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().
The first use case of bpf_get_file_xattr() is to implement file
verifications with asymmetric keys. Specificially, security applications
could use fsverity for file hashes and use xattr to store file signatures.
(kfunc for fsverity hash will be added in a separate commit.)
Currently, only xattrs with "user." prefix can be read with kfunc
bpf_get_file_xattr(). As use cases evolve, we may add a dedicated prefix
for bpf_get_file_xattr().
To avoid recursion, bpf_get_file_xattr can be only called from LSM hooks.
Signed-off-by: Song Liu <song@xxxxxxxxxx>
Acked-by: Christian Brauner <brauner@xxxxxxxxxx>
Acked-by: KP Singh <kpsingh@xxxxxxxxxx>
---
kernel/trace/bpf_trace.c | 67 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 67 insertions(+)
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index c284a4ad0315..1648bde28f01 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>
@@ -1431,6 +1432,72 @@ static int __init bpf_key_sig_kfuncs_init(void)
late_initcall(bpf_key_sig_kfuncs_init);
#endif /* CONFIG_KEYS */
+/* filesystem kfuncs */
+__bpf_kfunc_start_defs();
+
+/**
+ * bpf_get_file_xattr - get xattr of a file
+ * @file: file to get xattr from
+ * @name__str: name of the xattr
+ * @value_ptr: output buffer of the xattr value
+ *
+ * Get xattr *name__str* of *file* and store the output in *value_ptr*.
+ *
+ * For security reasons, only *name__str* with prefix "user." is allowed.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
+ struct bpf_dynptr_kern *value_ptr)
+{
+ struct dentry *dentry;
+ u32 value_len;
+ void *value;
+ int ret;
+
+ if (strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN))
+ return -EPERM;
+
+ value_len = __bpf_dynptr_size(value_ptr);
+ value = __bpf_dynptr_data_rw(value_ptr, value_len);
+ if (!value)
+ return -EINVAL;
+
+ dentry = file_dentry(file);
+ ret = inode_permission(&nop_mnt_idmap, dentry->d_inode, MAY_READ);
+ if (ret)
+ return ret;
+ return __vfs_getxattr(dentry, dentry->d_inode, name__str, value, value_len);
+}
+
+__bpf_kfunc_end_defs();
+
+BTF_SET8_START(fs_kfunc_set_ids)
+BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
+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;
+}
+
+static 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
