[PATCH v5 bpf-next 0/3] bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling

[PATCH v5 bpf-next 0/3] bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling

[David Windsor]

Many in-kernel LSMs (SELinux, Smack, IMA) store security labels in
extended attributes. For these LSMs, atomic labeling during inode
creation is critical: if the inode becomes accessible before its xattr
is set, it is briefly unlabeled, which can disrupt LSMs making policy
decisions based on file labels.

Existing LSMs solve this by setting xattrs directly in the
inode_init_security hook, which runs before the inode becomes
accessible. BPF LSM programs currently lack this capability because
the hook uses an output parameter (xattr_count) that BPF programs
cannot write to, and existing kfuncs like bpf_set_dentry_xattr
require a dentry that isn't available until after the inode is
accessible.

This series introduces the bpf_init_inode_xattr() kfunc, which takes
the combined inode_init_security xattr context argument and claims a
slot in it via the new security_lsmxattr_add() LSM helper.

v5:
  - make the kfunc non-sleepable to avoid an ext4 deadlock (sashiko)
  - move xattr creation logic to security_lsmxattr_add() (Paul)
  - use distinct xattr names in init_inode_xattr_slot_limit

v4:
  - introduce struct lsm_xattrs in separate patch (Alexei, Paul)
  - rename struct xattr_ctx to struct lsm_xattrs (Paul)
  - make lsm_xattrs.xattr_count unsigned int (Paul)
  - drop new_xattrs/xattr_count locals in
    security_inode_init_security() (Paul)
  - fold __bpf_init_inode_xattr() into bpf_init_inode_xattr() (Paul)
  - drop bpf_fs_kfuncs_filter() attach-point check; rely on verifier
    type enforcement (Alexei)
  - drop attach-time cap; enforce slot budget in the kfunc (Alexei)
  - allocate the combined xattr with GFP_NOFS (sashiko-bot)
  - replace init_inode_xattr_attach_cap selftest with runtime
    init_inode_xattr_slot_limit

v3:
  - rename struct lsm_xattr_ctx to struct xattr_ctx (Paul)
  - increase BPF_LSM_INODE_INIT_XATTRS to 4 (Song)
  - enforce per-hook attachment cap at attach time to prevent
    runtime rejection (Paul)
  - add init_inode_xattr_attach_cap selftest

v2:
  - pass the xattr state as a combined context object and drop the
    verifier fixup path (Kumar)
  - restrict bpf_init_inode_xattr labels to bpf.* namespace (Matt)
  - cap bpf_init_inode_xattr() at BPF_LSM_INODE_INIT_XATTRS slots per
    invocation (AI)

David Windsor (3):
  security: rework inode_init_security xattr handling
  bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling
  selftests/bpf: add tests for bpf_init_inode_xattr kfunc

 fs/bpf_fs_kfuncs.c                            |  36 +++++
 include/linux/bpf_lsm.h                       |   3 +
 include/linux/evm.h                           |   9 +-
 include/linux/lsm_hook_defs.h                 |   4 +-
 include/linux/lsm_hooks.h                     |  16 +--
 include/linux/security.h                      |  15 +++
 security/bpf/hooks.c                          |   1 +
 security/integrity/evm/evm_main.c             |   8 +-
 security/security.c                           | 108 +++++++++++++--
 security/selinux/hooks.c                      |   4 +-
 security/smack/smack_lsm.c                    |  27 ++--
 tools/testing/selftests/bpf/bpf_kfuncs.h      |   5 +
 .../selftests/bpf/prog_tests/fs_kfuncs.c      | 124 ++++++++++++++++++
 .../bpf/progs/test_init_inode_xattr.c         |  31 +++++
 14 files changed, 344 insertions(+), 47 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_init_inode_xattr.c


base-commit: 602701718649936eb287bf6c7ecf870ec54c6f71
-- 
2.53.0

[PATCH v5 bpf-next 1/3] security: rework inode_init_security xattr handling

[David Windsor]

In preparation for bpf_init_inode_xattr(), a kfunc that lets bpf LSM
programs atomically label new inodes, rework how inode_init_security
xattrs are managed.

inode_init_security receives the LSM xattr array and its count as
separate parameters. For better compatibility with the bpf verifier,
update inode_init_security and its callers to consolidate these
parameters into a single context object: struct lsm_xattrs.

