Re: [PATCH 4.4 1/3] bpf: fix branch pruning logic

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jan 31, 2018 at 06:03:45PM +0000, Ben Hutchings wrote:
> commit c131187db2d3fa2f8bf32fdf4e9a4ef805168467 upstream.
> 
> when the verifier detects that register contains a runtime constant
> and it's compared with another constant it will prune exploration
> of the branch that is guaranteed not to be taken at runtime.
> This is all correct, but malicious program may be constructed
> in such a way that it always has a constant comparison and
> the other branch is never taken under any conditions.
> In this case such path through the program will not be explored
> by the verifier. It won't be taken at run-time either, but since
> all instructions are JITed the malicious program may cause JITs
> to complain about using reserved fields, etc.
> To fix the issue we have to track the instructions explored by
> the verifier and sanitize instructions that are dead at run time
> with NOPs. We cannot reject such dead code, since llvm generates
> it for valid C code, since it doesn't do as much data flow
> analysis as the verifier does.
> 
> Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> Acked-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> [bwh: Backported to 4.4:
>  - s/bpf_verifier_env/verifier_env/
>  - Adjust context]
> Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
> ---
>  kernel/bpf/verifier.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)

Daniel already provided a backport of this, thanks.

greg k-h