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SubjectRe: KASAN: use-after-free Read in path_lookupat
On Mon, Mar 25, 2019 at 11:36 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> >
> > You mean, split ->destroy_inode() into immediate and RCU-delayed parts?
> > There are filesystems where both parts are non-empty - we can't just
> > switch all ->destroy_inode() work to call_rcu().
>
> Right. Not just move the existing destroy_inode() - because as you
> say, people may not be able to to do that in RCU contect, but split it
> up, and add a "final_free_inode()" callback or something for the RCU
> phase.

Something like the attached.

COMPLETELY UNTESTED. And no filesystems converted to actually use the
new rcu_destroy_inode() thing.

Hmm?

Linus
Documentation/filesystems/vfs.txt | 6 ++++++
fs/inode.c | 27 +++++++++++++++++++++------
include/linux/fs.h | 1 +
3 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 761c6fd24a53..60f7841a12e6 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -210,6 +210,7 @@ filesystem. As of kernel 2.6.22, the following members are defined:
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *);
+ int (*rcu_destroy_inode)(struct inode *);

void (*dirty_inode) (struct inode *, int flags);
int (*write_inode) (struct inode *, int);
@@ -248,6 +249,11 @@ or bottom half).
->alloc_inode was defined and simply undoes anything done by
->alloc_inode.

+ rcu_destroy_inode: this method is called after the RCU delay by
+ destroy_inode() to release resources allocated for struct inode.
+ If it returns a non-zero value, it means that it has free'd the
+ inode, otherwise the inode layer will free it.
+
dirty_inode: this method is called by the VFS to mark an inode dirty.

write_inode: this method is called when the VFS needs to write an
diff --git a/fs/inode.c b/fs/inode.c
index e9d97add2b36..b85457baad20 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -215,9 +215,13 @@ static struct inode *alloc_inode(struct super_block *sb)
return NULL;

if (unlikely(inode_init_always(sb, inode))) {
- if (inode->i_sb->s_op->destroy_inode)
+ if (inode->i_sb->s_op->destroy_inode) {
inode->i_sb->s_op->destroy_inode(inode);
- else
+ if (!inode->i_sb->s_op->rcu_destroy_inode)
+ return NULL;
+ }
+ if (!inode->i_sb->s_op->rcu_destroy_inode ||
+ !inode->i_sb->s_op->rcu_destroy_inode(inode))
kmem_cache_free(inode_cachep, inode);
return NULL;
}
@@ -256,17 +260,28 @@ EXPORT_SYMBOL(__destroy_inode);
static void i_callback(struct rcu_head *head)
{
struct inode *inode = container_of(head, struct inode, i_rcu);
- kmem_cache_free(inode_cachep, inode);
+
+ if (!inode->i_sb->s_op->rcu_destroy_inode ||
+ !inode->i_sb->s_op->rcu_destroy_inode(inode))
+ kmem_cache_free(inode_cachep, inode);
}

static void destroy_inode(struct inode *inode)
{
BUG_ON(!list_empty(&inode->i_lru));
__destroy_inode(inode);
- if (inode->i_sb->s_op->destroy_inode)
+
+ /*
+ * If we have a 'destroy_inode' but no 'rcu_destroy_inode'
+ * then the filesystem handles the RCU-delayed destruction
+ * on its own, and we don't do any RCU callbacks.
+ */
+ if (inode->i_sb->s_op->destroy_inode) {
inode->i_sb->s_op->destroy_inode(inode);
- else
- call_rcu(&inode->i_rcu, i_callback);
+ if (!inode->i_sb->s_op->rcu_destroy_inode)
+ return;
+ }
+ call_rcu(&inode->i_rcu, i_callback);
}

/**
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8b42df09b04c..727561ecbc23 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1900,6 +1900,7 @@ extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *);
+ int (*rcu_destroy_inode)(struct inode *);

void (*dirty_inode) (struct inode *, int flags);
int (*write_inode) (struct inode *, struct writeback_control *wbc);
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