Also, add security_lsmxattr_add(), which claims a slot in the
inode_init_security xattr array on behalf of the calling LSM and
fills it with a copy of the given name and value.

Suggested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 include/linux/bpf_lsm.h           |   3 +
 include/linux/evm.h               |   9 +--
 include/linux/lsm_hook_defs.h     |   4 +-
 include/linux/lsm_hooks.h         |  16 ++---
 include/linux/security.h          |  15 +++++
 security/bpf/hooks.c              |   1 +
 security/integrity/evm/evm_main.c |   8 ++-
 security/security.c               | 108 ++++++++++++++++++++++++++----
 security/selinux/hooks.c          |   4 +-
 security/smack/smack_lsm.c        |  27 ++++----
 10 files changed, 148 insertions(+), 47 deletions(-)

diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
index dda272d78f01..374c50801a8c 100644
--- a/include/linux/bpf_lsm.h
+++ b/include/linux/bpf_lsm.h
@@ -21,6 +21,9 @@ extern bool bpf_lsm_initialized __ro_after_init;
 #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/include/linux/evm.h b/include/linux/evm.h
index 913f4573b203..528f360f3308 100644
--- a/include/linux/evm.h
+++ b/include/linux/evm.h
@@ -12,6 +12,8 @@
 #include <linux/integrity.h>
 #include <linux/xattr.h>
 
+struct lsm_xattrs;
+
 #ifdef CONFIG_EVM
 extern int evm_set_key(void *key, size_t keylen);
 extern enum integrity_status evm_verifyxattr(struct dentry *dentry,
@@ -21,8 +23,8 @@ extern enum integrity_status evm_verifyxattr(struct dentry *dentry,
 int evm_fix_hmac(struct dentry *dentry, const char *xattr_name,
 		 const char *xattr_value, size_t xattr_value_len);
 int evm_inode_init_security(struct inode *inode, struct inode *dir,
-			    const struct qstr *qstr, struct xattr *xattrs,
-			    int *xattr_count);
+			    const struct qstr *qstr,
+			    struct lsm_xattrs *xattrs);
 extern bool evm_revalidate_status(const char *xattr_name);
 extern int evm_protected_xattr_if_enabled(const char *req_xattr_name);
 extern int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
@@ -63,8 +65,7 @@ static inline int evm_fix_hmac(struct dentry *dentry, const char *xattr_name,
 
 static inline int evm_inode_init_security(struct inode *inode, struct inode *dir,
 					  const struct qstr *qstr,
-					  struct xattr *xattrs,
-					  int *xattr_count)
+					  struct lsm_xattrs *xattrs)
 {
 	return 0;
 }
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 65c9609ec207..5b2de7865ce8 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -116,8 +116,8 @@ LSM_HOOK(int, 0, inode_alloc_security, struct inode *inode)
 LSM_HOOK(void, LSM_RET_VOID, inode_free_security, struct inode *inode)
 LSM_HOOK(void, LSM_RET_VOID, inode_free_security_rcu, void *inode_security)
 LSM_HOOK(int, -EOPNOTSUPP, inode_init_security, struct inode *inode,
-	 struct inode *dir, const struct qstr *qstr, struct xattr *xattrs,
-	 int *xattr_count)
+	 struct inode *dir, const struct qstr *qstr,
+	 struct lsm_xattrs *xattrs)
 LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
 	 const struct qstr *name, const struct inode *context_inode)
 LSM_HOOK(int, 0, inode_create, struct inode *dir, struct dentry *dentry,
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index b4f8cad53ddb..7afe06a8d4c6 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -200,20 +200,18 @@ extern struct lsm_static_calls_table static_calls_table __ro_after_init;
 
