Add bpf_init_inode_xattr() kfunc for BPF LSM programs to atomically set
xattrs via the inode_init_security hook using lsm_get_xattr_slot(). The
hook now passes its xattr state as a single struct lsm_xattrs object,
which the kfunc takes directly.
The kfunc is only usable from lsm/inode_init_security programs: no other
hook exposes a struct lsm_xattrs argument, so the verifier rejects calls
from elsewhere. Restrict the xattr names that may be set via this kfunc
to the bpf.* namespace.
BPF reserves BPF_LSM_INODE_INIT_XATTRS slots via lbs_xattr_count, and the
kfunc enforces that BPF never consumes more slots than it reserved,
returning -ENOSPC once the budget is exhausted.
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]
Signed-off-by: David Windsor <[email protected]>
---
fs/bpf_fs_kfuncs.c | 79 +++++++++++++++++++++++++++++++++++++++++
include/linux/bpf_lsm.h | 3 ++
kernel/bpf/bpf_lsm.c | 1 +
security/bpf/hooks.c | 1 +
4 files changed, 84 insertions(+)
diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index 768aca2dc0f0..c4023c82f21e 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -10,6 +10,7 @@
#include <linux/fsnotify.h>
#include <linux/file.h>
#include <linux/kernfs.h>
+#include <linux/lsm_hooks.h>
#include <linux/mm.h>
#include <linux/xattr.h>
@@ -374,6 +375,83 @@ __bpf_kfunc struct inode *bpf_real_inode(struct dentry
*dentry)
return d_real_inode(dentry);
}
+static int bpf_xattrs_used(const struct lsm_xattrs *ctx)
+{
+ const size_t prefix_len = sizeof(XATTR_BPF_LSM_SUFFIX) - 1;
+ unsigned int i, n = 0;
+
+ for (i = 0; i < ctx->xattr_count; i++) {
+ const char *name = ctx->xattrs[i].name;
+
+ if (name && !strncmp(name, XATTR_BPF_LSM_SUFFIX, prefix_len))
+ n++;
+ }
+ return n;
+}
+
+/**
+ * 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;
+ size_t name_len;
+ void *xattr_value;
+ struct xattr *xattr;
+ const void *value;
+ u32 value_len;
+
+ if (!xattrs || !xattrs->xattrs || !name__str)
+ return -EINVAL;
+ if (bpf_xattrs_used(xattrs) >= BPF_LSM_INODE_INIT_XATTRS)
+ return -ENOSPC;
+
+ name_len = strlen(name__str);
+ if (name_len == 0 || name_len > XATTR_NAME_MAX)
+ 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);
+ if (value_len == 0 || value_len > XATTR_SIZE_MAX)
+ return -EINVAL;
+
+ value = __bpf_dynptr_data(value_ptr, value_len);
+ if (!value)
+ return -EINVAL;
+
+ /* Combine xattr value + name into one allocation. */
+ xattr_value = kmalloc(value_len + name_len + 1, GFP_NOFS);
+ if (!xattr_value)
+ return -ENOMEM;
+
+ memcpy(xattr_value, value, value_len);
+ memcpy(xattr_value + value_len, name__str, name_len);
+ ((char *)xattr_value)[value_len + name_len] = '\0';
+
+ xattr = lsm_get_xattr_slot(xattrs);
+ if (!xattr) {
+ kfree(xattr_value);
+ return -ENOSPC;
+ }
+
+ xattr->value = xattr_value;
+ xattr->name = (const char *)xattr_value + value_len;
+ xattr->value_len = value_len;
+
+ return 0;
+}
+
__bpf_kfunc_end_defs();
BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
@@ -385,6 +463,7 @@ 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, KF_SLEEPABLE)
BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
index 143775a27a2a..b655c708818e 100644
--- a/include/linux/bpf_lsm.h
+++ b/include/linux/bpf_lsm.h
@@ -19,6 +19,9 @@
#include <linux/lsm_hook_defs.h>
#undef LSM_HOOK
+/* max bpf xattrs per inode */
+#define BPF_LSM_INODE_INIT_XATTRS 4
+
struct bpf_storage_blob {
struct bpf_local_storage __rcu *storage;
};
diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
index 564071a92d7d..1c3f84a92420 100644
--- a/kernel/bpf/bpf_lsm.c
+++ b/kernel/bpf/bpf_lsm.c
@@ -315,6 +315,7 @@ BTF_ID(func, bpf_lsm_inode_create)
BTF_ID(func, bpf_lsm_inode_free_security)
BTF_ID(func, bpf_lsm_inode_getattr)
BTF_ID(func, bpf_lsm_inode_getxattr)
+BTF_ID(func, bpf_lsm_inode_init_security)
BTF_ID(func, bpf_lsm_inode_mknod)
BTF_ID(func, bpf_lsm_inode_need_killpriv)
BTF_ID(func, bpf_lsm_inode_post_setxattr)
diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
index 40efde233f3a..d7c44c5c0e30 100644
--- a/security/bpf/hooks.c
+++ b/security/bpf/hooks.c
@@ -30,6 +30,7 @@ static int __init bpf_lsm_init(void)
struct lsm_blob_sizes bpf_lsm_blob_sizes __ro_after_init = {
.lbs_inode = sizeof(struct bpf_storage_blob),
+ .lbs_xattr_count = BPF_LSM_INODE_INIT_XATTRS,
};
DEFINE_LSM(bpf) = {
--
2.53.0
