The legacy bpf_d_path() helper didn't operate on trusted pointer arguments, therefore under certain circumstances was susceptible to memory corruption issues [0]. This new d_path() based BPF kfunc bpf_path_d_path() makes use of trusted pointer arguments and therefore is subject to the BPF verifier constraints associated with KF_TRUSTED_ARGS semantics. Making use of such trusted pointer semantics will allow d_path() to be called safely from the contexts of a BPF program. For now, we restrict bpf_path_d_path() to BPF LSM program types, but this may be relaxed in the future. [0] https://lore.kernel.org/bpf/CAG48ez0ppjcT=QxU-jtCUfb5xQb3mLr=5FcwddF_VKfEBPs_Dg@xxxxxxxxxxxxxx/ Signed-off-by: Matt Bobrowski <mattbobrowski@xxxxxxxxxx> --- kernel/trace/bpf_trace.c | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+) diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index 2bb7766337ca..57a7b4aae8d5 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -1635,6 +1635,36 @@ __bpf_kfunc void bpf_put_path(struct path *path) path_put(path); } +/** + * bpf_path_d_path - resolve the pathname for a given path + * @path: path to resolve the pathname for + * @buf: buffer to return the resolved path value in + * @buflen: length of the supplied buffer + * + * Resolve the pathname for the supplied trusted *path* in *buf*. This kfunc is + * the trusted/safer variant of the legacy bpf_d_path() helper. + * + * Return: A strictly positive integer corresponding to the length of the string + * representing the resolved pathname, including the NUL termination + * character. On error, a negative integer is returned. + */ +__bpf_kfunc int bpf_path_d_path(struct path *path, char *buf, int buflen) +{ + int len; + char *ret; + + if (buflen <= 0) + return -EINVAL; + + ret = d_path(path, buf, buflen); + if (IS_ERR(ret)) + return PTR_ERR(ret); + + len = buf + buflen - ret; + memmove(buf, ret, len); + return len; +} + __bpf_kfunc_end_defs(); BTF_KFUNCS_START(lsm_kfunc_set_ids) @@ -1651,6 +1681,7 @@ BTF_ID_FLAGS(func, bpf_get_task_fs_root, BTF_ID_FLAGS(func, bpf_get_task_fs_pwd, KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL) BTF_ID_FLAGS(func, bpf_put_path, KF_RELEASE | KF_SLEEPABLE) +BTF_ID_FLAGS(func, bpf_path_d_path, KF_TRUSTED_ARGS | KF_SLEEPABLE) BTF_KFUNCS_END(lsm_kfunc_set_ids) static int bpf_lsm_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id) -- 2.44.0.rc0.258.g7320e95886-goog /M
