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)
