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bpf: Fix the off-by-two error in range markings
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commit 2fa7d94 upstream.

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 android-rpi#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 android-rpi#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: fb2a311 ("bpf: fix off by one for range markings with L{T, E} patterns")
Fixes: b37242c ("bpf: add test cases to bpf selftests to cover all access tests")
Signed-off-by: Maxim Mikityanskiy <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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@nvmmax @gregkh
nvmmax authored and gregkh committed Dec 14, 2021
1 parent 15f9874 commit 4174bd4
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Showing 2 changed files with 17 additions and 17 deletions.
2 changes: 1 addition & 1 deletion kernel/bpf/verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5372,7 +5372,7 @@ 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:
*
Expand Down
32 changes: 16 additions & 16 deletions tools/testing/selftests/bpf/verifier/xdp_direct_packet_access.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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(),
},
Expand Down Expand Up @@ -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(),
},
Expand Down Expand Up @@ -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(),
},
Expand Down Expand Up @@ -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(),
},
Expand Down Expand Up @@ -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(),
},
Expand Down Expand Up @@ -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(),
},
Expand Down Expand Up @@ -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(),
},
Expand Down Expand Up @@ -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(),
},
Expand Down

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