on initially
> > read-only mapping.
> >
> > Alternatively, we could treat any memory-mapping on unfrozen map as writable
> > and bump writecnt instead. But there is little legitimate reason to map
> > BPF map as read-only and then re-mmap() it as writable through mprotect(),
> > instead of just mmap()'ing it as read/write from the very beginning.
> >
> > Also, at the suggestion of Jann Horn, drop unnecessary refcounting in mmap
> > operations. We can just rely on VMA holding reference to BPF map's file
> > properly.
> >
> > Fixes: fc9702273e2e ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY")
> > Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> > Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
>
> Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
>
> (in the sense that I think this patch is good and correct, but does
> not fix the entire problem in the bigger picture)
I agree, we'll continue discussion on the other thread, but this
should be applied as a bug fix anyways.
