[RFC] lsm: fs: Use i_callback to free i_security in RCU callback
From: Song Liu
Date: Mon Dec 16 2024 - 18:44:04 EST
inode->i_security needes to be freed from RCU callback. A rcu_head was
added to i_security to call the RCU callback. However, since struct inode
already has i_rcu, the extra rcu_head is wasteful. Specifically, when any
LSM uses i_security, a rcu_head (two pointers) is allocated for each
inode.
Add security_inode_free_rcu() to i_callback to free i_security so that
a rcu_head is saved for each inode. Special care are needed for file
systems that provide a destroy_inode() callback, but not a free_inode()
callback. Specifically, the following logic are added to handle such
cases:
- XFS recycles inode after destroy_inode. The inodes are freed from
recycle logic. Let xfs_inode_free_callback() and xfs_inode_alloc()
call security_inode_free_rcu() before freeing the inode.
- Let pipe free inode from a RCU callback.
- Let btrfs-test free inode from a RCU callback.
Signed-off-by: Song Liu <song@xxxxxxxxxx>
---
Documentation/filesystems/vfs.rst | 8 ++++-
fs/btrfs/fs.h | 1 +
fs/btrfs/inode.c | 4 +++
fs/btrfs/tests/btrfs-tests.c | 1 +
fs/inode.c | 2 ++
fs/pipe.c | 1 -
fs/xfs/xfs_icache.c | 3 ++
include/linux/security.h | 4 +++
security/security.c | 49 +++++++++++++++++++------------
9 files changed, 53 insertions(+), 20 deletions(-)
diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 0b18af3f954e..af62cdd0bb7a 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -306,7 +306,13 @@ or bottom half).
``free_inode``
this method is called from RCU callback. If you use call_rcu()
in ->destroy_inode to free 'struct inode' memory, then it's
- better to release memory in this method.
+ better to release memory in this method. LSMs leverage the
+ vfs RCU callback (i_callback) to free inode->i_security after a
+ RCU grace period. If the filesystem provides a destroy_inode()
+ callback but not a free_inode() callback, it is responsible to
+ call security_inode_free_rcu() in the filesystem's RCU callback.
+ For example, xfs calls security_inode_free_rcu() from
+ xfs_inode_free_callback().
``dirty_inode``
this method is called by the VFS when an inode is marked dirty.
diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
index 79a1a3d6f04d..f606ea2a14ab 100644
--- a/fs/btrfs/fs.h
+++ b/fs/btrfs/fs.h
@@ -1068,6 +1068,7 @@ static inline int btrfs_is_testing(const struct btrfs_fs_info *fs_info)
}
void btrfs_test_destroy_inode(struct inode *inode);
+void btrfs_test_free_inode(struct inode *inode);
#else
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 03fe0de2cd0d..62ee8394cf6c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7707,6 +7707,10 @@ void btrfs_test_destroy_inode(struct inode *inode)
{
btrfs_drop_extent_map_range(BTRFS_I(inode), 0, (u64)-1, false);
kfree(BTRFS_I(inode)->file_extent_tree);
+}
+
+void btrfs_test_free_inode(struct inode *inode)
+{
kmem_cache_free(btrfs_inode_cachep, BTRFS_I(inode));
}
#endif
diff --git a/fs/btrfs/tests/btrfs-tests.c b/fs/btrfs/tests/btrfs-tests.c
index e607b5d52fb1..581b518b99b7 100644
--- a/fs/btrfs/tests/btrfs-tests.c
+++ b/fs/btrfs/tests/btrfs-tests.c
@@ -35,6 +35,7 @@ const char *test_error[] = {
static const struct super_operations btrfs_test_super_ops = {
.alloc_inode = btrfs_alloc_inode,
.destroy_inode = btrfs_test_destroy_inode,
+ .free_inode = btrfs_test_free_inode,
};
diff --git a/fs/inode.c b/fs/inode.c
index 6b4c77268fc0..4b1eac736730 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -321,6 +321,8 @@ EXPORT_SYMBOL(free_inode_nonrcu);
static void i_callback(struct rcu_head *head)
{
struct inode *inode = container_of(head, struct inode, i_rcu);
+
+ security_inode_free_rcu(inode);
if (inode->free_inode)
inode->free_inode(inode);
else
diff --git a/fs/pipe.c b/fs/pipe.c
index 12b22c2723b7..eb9c75ef5d80 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1422,7 +1422,6 @@ long pipe_fcntl(struct file *file, unsigned int cmd, unsigned int arg)
}
static const struct super_operations pipefs_ops = {
- .destroy_inode = free_inode_nonrcu,
.statfs = simple_statfs,
};
diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index 7b6c026d01a1..a85a661ad78f 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -30,6 +30,7 @@
#include "xfs_metafile.h"
#include <linux/iversion.h>
+#include <linux/security.h>
/* Radix tree tags for incore inode tree. */
@@ -97,6 +98,7 @@ xfs_inode_alloc(
ip = alloc_inode_sb(mp->m_super, xfs_inode_cache, GFP_KERNEL | __GFP_NOFAIL);
if (inode_init_always(mp->m_super, VFS_I(ip))) {
+ security_inode_free_rcu(VFS_I(ip));
kmem_cache_free(xfs_inode_cache, ip);
return NULL;
}
@@ -162,6 +164,7 @@ xfs_inode_free_callback(
ip->i_itemp = NULL;
}
+ security_inode_free_rcu(inode);
kmem_cache_free(xfs_inode_cache, ip);
}
diff --git a/include/linux/security.h b/include/linux/security.h
index 980b6c207cad..4983d6a3ccb3 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -400,6 +400,7 @@ int security_path_notify(const struct path *path, u64 mask,
unsigned int obj_type);
int security_inode_alloc(struct inode *inode, gfp_t gfp);
void security_inode_free(struct inode *inode);
+void security_inode_free_rcu(struct inode *inode);
int security_inode_init_security(struct inode *inode, struct inode *dir,
const struct qstr *qstr,
initxattrs initxattrs, void *fs_data);
@@ -860,6 +861,9 @@ static inline int security_inode_alloc(struct inode *inode, gfp_t gfp)
static inline void security_inode_free(struct inode *inode)
{ }
+static inline void security_inode_free_rcu(struct inode *inode)
+{ }
+
static inline int security_dentry_init_security(struct dentry *dentry,
int mode,
const struct qstr *name,
diff --git a/security/security.c b/security/security.c
index 7523d14f31fb..0ca9a41b7aca 100644
--- a/security/security.c
+++ b/security/security.c
@@ -265,12 +265,6 @@ static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)
lsm_set_blob_size(&needed->lbs_cred, &blob_sizes.lbs_cred);
lsm_set_blob_size(&needed->lbs_file, &blob_sizes.lbs_file);
lsm_set_blob_size(&needed->lbs_ib, &blob_sizes.lbs_ib);
- /*
- * The inode blob gets an rcu_head in addition to
- * what the modules might need.
- */
- if (needed->lbs_inode && blob_sizes.lbs_inode == 0)
- blob_sizes.lbs_inode = sizeof(struct rcu_head);
lsm_set_blob_size(&needed->lbs_inode, &blob_sizes.lbs_inode);
lsm_set_blob_size(&needed->lbs_ipc, &blob_sizes.lbs_ipc);
lsm_set_blob_size(&needed->lbs_key, &blob_sizes.lbs_key);
@@ -1688,23 +1682,26 @@ int security_path_notify(const struct path *path, u64 mask,
*/
int security_inode_alloc(struct inode *inode, gfp_t gfp)
{
- int rc = lsm_inode_alloc(inode, gfp);
+ int rc;
- if (unlikely(rc))
- return rc;
+ /*
+ * If the file system recycles inode, i_security may be already
+ * allocated. In this case, we need to call inode_alloc_security
+ * hooks again, so that the LSMs can reinitialize the inode blob
+ * properly.
+ */
+ if (!inode->i_security) {
+ rc = lsm_inode_alloc(inode, gfp);
+
+ if (unlikely(rc))
+ return rc;
+ }
rc = call_int_hook(inode_alloc_security, inode);
if (unlikely(rc))
security_inode_free(inode);
return rc;
}
-static void inode_free_by_rcu(struct rcu_head *head)
-{
- /* The rcu head is at the start of the inode blob */
- call_void_hook(inode_free_security_rcu, head);
- kmem_cache_free(lsm_inode_cache, head);
-}
-
/**
* security_inode_free() - Free an inode's LSM blob
* @inode: the inode
@@ -1724,9 +1721,25 @@ static void inode_free_by_rcu(struct rcu_head *head)
void security_inode_free(struct inode *inode)
{
call_void_hook(inode_free_security, inode);
- if (!inode->i_security)
+}
+
+/**
+ * security_inode_free_rcu() - Free an inode's LSM blob from i_callback
+ * @inode: the inode
+ *
+ * Release any LSM resources associated with @inode. This is called from
+ * i_callback after a RCU grace period. Therefore, it is safe to free
+ * everything now.
+ */
+void security_inode_free_rcu(struct inode *inode)
+{
+ void *inode_security = inode->i_security;
+
+ if (!inode_security)
return;
- call_rcu((struct rcu_head *)inode->i_security, inode_free_by_rcu);
+ inode->i_security = NULL;
+ call_void_hook(inode_free_security_rcu, inode_security);
+ kmem_cache_free(lsm_inode_cache, inode_security);
}
/**
--
2.43.5