[PATCH v5 bpf-next 2/3] bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling

Add bpf_init_inode_xattr() kfunc for BPF LSM programs to atomically set
xattrs via the inode_init_security hook using security_lsmxattr_add().
The hook now passes its xattr state as a single struct lsm_xattrs
object, which the kfunc takes directly.

This kfunc is only callable from inode_init_security; the verifier
rejects attempts to call it elsewhere.

A previous attempt [1] required a kmalloc string output protocol for
the xattr name. Since commit 6bcdfd2cac55 ("security: Allow all LSMs to
provide xattrs for inode_init_security hook") [2], the xattr name is no
longer allocated; it is a static constant.

Link: https://kernsec.org/pipermail/linux-security-module-archive/2022-October/034878.html [1]
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6bcdfd2cac55 [2]
Suggested-by: Song Liu <[email protected]>
Signed-off-by: David Windsor <[email protected]>
---
 fs/bpf_fs_kfuncs.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index 768aca2dc0f0..ad0025f2264a 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -11,7 +11,9 @@
 #include <linux/file.h>
 #include <linux/kernfs.h>
 #include <linux/mm.h>
+#include <linux/security.h>
 #include <linux/xattr.h>
+#include <uapi/linux/lsm.h>
 
 __bpf_kfunc_start_defs();
 
@@ -374,6 +376,39 @@ __bpf_kfunc struct inode *bpf_real_inode(struct dentry *dentry)
 	return d_real_inode(dentry);
 }
 
+/**
+ * bpf_init_inode_xattr - set an xattr on a new inode from inode_init_security
+ * @xattrs: inode_init_security xattr state from the hook context
+ * @name__str: xattr name (e.g., "bpf.file_label")
+ * @value_p: dynptr containing the xattr value
+ *
+ * Only callable from lsm/inode_init_security programs.
+ *
+ * Return: 0 on success, negative error on failure.
+ */
+__bpf_kfunc int bpf_init_inode_xattr(struct lsm_xattrs *xattrs,
+				     const char *name__str,
+				     const struct bpf_dynptr *value_p)
+{
+	struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p;
+	const void *value;
+	u32 value_len;
+
+	if (!name__str)
+		return -EINVAL;
+	if (strncmp(name__str, XATTR_BPF_LSM_SUFFIX,
+		    sizeof(XATTR_BPF_LSM_SUFFIX) - 1))
+		return -EPERM;
+
+	value_len = __bpf_dynptr_size(value_ptr);
+	value = __bpf_dynptr_data(value_ptr, value_len);
+	if (!value)
+		return -EINVAL;
+
+	return security_lsmxattr_add(xattrs, LSM_ID_BPF, name__str, value,
+				     value_len);
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
@@ -385,6 +420,7 @@ BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_set_dentry_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_remove_dentry_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_real_inode, KF_SLEEPABLE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_init_inode_xattr)
 BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
 
 static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
-- 
2.53.0