From c0c852dd1876dc1db4600ce951a92aadd3073b1c Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Sat, 3 Dec 2022 12:49:54 -0800 Subject: [PATCH] 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 Link: https://lore.kernel.org/r/20221203204954.2043348-1-yhs@fb.com Signed-off-by: Alexei Starovoitov --- include/linux/bpf_lsm.h | 6 ++++++ kernel/bpf/bpf_lsm.c | 16 ++++++++++++++++ kernel/bpf/btf.c | 2 ++ tools/testing/selftests/bpf/prog_tests/task_kfunc.c | 1 + tools/testing/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 4bcf76a..1de7ece 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 ae0267f..9ea42a4 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 d11cbf8..c80bd87 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -19,6 +19,7 @@ #include #include #include +#include #include #include #include @@ -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 ffd8ef4..18848c3 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 e310473..87fa1db 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; +} -- 2.7.4