[PATCH v5 bpf-next 4/9] bpf: Add kfunc bpf_get_file_xattr

Song Liu
2023-10-24T06:31:28Z

It is common practice for security solutions to store tags/labels in
xattrs. To implement similar functionalities in BPF LSM, add new kfunc
bpf_get_file_xattr().

To avoid recursion, bpf_get_file_xattr can be only called from LSM hooks.

Signed-off-by: Song Liu <song@kernel.org>
---
kernel/trace/bpf_trace.c | 59 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 59 insertions(+)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 2626706b6387..802181986ad3 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -24,6 +24,7 @@
#include <linux/key.h>
#include <linux/verification.h>
#include <linux/namei.h>
+#include <linux/fileattr.h>

#include <net/bpf_sk_storage.h>

@@ -1436,6 +1437,64 @@ static int __init bpf_key_sig_kfuncs_init(void)
late_initcall(bpf_key_sig_kfuncs_init);
#endif /* CONFIG_KEYS */

+/* filesystem kfuncs */
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+ "kfuncs which will be used in BPF programs");
+
+/**
+ * bpf_get_file_xattr - get xattr of a file
+ * @file: file to get xattr from
+ * @name__const_str: name of the xattr
+ * @value_ptr: output buffer of the xattr value
+ *
+ * Get xattr *name__const_str* of *file* and store the output in *value_ptr*.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__const_str,
+ struct bpf_dynptr_kern *value_ptr)
+{
+ struct dentry *dentry;
+ void *value;
+
+ value = bpf_dynptr_slice_rdwr(value_ptr, 0, NULL, 0);
+ if (IS_ERR_OR_NULL(value))
+ return PTR_ERR(value);
+
+ dentry = file_dentry(file);
+ return __vfs_getxattr(dentry, dentry->d_inode, name__const_str,
+ value, __bpf_dynptr_size(value_ptr));
+}
+
+__diag_pop();
+
+BTF_SET8_START(fs_kfunc_set_ids)
+BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE)
+BTF_SET8_END(fs_kfunc_set_ids)
+
+static int bpf_get_file_xattr_filter(const struct bpf_prog *prog, u32 kfunc_id)
+{
+ if (!btf_id_set8_contains(&fs_kfunc_set_ids, kfunc_id))
+ return 0;
+
+ /* Only allow to attach from LSM hooks, to avoid recursion */
+ return prog->type != BPF_PROG_TYPE_LSM ? -EACCES : 0;
+}
+
+const struct btf_kfunc_id_set bpf_fs_kfunc_set = {
+ .owner = THIS_MODULE,
+ .set = &fs_kfunc_set_ids,
+ .filter = bpf_get_file_xattr_filter,
+};
+
+static int __init bpf_fs_kfuncs_init(void)
+{
+ return register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fs_kfunc_set);
+}
+
+late_initcall(bpf_fs_kfuncs_init);
+
static const struct bpf_func_proto *
bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
{
--
2.34.1