The first commit cited below attempts to fix the off-by-one error that
appeared in some comparisons with an open range. Due to this error,
arithmetically equivalent pieces of code could get different verdicts
from the verifier, for example (pseudocode):
// 1. Passes the verifier:
if (data + 8 > data_end)
return early
read *(u64 *)data, i.e. [data; data+7]
// 2. Rejected by the verifier (should still pass):
if (data + 7 >= data_end)
return early
read *(u64 *)data, i.e. [data; data+7]
The attempted fix, however, shifts the range by one in a wrong
direction, so the bug not only remains, but also such piece of code
starts failing in the verifier:
// 3. Rejected by the verifier, but the check is stricter than in #1.
if (data + 8 >= data_end)
return early
read *(u64 *)data, i.e. [data; data+7]
The change performed by that fix converted an off-by-one bug into
off-by-two. The second commit cited below added the BPF selftests
written to ensure than code chunks like #3 are rejected, however,
they should be accepted.
This commit fixes the off-by-two error by adjusting new_range in the
right direction and fixes the tests by changing the range into the one
that should actually fail.
Fixes: fb2a311a31d3 ("bpf: fix off by one for range markings with L{T, E} patterns")
Fixes: b37242c773b2 ("bpf: add test cases to bpf selftests to cover all access tests")
Signed-off-by: Maxim Mikityanskiy <maximmi@xxxxxxxxxx>
---
After this patch is merged, I'm going to submit another patch to
bpf-next, that will add new selftests for this bug.
kernel/bpf/verifier.c | 2 +-
.../bpf/verifier/xdp_direct_packet_access.c | 32 +++++++++----------
2 files changed, 17 insertions(+), 17 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 50efda51515b..f3001937bbb9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8422,7 +8422,7 @@ static void find_good_pkt_pointers(struct bpf_verifier_state *vstate,
new_range = dst_reg->off;
if (range_right_open)
- new_range--;
+ new_range++;
/* Examples for register markings:
*
diff --git a/tools/testing/selftests/bpf/verifier/xdp_direct_packet_access.c b/tools/testing/selftests/bpf/verifier/xdp_direct_packet_access.c
index bfb97383e6b5..de172a5b8754 100644
--- a/tools/testing/selftests/bpf/verifier/xdp_direct_packet_access.c
+++ b/tools/testing/selftests/bpf/verifier/xdp_direct_packet_access.c
@@ -112,10 +112,10 @@
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
offsetof(struct xdp_md, data_end)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JGT, BPF_REG_3, BPF_REG_1, 1),
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
@@ -167,10 +167,10 @@
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
offsetof(struct xdp_md, data_end)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JLT, BPF_REG_1, BPF_REG_3, 1),
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
@@ -274,9 +274,9 @@
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
offsetof(struct xdp_md, data_end)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JGE, BPF_REG_1, BPF_REG_3, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
@@ -437,9 +437,9 @@
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
offsetof(struct xdp_md, data_end)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JLE, BPF_REG_3, BPF_REG_1, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
@@ -544,10 +544,10 @@
offsetof(struct xdp_md, data_meta)),
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1, offsetof(struct xdp_md, data)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JGT, BPF_REG_3, BPF_REG_1, 1),
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
@@ -599,10 +599,10 @@
offsetof(struct xdp_md, data_meta)),
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1, offsetof(struct xdp_md, data)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JLT, BPF_REG_1, BPF_REG_3, 1),
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
@@ -706,9 +706,9 @@
offsetof(struct xdp_md, data_meta)),
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1, offsetof(struct xdp_md, data)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JGE, BPF_REG_1, BPF_REG_3, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
@@ -869,9 +869,9 @@
offsetof(struct xdp_md, data_meta)),
BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1, offsetof(struct xdp_md, data)),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_2),
- BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 8),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 6),
BPF_JMP_REG(BPF_JLE, BPF_REG_3, BPF_REG_1, 1),
- BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8),
+ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -6),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),
},
--
2.30.2
