Commit 1b9b69ec authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by David S. Miller
Browse files

bpf: teach verifier to recognize imm += ptr pattern 



Humans don't write C code like:
  u8 *ptr = skb->data;
  int imm = 4;
  imm += ptr;
but from llvm backend point of view 'imm' and 'ptr' are registers and
imm += ptr may be preferred vs ptr += imm depending which register value
will be used further in the code, while verifier can only recognize ptr += imm.
That caused small unrelated changes in the C code of the bpf program to
trigger rejection by the verifier. Therefore teach the verifier to recognize
both ptr += imm and imm += ptr.
For example:
when R6=pkt(id=0,off=0,r=62) R7=imm22
after r7 += r6 instruction
will be R6=pkt(id=0,off=0,r=62) R7=pkt(id=0,off=22,r=62)

Fixes: 969bf05e ("bpf: direct packet access")
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent d91b28ed

kernel/bpf/verifier.c

+17 −1
Original line number Diff line number Diff line
@@ -1245,6 +1245,7 @@ static int check_packet_ptr_add(struct verifier_env *env, struct bpf_insn *insn) 
	struct reg_state *regs = env->cur_state.regs;

	struct reg_state *dst_reg = &regs[insn->dst_reg];

	struct reg_state *src_reg = &regs[insn->src_reg];

	struct reg_state tmp_reg;

	s32 imm;



	if (BPF_SRC(insn->code) == BPF_K) {
@@ -1267,6 +1268,19 @@ add_imm: 
		 */

		dst_reg->off += imm;

	} else {

		if (src_reg->type == PTR_TO_PACKET) {

			/* R6=pkt(id=0,off=0,r=62) R7=imm22; r7 += r6 */

			tmp_reg = *dst_reg;  /* save r7 state */

			*dst_reg = *src_reg; /* copy pkt_ptr state r6 into r7 */

			src_reg = &tmp_reg;  /* pretend it's src_reg state */

			/* if the checks below reject it, the copy won't matter,

			 * since we're rejecting the whole program. If all ok,

			 * then imm22 state will be added to r7

			 * and r7 will be pkt(id=0,off=22,r=62) while

			 * r6 will stay as pkt(id=0,off=0,r=62)

			 */

		}



		if (src_reg->type == CONST_IMM) {

			/* pkt_ptr += reg where reg is known constant */

			imm = src_reg->imm;
@@ -1565,7 +1579,9 @@ static int check_alu_op(struct verifier_env *env, struct bpf_insn *insn) 
			return 0;

		} else if (opcode == BPF_ADD &&

			   BPF_CLASS(insn->code) == BPF_ALU64 &&

			   dst_reg->type == PTR_TO_PACKET) {

			   (dst_reg->type == PTR_TO_PACKET ||

			    (BPF_SRC(insn->code) == BPF_X &&

			     regs[insn->src_reg].type == PTR_TO_PACKET))) {

			/* ptr_to_packet += K|X */

			return check_packet_ptr_add(env, insn);

		} else if (BPF_CLASS(insn->code) == BPF_ALU64 &&

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