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@xxxxxxxxx>
---
kernel/bpf/verifier.c | 6 ++++++
tools/testing/selftests/bpf/verifier/bounds_deduction.c | 2 +-
tools/testing/selftests/bpf/verifier/ctx.c | 8 ++++----
3 files changed, 11 insertions(+), 5 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e37eb6020253..455b4ab69e47 100644
--- a/kernel/bpf/verifier.c
+++ b/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);
diff --git a/tools/testing/selftests/bpf/verifier/bounds_deduction.c b/tools/testing/selftests/bpf/verifier/bounds_deduction.c
index 91869aea6d64..3931c481e30c 100644
--- a/tools/testing/selftests/bpf/verifier/bounds_deduction.c
+++ b/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,
},
diff --git a/tools/testing/selftests/bpf/verifier/ctx.c b/tools/testing/selftests/bpf/verifier/ctx.c
index 60f6fbe03f19..c8eaf0536c24 100644
--- a/tools/testing/selftests/bpf/verifier/ctx.c
+++ b/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)",
--
2.35.1