 /**
  * lsm_get_xattr_slot - Return the next available slot and increment the index
- * @xattrs: array storing LSM-provided xattrs
- * @xattr_count: number of already stored xattrs (updated)
+ * @ctx: xattr state shared by inode_init_security hooks
  *
- * Retrieve the first available slot in the @xattrs array to fill with an xattr,
- * and increment @xattr_count.
+ * Retrieve the first available slot in the @ctx->xattrs array to fill with an
+ * xattr, and increment @ctx->xattr_count.
  *
- * Return: The slot to fill in @xattrs if non-NULL, NULL otherwise.
+ * Return: The slot to fill in @ctx->xattrs if non-NULL, NULL otherwise.
  */
-static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
-					       int *xattr_count)
+static inline struct xattr *lsm_get_xattr_slot(struct lsm_xattrs *ctx)
 {
-	if (unlikely(!xattrs))
+	if (unlikely(!ctx || !ctx->xattrs))
 		return NULL;
-	return &xattrs[(*xattr_count)++];
+	return &ctx->xattrs[ctx->xattr_count++];
 }
 
 #endif /* ! __LINUX_LSM_HOOKS_H */
diff --git a/include/linux/security.h b/include/linux/security.h
index 153e9043058f..647f7b88358b 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -68,6 +68,11 @@ struct watch;
 struct watch_notification;
 struct lsm_ctx;
 
+struct lsm_xattrs {
+	struct xattr *xattrs;
+	unsigned int xattr_count;
+};
+
 /* Default (no) options for the capable function */
 #define CAP_OPT_NONE 0x0
 /* If capable should audit the security request */
@@ -401,6 +406,9 @@ void security_inode_free(struct inode *inode);
 int security_inode_init_security(struct inode *inode, struct inode *dir,
 				 const struct qstr *qstr,
 				 initxattrs initxattrs, void *fs_data);
+int security_lsmxattr_add(struct lsm_xattrs *xattrs, u64 lsm_id,
+			  const char *name, const void *value,
+			  size_t value_len);
 int security_inode_init_security_anon(struct inode *inode,
 				      const struct qstr *name,
 				      const struct inode *context_inode);
@@ -895,6 +903,13 @@ static inline int security_inode_init_security(struct inode *inode,
 	return 0;
 }
 
+static inline int security_lsmxattr_add(struct lsm_xattrs *xattrs, u64 lsm_id,
+					const char *name, const void *value,
+					size_t value_len)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int security_inode_init_security_anon(struct inode *inode,
 						    const struct qstr *name,
 						    const struct inode *context_inode)
diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
index 7b98f5d1e2be..8f8c3de3035f 100644
--- a/security/bpf/hooks.c
+++ b/security/bpf/hooks.c
@@ -33,6 +33,7 @@ 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) = {
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index b59e3f121b8a..b7158fc63543 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -1062,14 +1062,16 @@ static int evm_inode_copy_up_xattr(struct dentry *src, const char *name)
  * evm_inode_init_security - initializes security.evm HMAC value
  */
 int evm_inode_init_security(struct inode *inode, struct inode *dir,
-			    const struct qstr *qstr, struct xattr *xattrs,
-			    int *xattr_count)
+			    const struct qstr *qstr,
+			    struct lsm_xattrs *lsm_xattrs)
 {
 	struct evm_xattr *xattr_data;
 	struct xattr *xattr, *evm_xattr;
+	struct xattr *xattrs;
 	bool evm_protected_xattrs = false;
 	int rc;
 
+	xattrs = lsm_xattrs ? lsm_xattrs->xattrs : NULL;
 	if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs)
 		return 0;
 
@@ -1087,7 +1089,7 @@ int evm_inode_init_security(struct inode *inode, struct inode *dir,
 	if (!evm_protected_xattrs)
 		return 0;
 
-	evm_xattr = lsm_get_xattr_slot(xattrs, xattr_count);
+	evm_xattr = lsm_get_xattr_slot(lsm_xattrs);
 	/*
 	 * Array terminator (xattr name = NULL) must be the first non-filled
 	 * xattr slot.
diff --git a/security/security.c b/security/security.c
index 71aea8fdf014..261f68e17cfd 100644
--- a/security/security.c
+++ b/security/security.c
@@ -12,6 +12,7 @@
 #define pr_fmt(fmt) "LSM: " fmt
 
