[PATCH v2 bpf-next 8/9] bpf: add trusted d_path() based BPF kfunc bpf_path_d_path()

[Matt Bobrowski <mattbobrowski@google.com>]

The legacy bpf_d_path() helper did not 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 the trusted pointer argument constraint
KF_TRUSTED_ARGS. Making use of the KF_TRUSTED_ARGS constraint will
ensure that d_path() may only be called when and underlying BPF
program holds a stable handle for a given path.

For now, we restrict bpf_path_d_path() to BPF LSM program types, but
this may be relaxed in the future.

Notably, we are consciously not retroactively enforcing the
KF_TRUSTED_ARGS constraint onto the legacy bpf_d_path() helper, as
that would lead to wide-scale BPF program breakage.

[0]
https://lore.kernel.org/bpf/CAG48ez0ppjcT=QxU-jtCUfb5xQb3mLr=5FcwddF_VKfEBPs_Dg@mail.gmail.com/

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
---
 kernel/trace/bpf_trace.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 84fd87ead20c..4989639153cd 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1647,6 +1647,37 @@ __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 and should be
+ * used in place of bpf_d_path() whenever possible.
+ *
+ * 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)
@@ -1663,6 +1694,7 @@ 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)
-- 
2.44.0.278.ge034bb2e1d-goog

/M