bpf: Disallow negative offset in check_ptr_off_reg

check_ptr_off_reg only allows fixed offset to be set for PTR_TO_BTF_ID,
where reg->off < 0 doesn't make sense. This would shift the pointer
backwards, and fails later in btf_struct_ids_match or btf_struct_walk
due to out of bounds access (since offset is interpreted as unsigned).

Improve the verifier by rejecting this case by using a better error
message for BPF helpers and kfunc, by putting a check inside the
check_func_arg_reg_off function.

Also, update existing verifier selftests to work with new error string.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220304224645.3677453-4-memxor@gmail.com

kernel/bpf/verifier.c

@@ -3990,6 +3990,12 @@ static int __check_ptr_off_reg(struct bpf_verifier_env *env,
 	 * is only allowed in its original, unmodified form.
 	 */
 
+	if (reg->off < 0) {
+		verbose(env, "negative offset %s ptr R%d off=%d disallowed\n",
+			reg_type_str(env, reg->type), regno, reg->off);
+		return -EACCES;
+	}
+
 	if (!fixed_off_ok && reg->off) {
 		verbose(env, "dereference of modified %s ptr R%d off=%d disallowed\n",
 			reg_type_str(env, reg->type), regno, reg->off);

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

@@ -105,7 +105,7 @@
 		BPF_EXIT_INSN(),
 	},
 	.errstr_unpriv = "R1 has pointer with unsupported alu operation",
-	.errstr = "dereference of modified ctx ptr",
+	.errstr = "negative offset ctx ptr R1 off=-1 disallowed",
 	.result = REJECT,
 	.flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,
 },

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

@@ -58,7 +58,7 @@
 	},
 	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 	.result = REJECT,
-	.errstr = "dereference of modified ctx ptr",
+	.errstr = "negative offset ctx ptr R1 off=-612 disallowed",
 },
 {
 	"pass modified ctx pointer to helper, 2",
@@ -71,8 +71,8 @@
 	},
 	.result_unpriv = REJECT,
 	.result = REJECT,
-	.errstr_unpriv = "dereference of modified ctx ptr",
-	.errstr = "dereference of modified ctx ptr",
+	.errstr_unpriv = "negative offset ctx ptr R1 off=-612 disallowed",
+	.errstr = "negative offset ctx ptr R1 off=-612 disallowed",
 },
 {
 	"pass modified ctx pointer to helper, 3",
@@ -141,7 +141,7 @@
 	.prog_type = BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
 	.expected_attach_type = BPF_CGROUP_UDP6_SENDMSG,
 	.result = REJECT,
-	.errstr = "dereference of modified ctx ptr",
+	.errstr = "negative offset ctx ptr R1 off=-612 disallowed",
 },
 {
 	"pass ctx or null check, 5: null (connect)",