 #include <linux/bpf.h>
+#include <linux/bpf_lsm.h>
 #include <linux/capability.h>
 #include <linux/dcache.h>
 #include <linux/export.h>
@@ -1333,8 +1334,8 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
 				 const initxattrs initxattrs, void *fs_data)
 {
 	struct lsm_static_call *scall;
-	struct xattr *new_xattrs = NULL;
-	int ret = -EOPNOTSUPP, xattr_count = 0;
+	struct lsm_xattrs xattrs = {};
+	int ret = -EOPNOTSUPP;
 
 	if (unlikely(IS_PRIVATE(inode)))
 		return 0;
@@ -1344,15 +1345,15 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
 
 	if (initxattrs) {
 		/* Allocate +1 as terminator. */
-		new_xattrs = kcalloc(blob_sizes.lbs_xattr_count + 1,
-				     sizeof(*new_xattrs), GFP_NOFS);
-		if (!new_xattrs)
+		xattrs.xattrs = kcalloc(blob_sizes.lbs_xattr_count + 1,
+					sizeof(*xattrs.xattrs), GFP_NOFS);
+		if (!xattrs.xattrs)
 			return -ENOMEM;
 	}
 
 	lsm_for_each_hook(scall, inode_init_security) {
-		ret = scall->hl->hook.inode_init_security(inode, dir, qstr, new_xattrs,
-						  &xattr_count);
+		ret = scall->hl->hook.inode_init_security(inode, dir, qstr,
+							  &xattrs);
 		if (ret && ret != -EOPNOTSUPP)
 			goto out;
 		/*
@@ -1364,18 +1365,101 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
 	}
 
 	/* If initxattrs() is NULL, xattr_count is zero, skip the call. */
-	if (!xattr_count)
+	if (!xattrs.xattr_count)
 		goto out;
 
-	ret = initxattrs(inode, new_xattrs, fs_data);
+	ret = initxattrs(inode, xattrs.xattrs, fs_data);
 out:
-	for (; xattr_count > 0; xattr_count--)
-		kfree(new_xattrs[xattr_count - 1].value);
-	kfree(new_xattrs);
+	for (; xattrs.xattr_count > 0; xattrs.xattr_count--)
+		kfree(xattrs.xattrs[xattrs.xattr_count - 1].value);
+	kfree(xattrs.xattrs);
 	return (ret == -EOPNOTSUPP) ? 0 : ret;
 }
 EXPORT_SYMBOL(security_inode_init_security);
 
+#ifdef CONFIG_BPF_LSM
+static unsigned int lsm_xattrs_used(const struct lsm_xattrs *xattrs,
+				    const char *prefix)
+{
+	size_t prefix_len = strlen(prefix);
+	unsigned int i, n = 0;
+
+	for (i = 0; i < xattrs->xattr_count; i++) {
+		const char *name = xattrs->xattrs[i].name;
+
+		if (name && !strncmp(name, prefix, prefix_len))
+			n++;
+	}
+	return n;
+}
+#endif /* CONFIG_BPF_LSM */
+
+/**
+ * security_lsmxattr_add() - Add an xattr during inode_init_security
+ * @xattrs: xattr state shared by inode_init_security hooks
+ * @lsm_id: LSM_ID_* value identifying the calling LSM
+ * @name: xattr name suffix
+ * @value: xattr value
+ * @value_len: length of @value
+ *
+ * Claim an xattr slot in @xattrs on behalf of the LSM identified by
+ * @lsm_id and fill it with a copy of @name and @value. Callers can invoke
+ * this function from non-sleepable context.
+ *
+ * Return: Returns 0 on success, -ENOSPC if the calling LSM's slot budget
+ *         is exhausted, negative values on other errors.
+ */
+int security_lsmxattr_add(struct lsm_xattrs *xattrs, u64 lsm_id,
+			  const char *name, const void *value,
+			  size_t value_len)
+{
+	struct xattr *xattr;
+	void *xattr_value;
+	size_t name_len;
+
+	if (!xattrs || !xattrs->xattrs || !name || !value)
+		return -EINVAL;
+
+	name_len = strlen(name);
+	if (name_len == 0 || name_len > XATTR_NAME_MAX)
+		return -EINVAL;
+	if (value_len == 0 || value_len > XATTR_SIZE_MAX)
+		return -EINVAL;
+
+	switch (lsm_id) {
+#ifdef CONFIG_BPF_LSM
+	case LSM_ID_BPF:
+		if (lsm_xattrs_used(xattrs, XATTR_BPF_LSM_SUFFIX) >=
+		    BPF_LSM_INODE_INIT_XATTRS)
+			return -ENOSPC;
+		break;
+#endif /* CONFIG_BPF_LSM */
+	default:
+		return -EINVAL;
+	}
+
+	/* Combine xattr value + name into one allocation. */
+	xattr_value = kmalloc(value_len + name_len + 1, GFP_NOWAIT);
+	if (!xattr_value)
+		return -ENOMEM;
+
+	memcpy(xattr_value, value, value_len);
+	memcpy(xattr_value + value_len, name, 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;
+}
+
 /**
  * security_inode_init_security_anon() - Initialize an anonymous inode
  * @inode: the inode
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 1a713d96206f..6bba6b212e17 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2962,7 +2962,7 @@ static int selinux_dentry_create_files_as(struct dentry *dentry, int mode,
 
