Re: [PATCH bpf] riscv, bpf: Fix offset range checking for auipc+jalr on RV64

[Daniel Borkmann]

On 4/7/20 12:16 AM, Luke Nelson wrote:
> The existing code in emit_call on RV64 checks that the PC-relative offset
> to the function fits in 32 bits before calling emit_jump_and_link to emit
> an auipc+jalr pair. However, this check is incorrect because offsets in
> the range [2^31 - 2^11, 2^31 - 1] cannot be encoded using auipc+jalr on
> RV64 (see discussion [1]). The RISC-V spec has recently been updated
> to reflect this fact [2, 3].
>
> This patch fixes the problem by moving the check on the offset into
> emit_jump_and_link and modifying it to the correct range of encodable
> offsets, which is [-2^31 - 2^11, 2^31 - 2^11). This also enforces the
> check on the offset to other uses of emit_jump_and_link (e.g., BPF_JA)
> as well.
>
> Currently, this bug is unlikely to be triggered, because the memory
> region from which JITed images are allocated is close enough to kernel
> text for the offsets to not become too large; and because the bounds on
> BPF program size are small enough. This patch prevents this problem from
> becoming an issue if either of these change.
>
> [1]: https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/bwWFhBnnZFQ
> [2]: https://github.com/riscv/riscv-isa-manual/commit/b1e42e09ac55116dbf9de5e4fb326a5a90e4a993
> [3]: https://github.com/riscv/riscv-isa-manual/commit/4c1b2066ebd2965a422e41eb262d0a208a7fea07
>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>

Applied, thanks!