[PATCH bpf 05/11] bpf: Reject pkt arguments in mutating subprogs

[Emil Tsalapatis <emil@etsalapatis.com>]

The verifier tracks changes in how PTR_TO_PACKET registers'
bounds are modified across subprog boundaries. PTR_TO_PACKET
registers are actually passed as PTR_TO_MEM, which is assumed
valid for the entire call. This is not the case with packet memory,
where a pskb_* call may invalidate its memory region.

Reject BPF code that passes PTR_TO_PACKET pointers to subprogs that
may mutate a packet. We cannot pass the pointer as a true PTR_TO_PACKET
because we would also need to somehow pass the PTR_TO_PACKET_META
or PTR_TO_PACKET_END to the subprog. Since we cannot avoid representing
the pointer in the subprog as PTR_TO_MEM, only permit it if the
subprog is guaranteed not to mutate the packet.

Fixes: 80f281664f5a ("bpf: Support pointers in global func args")
Reported-by: Nicholas Carlini <nicholas@carlini.com>
Suggested-by: Nicholas Carlini <nicholas@carlini.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
---
 kernel/bpf/verifier.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6c6b8d852..507bc14b4 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10375,6 +10375,16 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
 			if (check_mem_reg(env, reg, argno, arg->mem_size, BPF_READ | BPF_WRITE, NULL,
 					  NULL))
 				return -EINVAL;
+			/*
+			 * PTR_TO_PACKET get passed as PTR_TO_MEM, preventing
+			 * us from adjusting bounds tracking info.
+			 */
+			if ((reg_is_pkt_pointer_any(reg) || reg_is_dynptr_slice_pkt(reg)) &&
+			    sub->changes_pkt_data) {
+				bpf_log(log, "%s is a packet pointer, but func#%d may change packet data\n",
+						reg_arg_name(env, argno), subprog);
+				return -EINVAL;
+			}
 			if (!(arg->arg_type & PTR_MAYBE_NULL) &&
 			    (type_may_be_null(reg->type) || bpf_register_is_null(reg))) {
 				bpf_log(log, "%s is expected to be non-NULL\n",
-- 
2.54.0