 static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
 				       const struct qstr *qstr,
-				       struct xattr *xattrs, int *xattr_count)
+				       struct lsm_xattrs *xattrs)
 {
 	const struct cred_security_struct *crsec = selinux_cred(current_cred());
 	struct superblock_security_struct *sbsec;
@@ -2992,7 +2992,7 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
 	    !(sbsec->flags & SBLABEL_MNT))
 		return -EOPNOTSUPP;
 
-	xattr = lsm_get_xattr_slot(xattrs, xattr_count);
+	xattr = lsm_get_xattr_slot(xattrs);
 	if (xattr) {
 		rc = security_sid_to_context_force(newsid,
 						   &context, &clen);
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index ff115068c5c0..4501078430ca 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -981,10 +981,10 @@ smk_rule_transmutes(struct smack_known *subject,
 }
 
 static int
-xattr_dupval(struct xattr *xattrs, int *xattr_count,
+xattr_dupval(struct lsm_xattrs *xattrs,
 	     const char *name, const void *value, unsigned int vallen)
 {
-	struct xattr * const xattr = lsm_get_xattr_slot(xattrs, xattr_count);
+	struct xattr * const xattr = lsm_get_xattr_slot(xattrs);
 
 	if (!xattr)
 		return 0;
@@ -1003,14 +1003,13 @@ xattr_dupval(struct xattr *xattrs, int *xattr_count,
  * @inode: the newly created inode
  * @dir: containing directory object
  * @qstr: unused
- * @xattrs: where to put the attributes
- * @xattr_count: current number of LSM-provided xattrs (updated)
+ * @xattrs: where to put attributes and update count
  *
  * Returns 0 if it all works out, -ENOMEM if there's no memory
  */
 static int smack_inode_init_security(struct inode *inode, struct inode *dir,
 				     const struct qstr *qstr,
-				     struct xattr *xattrs, int *xattr_count)
+				     struct lsm_xattrs *xattrs)
 {
 	struct task_smack *tsp = smack_cred(current_cred());
 	struct inode_smack * const issp = smack_inode(inode);
@@ -1057,21 +1056,19 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
 		if (S_ISDIR(inode->i_mode)) {
 			transflag = SMK_INODE_TRANSMUTE;
 
-			if (xattr_dupval(xattrs, xattr_count,
-				XATTR_SMACK_TRANSMUTE,
-				TRANS_TRUE,
-				TRANS_TRUE_SIZE
-			))
+			if (xattr_dupval(xattrs,
+					 XATTR_SMACK_TRANSMUTE,
+					 TRANS_TRUE,
+					 TRANS_TRUE_SIZE))
 				rc = -ENOMEM;
 		}
 	}
 
 	if (rc == 0)
-		if (xattr_dupval(xattrs, xattr_count,
-			    XATTR_SMACK_SUFFIX,
-			    issp->smk_inode->smk_known,
-		     strlen(issp->smk_inode->smk_known)
-		))
+		if (xattr_dupval(xattrs,
+				 XATTR_SMACK_SUFFIX,
+				 issp->smk_inode->smk_known,
+				 strlen(issp->smk_inode->smk_known)))
 			rc = -ENOMEM;
 instant_inode:
 	issp->smk_flags |= (SMK_INODE_INSTANT | transflag);
-- 
2.53.0

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

[David Windsor]

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 <song@kernel.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 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

[PATCH v5 bpf-next 3/3] selftests/bpf: add tests for bpf_init_inode_xattr kfunc

[David Windsor]

Test bpf atomic inode xattr labeling in inode_init_security.

Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 tools/testing/selftests/bpf/bpf_kfuncs.h      |   5 +
 .../selftests/bpf/prog_tests/fs_kfuncs.c      | 124 ++++++++++++++++++
 .../bpf/progs/test_init_inode_xattr.c         |  31 +++++
 3 files changed, 160 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/test_init_inode_xattr.c

diff --git a/tools/testing/selftests/bpf/bpf_kfuncs.h b/tools/testing/selftests/bpf/bpf_kfuncs.h
index ae71e9b69051..2639f9f94195 100644
--- a/tools/testing/selftests/bpf/bpf_kfuncs.h
+++ b/tools/testing/selftests/bpf/bpf_kfuncs.h
@@ -92,4 +92,9 @@ extern int bpf_set_dentry_xattr(struct dentry *dentry, const char *name__str,
 				const struct bpf_dynptr *value_p, int flags) __ksym __weak;
 extern int bpf_remove_dentry_xattr(struct dentry *dentry, const char *name__str) __ksym __weak;
 
+struct lsm_xattrs;
+extern int bpf_init_inode_xattr(struct lsm_xattrs *xattrs,
+				const char *name__str,
+				const struct bpf_dynptr *value_p) __ksym __weak;
+
 #endif
diff --git a/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c b/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c
index 43a26ec69a8e..10be0ab91e70 100644
--- a/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c
+++ b/tools/testing/selftests/bpf/prog_tests/fs_kfuncs.c
@@ -10,6 +10,7 @@
 #include "test_get_xattr.skel.h"
 #include "test_set_remove_xattr.skel.h"
 #include "test_fsverity.skel.h"
+#include "test_init_inode_xattr.skel.h"
 
 static const char testfile[] = "/tmp/test_progs_fs_kfuncs";
 
