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: Huapeng Zhou <hzhou@fb.com>
Fixes: 969bf05eb3ce ("bpf: direct packet access")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 23 insertions, 2 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 796b68d00119..5e6202e62265 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -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);
         }
 }
 
diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index d1555e4240c0..7d761d4cc759 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -3418,6 +3418,26 @@ static struct bpf_test tests[] = {
                 .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 = {
                         BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,