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Message-Id: <20211213092946.373740164@linuxfoundation.org>
Date:   Mon, 13 Dec 2021 10:29:14 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Maxim Mikityanskiy <maximmi@...dia.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH 5.15 039/171] bpf: Fix the off-by-two error in range markings

From: Maxim Mikityanskiy <maximmi@...dia.com>

commit 2fa7d94afc1afbb4d702760c058dc2d7ed30f226 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 #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@...dia.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Link: https://lore.kernel.org/bpf/20211130181607.593149-1-maximmi@nvidia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/bpf/verifier.c                                           |    2 
 tools/testing/selftests/bpf/verifier/xdp_direct_packet_access.c |   32 +++++-----
 2 files changed, 17 insertions(+), 17 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8228,7 +8228,7 @@ static void find_good_pkt_pointers(struc
 
 	new_range = dst_reg->off;
 	if (range_right_open)
-		new_range--;
+		new_range++;
 
 	/* Examples for register markings:
 	 *
--- 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(),
 	},

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