@@ -268,6 +269,123 @@ static void test_fsverity(void)
 	remove(testfile);
 }
 
+static void test_init_inode_xattr(void)
+{
+	struct test_init_inode_xattr *skel = NULL;
+	int fd = -1, err;
+	char value_out[64];
+	const char *testfile_new = "/tmp/test_progs_fs_kfuncs_new";
+
+	skel = test_init_inode_xattr__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_init_inode_xattr__open_and_load"))
+		return;
+
+	skel->bss->monitored_pid = getpid();
+	err = test_init_inode_xattr__attach(skel);
+	if (!ASSERT_OK(err, "test_init_inode_xattr__attach"))
+		goto out;
+
+	/* Trigger inode_init_security */
+	fd = open(testfile_new, O_CREAT | O_RDWR, 0644);
+	if (!ASSERT_GE(fd, 0, "create_file"))
+		goto out;
+
+	ASSERT_EQ(skel->data->init_result, 0, "init_result");
+
+	/* initxattrs prepends "security." to the name. */
+	err = getxattr(testfile_new, "security.bpf.test_label", value_out,
+		       sizeof(value_out));
+	if (err < 0 && errno == ENODATA) {
+		printf("%s:SKIP:filesystem did not apply LSM xattrs\n",
+		       __func__);
+		test__skip();
+		goto out;
+	}
+	if (!ASSERT_GE(err, 0, "getxattr"))
+		goto out;
+
+	ASSERT_EQ(err, (int)sizeof(skel->data->xattr_value), "xattr_size");
+	ASSERT_EQ(strncmp(value_out, "unconfined_u:object_r:user_home_t:s0",
+			  sizeof("unconfined_u:object_r:user_home_t:s0")), 0,
+		  "xattr_value");
+
+out:
+	close(fd);
+	test_init_inode_xattr__destroy(skel);
+	remove(testfile_new);
+}
+
+/* Keep in sync with BPF_LSM_INODE_INIT_XATTRS in include/linux/bpf_lsm.h. */
+#define INIT_INODE_XATTR_MAX 4
+
+/*
+ * Programs may attach to inode_init_security without an attach-time limit, but
+ * the kfunc only lets BPF claim INIT_INODE_XATTR_MAX xattr slots per inode.
+ * Calls beyond that budget are rejected at runtime with -ENOSPC.
+ */
+static void test_init_inode_xattr_slot_limit(void)
+{
+	struct test_init_inode_xattr *skel[INIT_INODE_XATTR_MAX + 1] = {};
+	struct bpf_link *link[INIT_INODE_XATTR_MAX + 1] = {};
+	const char *testfile_slot = "/tmp/test_progs_fs_kfuncs_slot";
+	int ok = 0, nospc = 0, other = 0;
+	int i, fd = -1;
+
+	/* All programs attach successfully; there is no attach-time cap. */
+	for (i = 0; i <= INIT_INODE_XATTR_MAX; i++) {
+		skel[i] = test_init_inode_xattr__open();
+		if (!ASSERT_OK_PTR(skel[i], "open"))
+			goto out;
+
+		snprintf(skel[i]->rodata->xattr_name,
+			 sizeof(skel[i]->rodata->xattr_name), "bpf.label_%d",
+			 i);
+
+		if (!ASSERT_OK(test_init_inode_xattr__load(skel[i]), "load"))
+			goto out;
+
+		skel[i]->bss->monitored_pid = getpid();
+
+		link[i] = bpf_program__attach_lsm(skel[i]->progs.test_init_inode_xattr);
+		if (!ASSERT_OK_PTR(link[i], "attach"))
+			goto out;
+	}
+
+	/* Trigger inode_init_security once with all programs attached. */
+	fd = open(testfile_slot, O_CREAT | O_RDWR, 0644);
+	if (!ASSERT_GE(fd, 0, "create_file"))
+		goto out;
+
+	/*
+	 * Exactly INIT_INODE_XATTR_MAX programs claim a slot; the program past
+	 * the budget gets -ENOSPC. The order in which programs run is not
+	 * guaranteed, so count results instead of indexing.
+	 */
+	for (i = 0; i <= INIT_INODE_XATTR_MAX; i++) {
+		int res = skel[i]->data->init_result;
+
+		if (res == 0)
+			ok++;
+		else if (res == -ENOSPC)
+			nospc++;
+		else
+			other++;
+	}
+
+	ASSERT_EQ(ok, INIT_INODE_XATTR_MAX, "slots_within_budget");
+	ASSERT_EQ(nospc, 1, "slot_over_budget");
+	ASSERT_EQ(other, 0, "unexpected_result");
+
+out:
+	if (fd >= 0)
+		close(fd);
+	for (i = 0; i <= INIT_INODE_XATTR_MAX; i++) {
+		bpf_link__destroy(link[i]);
+		test_init_inode_xattr__destroy(skel[i]);
+	}
+	remove(testfile_slot);
+}
+
 void test_fs_kfuncs(void)
 {
 	/* Matches xattr_names in progs/test_get_xattr.c */
@@ -288,4 +406,10 @@ void test_fs_kfuncs(void)
 
 	if (test__start_subtest("fsverity"))
 		test_fsverity();
+
+	if (test__start_subtest("init_inode_xattr"))
+		test_init_inode_xattr();
+
+	if (test__start_subtest("init_inode_xattr_slot_limit"))
+		test_init_inode_xattr_slot_limit();
 }
diff --git a/tools/testing/selftests/bpf/progs/test_init_inode_xattr.c b/tools/testing/selftests/bpf/progs/test_init_inode_xattr.c
new file mode 100644
index 000000000000..caade53b87e3
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_init_inode_xattr.c
@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Cisco Systems, Inc. */
+
+#include "vmlinux.h"
+#include <bpf/bpf_tracing.h>
+#include "bpf_kfuncs.h"
+
+char _license[] SEC("license") = "GPL";
+
+__u32 monitored_pid;
+int init_result = -1;
+
+const char xattr_name[16] = "bpf.test_label";
+char xattr_value[] = "unconfined_u:object_r:user_home_t:s0";
+
+SEC("lsm/inode_init_security")
+int BPF_PROG(test_init_inode_xattr, struct inode *inode, struct inode *dir,
+	     const struct qstr *qstr, struct lsm_xattrs *xattrs)
+{
+	struct bpf_dynptr value_ptr;
+	__u32 pid;
+
+	pid = bpf_get_current_pid_tgid() >> 32;
+	if (pid != monitored_pid)
+		return 0;
+
+	bpf_dynptr_from_mem(xattr_value, sizeof(xattr_value), 0, &value_ptr);
+	init_result = bpf_init_inode_xattr(xattrs, xattr_name, &value_ptr);
+
+	return 0;
+}
-- 
2.53.0

