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Commit b1977682 authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by David S. Miller
Browse files

bpf: improve verifier packet range checks 



llvm can optimize the 'if (ptr > data_end)' checks to be in the order
slightly different than the original C code which will confuse verifier.
Like:
if (ptr + 16 > data_end)
  return TC_ACT_SHOT;
// may be followed by
if (ptr + 14 > data_end)
  return TC_ACT_SHOT;
while llvm can see that 'ptr' is valid for all 16 bytes,
the verifier could not.
Fix verifier logic to account for such case and add a test.

Reported-by: default avatarHuapeng Zhou <hzhou@fb.com>
Fixes: 969bf05e ("bpf: direct packet access")
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarMartin KaFai Lau <kafai@fb.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 43a66845

kernel/bpf/verifier.c

+3 −2
Original line number Diff line number Diff line
@@ -1973,14 +1973,15 @@ static void find_good_pkt_pointers(struct bpf_verifier_state *state, 


	for (i = 0; i < MAX_BPF_REG; i++)

		if (regs[i].type == PTR_TO_PACKET && regs[i].id == dst_reg->id)

			regs[i].range = dst_reg->off;

			/* keep the maximum range already checked */

			regs[i].range = max(regs[i].range, dst_reg->off);



	for (i = 0; i < MAX_BPF_STACK; i += BPF_REG_SIZE) {

		if (state->stack_slot_type[i] != STACK_SPILL)

			continue;

		reg = &state->spilled_regs[i / BPF_REG_SIZE];

		if (reg->type == PTR_TO_PACKET && reg->id == dst_reg->id)

			reg->range = dst_reg->off;

			reg->range = max(reg->range, dst_reg->off);

	}

}

tools/testing/selftests/bpf/test_verifier.c

+20 −0
Original line number Diff line number Diff line
@@ -3417,6 +3417,26 @@ static struct bpf_test tests[] = { 
		.result = ACCEPT,

		.prog_type = BPF_PROG_TYPE_LWT_XMIT,

	},

	{

		"overlapping checks for direct packet access",

		.insns = {

			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,

				    offsetof(struct __sk_buff, data)),

			BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,

				    offsetof(struct __sk_buff, data_end)),

			BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),

			BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 8),

			BPF_JMP_REG(BPF_JGT, BPF_REG_0, BPF_REG_3, 4),

			BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),

			BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),

			BPF_JMP_REG(BPF_JGT, BPF_REG_1, BPF_REG_3, 1),

			BPF_LDX_MEM(BPF_H, BPF_REG_0, BPF_REG_2, 6),

			BPF_MOV64_IMM(BPF_REG_0, 0),

			BPF_EXIT_INSN(),

		},

		.result = ACCEPT,

		.prog_type = BPF_PROG_TYPE_LWT_XMIT,

	},

	{

		"invalid access of tc_classid for LWT_IN",

		.insns = {