bpf: Do not mark certain LSM hook arguments as trusted

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@fb.com>
Link: https://lore.kernel.org/r/20221203204954.2043348-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 16 insertions, 0 deletions

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 = {
 };