Re: [PATCH v5 bpf-next 0/3] bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling

[Kumar Kartikeya Dwivedi]

On Wed Jul 8, 2026 at 2:09 AM CEST, David Windsor wrote:
> Many in-kernel LSMs (SELinux, Smack, IMA) store security labels in
> extended attributes. For these LSMs, atomic labeling during inode
> creation is critical: if the inode becomes accessible before its xattr
> is set, it is briefly unlabeled, which can disrupt LSMs making policy
> decisions based on file labels.
>
> Existing LSMs solve this by setting xattrs directly in the
> inode_init_security hook, which runs before the inode becomes
> accessible. BPF LSM programs currently lack this capability because
> the hook uses an output parameter (xattr_count) that BPF programs
> cannot write to, and existing kfuncs like bpf_set_dentry_xattr
> require a dentry that isn't available until after the inode is
> accessible.
>
> This series introduces the bpf_init_inode_xattr() kfunc, which takes
> the combined inode_init_security xattr context argument and claims a
> slot in it via the new security_lsmxattr_add() LSM helper.
>

There are various CI failures in newly added tests, I don't think any of them
are passing. Please fix before respinning.

https://github.com/kernel-patches/bpf/pull/12730

E.g. both test_init_inode_xattr and test_init_inode_xattr_slot do not have
expected results.

pw-bot: cr

> [...]

Re: [PATCH v5 bpf-next 0/3] bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling

[David Windsor]

On Tue, Jul 7, 2026 at 11:12 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> On Wed Jul 8, 2026 at 2:09 AM CEST, David Windsor wrote:
> > Many in-kernel LSMs (SELinux, Smack, IMA) store security labels in
> > extended attributes. For these LSMs, atomic labeling during inode
> > creation is critical: if the inode becomes accessible before its xattr
> > is set, it is briefly unlabeled, which can disrupt LSMs making policy
> > decisions based on file labels.
> >
> > Existing LSMs solve this by setting xattrs directly in the
> > inode_init_security hook, which runs before the inode becomes
> > accessible. BPF LSM programs currently lack this capability because
> > the hook uses an output parameter (xattr_count) that BPF programs
> > cannot write to, and existing kfuncs like bpf_set_dentry_xattr
> > require a dentry that isn't available until after the inode is
> > accessible.
> >
> > This series introduces the bpf_init_inode_xattr() kfunc, which takes
> > the combined inode_init_security xattr context argument and claims a
> > slot in it via the new security_lsmxattr_add() LSM helper.
> >
>
> There are various CI failures in newly added tests, I don't think any of them
> are passing. Please fix before respinning.
>
> https://github.com/kernel-patches/bpf/pull/12730
>
> E.g. both test_init_inode_xattr and test_init_inode_xattr_slot do not have
> expected results.
>

This is happening because the path "/tmp" in bpf ci isn't mounted on
tmpfs, rather on 9p:

[ 5.813072] VFS: Mounted root (9p filesystem) on device 0:22.
[ 5.924390] vmtest: Mounting tmpfs at /dev/shm
[ 5.971012] vmtest: Mounting tmpfs at /run
[ 6.096271] vmtest: Mounting tmpfs at /mnt

... which doesn't call inode_init_security. Changing the test to
create its file in /dev/shm, which has a real tmpfs, fixes this. We
can update the test to skip on filesystems where the hook doesn't
fire.

I'll wait to gather feedback from Alexei and Paul before sending v6.

> pw-bot: cr
>
> > [...]