selftests/bpf: Add test for signed 32-bit bound check bug

After a 32-bit load followed by a branch, the verifier would reduce the
maximum bound of the register to 0x7fffffff, allowing a user to bypass
bound checks. Ensure such a program is rejected.

In the second test, the 64-bit compare should not sufficient to
determine whether the signed 32-bit lower bound is 0, so the verifier
should reject the second branch.
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/verifier/bounds.c

@@ -703,3 +703,44 @@
 	.fixup_map_hash_8b = { 3 },
 	.result = ACCEPT,
 },
+{
+	"bounds checks after 32-bit truncation. test 1",
+	.insns = {
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 4),
+	BPF_LDX_MEM(BPF_W, BPF_REG_1, BPF_REG_0, 0),
+	/* This used to reduce the max bound to 0x7fffffff */
+	BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0, 1),
+	BPF_JMP_IMM(BPF_JGT, BPF_REG_1, 0x7fffffff, 1),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 3 },
+	.errstr_unpriv = "R0 leaks addr",
+	.result_unpriv = REJECT,
+	.result = ACCEPT,
+},
+{
+	"bounds checks after 32-bit truncation. test 2",
+	.insns = {
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 4),
+	BPF_LDX_MEM(BPF_W, BPF_REG_1, BPF_REG_0, 0),
+	BPF_JMP_IMM(BPF_JSLT, BPF_REG_1, 1, 1),
+	BPF_JMP32_IMM(BPF_JSLT, BPF_REG_1, 0, 1),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 3 },
+	.errstr_unpriv = "R0 leaks addr",
+	.result_unpriv = REJECT,
+	.result = ACCEPT,
+},