[PATCH bpf-next 1/2] bpf: let verifier to calculate and record max_pkt_offset

[Jiong Wang <jiong.wang@netronome.com>]

In check_packet_access, update max_pkt_offset after the offset has passed
__check_packet_access.

It should be safe to use u32 for max_pkt_offset as explained in code
comment.

Also, when there is tail call, the max_pkt_offset of the called program is
unknown, so conservatively set max_pkt_offset to MAX_PACKET_OFF for such
case.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
---
 include/linux/bpf.h   |  1 +
 kernel/bpf/verifier.c | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 33014ae..b6a296e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -293,6 +293,7 @@ struct bpf_prog_aux {
 	atomic_t refcnt;
 	u32 used_map_cnt;
 	u32 max_ctx_offset;
+	u32 max_pkt_offset;
 	u32 stack_depth;
 	u32 id;
 	u32 func_cnt;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 98fa0be..6a248d8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1452,6 +1452,17 @@ static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off,
 		verbose(env, "R%d offset is outside of the packet\n", regno);
 		return err;
 	}
+
+	/* __check_packet_access has made sure "off + size - 1" is within u16.
+	 * reg->umax_value can't be bigger than MAX_PACKET_OFF which is 0xffff,
+	 * otherwise find_good_pkt_pointers would have refused to set range info
+	 * that __check_packet_access would have rejected this pkt access.
+	 * Therefore, "off + reg->umax_value + size - 1" won't overflow u32.
+	 */
+	env->prog->aux->max_pkt_offset =
+		max_t(u32, env->prog->aux->max_pkt_offset,
+		      off + reg->umax_value + size - 1);
+
 	return err;
 }
 
@@ -6128,6 +6139,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			 */
 			prog->cb_access = 1;
 			env->prog->aux->stack_depth = MAX_BPF_STACK;
+			env->prog->aux->max_pkt_offset = MAX_PACKET_OFF;
 
 			/* mark bpf_tail_call as different opcode to avoid
 			 * conditional branch in the interpeter for every normal
-- 
2.7.4