Martin mentioned that the verifier cannot assume arguments from
LSM hook sk_alloc_security being trusted since after the hook
is called, the sk ref_count is set to 1. This will overwrite
the ref_count changed by the bpf program and may cause ref_count
underflow later on.
I then further checked some other hooks. For example,
for bpf_lsm_file_alloc() hook in fs/file_table.c,
f->f_cred = get_cred(cred);
error = security_file_alloc(f);
if (unlikely(error)) {
file_free_rcu(&f->f_rcuhead);
return ERR_PTR(error);
}
atomic_long_set(&f->f_count, 1);
The input parameter 'f' to security_file_alloc() cannot be trusted
as well.
Specifically, I investiaged bpf_map/bpf_prog/file/sk/task alloc/free
lsm hooks. Except bpf_map_alloc and task_alloc, arguments for all other
hooks should not be considered as trusted. This may not be a complete
list, but it covers common usage for sk and task.
Fixes: 3f00c5239344 ("bpf: Allow trusted pointers to be passed to KF_TRUSTED_ARGS kfuncs")
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
include/linux/bpf_lsm.h | 6 ++++++
kernel/bpf/bpf_lsm.c | 16 ++++++++++++++++
kernel/bpf/btf.c | 2 ++
.../selftests/bpf/prog_tests/task_kfunc.c | 1 +
.../selftests/bpf/progs/task_kfunc_failure.c | 11 +++++++++++
5 files changed, 36 insertions(+)
diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
index 4bcf76a9bb06..1de7ece5d36d 100644
--- a/include/linux/bpf_lsm.h
+++ b/include/linux/bpf_lsm.h
@@ -28,6 +28,7 @@ int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
const struct bpf_prog *prog);
bool bpf_lsm_is_sleepable_hook(u32 btf_id);
+bool bpf_lsm_is_trusted(const struct bpf_prog *prog);
static inline struct bpf_storage_blob *bpf_inode(
const struct inode *inode)
@@ -51,6 +52,11 @@ static inline bool bpf_lsm_is_sleepable_hook(u32 btf_id)
return false;
}
+static inline bool bpf_lsm_is_trusted(const struct bpf_prog *prog)
+{
+ return false;
+}
+
static inline int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
const struct bpf_prog *prog)
{
diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
index ae0267f150b5..9ea42a45da47 100644
--- a/kernel/bpf/bpf_lsm.c
+++ b/kernel/bpf/bpf_lsm.c
@@ -345,11 +345,27 @@ BTF_ID(func, bpf_lsm_task_to_inode)
BTF_ID(func, bpf_lsm_userns_create)
BTF_SET_END(sleepable_lsm_hooks)
+BTF_SET_START(untrusted_lsm_hooks)
+BTF_ID(func, bpf_lsm_bpf_map_free_security)
+BTF_ID(func, bpf_lsm_bpf_prog_alloc_security)
+BTF_ID(func, bpf_lsm_bpf_prog_free_security)
+BTF_ID(func, bpf_lsm_file_alloc_security)
+BTF_ID(func, bpf_lsm_file_free_security)
+BTF_ID(func, bpf_lsm_sk_alloc_security)
+BTF_ID(func, bpf_lsm_sk_free_security)
+BTF_ID(func, bpf_lsm_task_free)
+BTF_SET_END(untrusted_lsm_hooks)
+
bool bpf_lsm_is_sleepable_hook(u32 btf_id)
{
return btf_id_set_contains(&sleepable_lsm_hooks, btf_id);
}
+bool bpf_lsm_is_trusted(const struct bpf_prog *prog)
+{
+ return !btf_id_set_contains(&untrusted_lsm_hooks, prog->aux->attach_btf_id);
+}
+
const struct bpf_prog_ops lsm_prog_ops = {
};
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index d11cbf8cece7..c80bd8709e69 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -19,6 +19,7 @@
#include <linux/bpf_verifier.h>
#include <linux/btf.h>
#include <linux/btf_ids.h>
+#include <linux/bpf_lsm.h>
#include <linux/skmsg.h>
#include <linux/perf_event.h>
#include <linux/bsearch.h>
@@ -5829,6 +5830,7 @@ static bool prog_args_trusted(const struct bpf_prog *prog)
case BPF_PROG_TYPE_TRACING:
return atype == BPF_TRACE_RAW_TP || atype == BPF_TRACE_ITER;
case BPF_PROG_TYPE_LSM:
+ return bpf_lsm_is_trusted(prog);
case BPF_PROG_TYPE_STRUCT_OPS:
return true;
default:
diff --git a/tools/testing/selftests/bpf/prog_tests/task_kfunc.c b/tools/testing/selftests/bpf/prog_tests/task_kfunc.c
index ffd8ef4303c8..18848c31e36f 100644
--- a/tools/testing/selftests/bpf/prog_tests/task_kfunc.c
+++ b/tools/testing/selftests/bpf/prog_tests/task_kfunc.c
@@ -103,6 +103,7 @@ static struct {
{"task_kfunc_release_null", "arg#0 is ptr_or_null_ expected ptr_ or socket"},
{"task_kfunc_release_unacquired", "release kernel function bpf_task_release expects"},
{"task_kfunc_from_pid_no_null_check", "arg#0 is ptr_or_null_ expected ptr_ or socket"},
+ {"task_kfunc_from_lsm_task_free", "reg type unsupported for arg#0 function"},
};
static void verify_fail(const char *prog_name, const char *expected_err_msg)
diff --git a/tools/testing/selftests/bpf/progs/task_kfunc_failure.c b/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
index e310473190d5..87fa1db9d9b5 100644
--- a/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
+++ b/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
@@ -271,3 +271,14 @@ int BPF_PROG(task_kfunc_from_pid_no_null_check, struct task_struct *task, u64 cl
return 0;
}
+
+SEC("lsm/task_free")
+int BPF_PROG(task_kfunc_from_lsm_task_free, struct task_struct *task)
+{
+ struct task_struct *acquired;
+
+ /* the argument of lsm task_free hook is untrusted. */
+ acquired = bpf_task_acquire(task);
+ bpf_task_release(acquired);
+ return 0;
+}
--
2.30.2
