On Thu, Mar 17, 2022 at 05:58:24PM +0100, Jakub Sitnicki wrote:
> In commit 9a69e2b385f4 ("bpf: Make remote_port field in struct
> bpf_sk_lookup 16-bit wide") the remote_port field has been split up and
> re-declared from u32 to be16.
>
> However, the accompanying changes to the context access converter have not
> been well thought through when it comes big-endian platforms.
>
> Today 2-byte wide loads from offsetof(struct bpf_sk_lookup, remote_port)
> are handled as narrow loads from a 4-byte wide field.
>
> This by itself is not enough to create a problem, but when we combine
>
> 1. 32-bit wide access to ->remote_port backed by a 16-wide wide load, with
> 2. inherent difference between litte- and big-endian in how narrow loads
> need have to be handled (see bpf_ctx_narrow_access_offset),
>
> we get inconsistent results for a 2-byte loads from &ctx->remote_port on LE
> and BE architectures. This in turn makes BPF C code for the common case of
> 2-byte load from ctx->remote_port not portable.
>
> To rectify it, inform the context access converter that remote_port is
> 2-byte wide field, and only 1-byte loads need to be treated as narrow
> loads.
>
> At the same time, we special-case the 4-byte load from &ctx->remote_port to
> continue handling it the same way as do today, in order to keep the
> existing BPF programs working.
>
> Fixes: 9a69e2b385f4 ("bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide")
> Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> ---
> net/core/filter.c | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 03655f2074ae..9b1e453baf6d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -10989,13 +10989,24 @@ static bool sk_lookup_is_valid_access(int off, int size,
> case bpf_ctx_range(struct bpf_sk_lookup, local_ip4):
> case bpf_ctx_range_till(struct bpf_sk_lookup, remote_ip6[0], remote_ip6[3]):
> case bpf_ctx_range_till(struct bpf_sk_lookup, local_ip6[0], local_ip6[3]):
> - case offsetof(struct bpf_sk_lookup, remote_port) ...
> - offsetof(struct bpf_sk_lookup, local_ip4) - 1:
> case bpf_ctx_range(struct bpf_sk_lookup, local_port):
> case bpf_ctx_range(struct bpf_sk_lookup, ingress_ifindex):
> bpf_ctx_record_field_size(info, sizeof(__u32));
> return bpf_ctx_narrow_access_ok(off, size, sizeof(__u32));
>
> + case bpf_ctx_range(struct bpf_sk_lookup, remote_port):
> + /* Allow 4-byte access to 2-byte field for backward compatibility */
> + if (size == sizeof(__u32))
> + return off == offsetof(struct bpf_sk_lookup, remote_port);
nit. The bad "off" value should have been rejected earlier in the
"if (off % size != 0)" check?
> + bpf_ctx_record_field_size(info, sizeof(__be16));
> + return bpf_ctx_narrow_access_ok(off, size, sizeof(__be16));
> +
> + case offsetofend(struct bpf_sk_lookup, remote_port) ...
> + offsetof(struct bpf_sk_lookup, local_ip4) - 1:
> + /* Allow access to zero padding for backward compatibility */
> + bpf_ctx_record_field_size(info, sizeof(__u16));
> + return bpf_ctx_narrow_access_ok(off, size, sizeof(__u16));
> +
> default:
> return false;
> }
> @@ -11077,6 +11088,11 @@ static u32 sk_lookup_convert_ctx_access(enum bpf_access_type type,
> sport, 2, target_size));
> break;
>
> + case offsetofend(struct bpf_sk_lookup, remote_port):
> + *target_size = 2;
> + *insn++ = BPF_MOV32_IMM(si->dst_reg, 0);
> + break;
> +
> case offsetof(struct bpf_sk_lookup, local_port):
> *insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
> bpf_target_off(struct bpf_sk_lookup_kern,
> --
> 2.35.1
>
