Re: [PATCH] mm: list_lru: Fix NULL pointer dereference in list_lru_add()

[Youling Tang]

Hi, Kent

On 12/07/2024 23:49, Kent Overstreet wrote:
> On Fri, Jul 12, 2024 at 12:28:57PM GMT, Youling Tang wrote:
> > Hi, Kent
> >
> > On 12/07/2024 12:07, Kent Overstreet wrote:
> > > On Fri, Jul 12, 2024 at 11:25:54AM GMT, Youling Tang wrote:
> > > > From: Youling Tang <tangyouling@xxxxxxxxxx>
> > > >
> > > > Note that list_lru_from_memcg_idx() may return NULL, so it is necessary
> > > > to error handle the return value to avoid triggering NULL pointer
> > > > dereference BUG.
> > > >
> > > > The issue was triggered for discussion [1],
> > > > Link [1]: https://lore.kernel.org/linux-bcachefs/84de6cb1-57bd-42f7-8029-4203820ef0b4@xxxxxxxxx/T/#m901bb26cdb1d9d4bacebf0d034f0a5a712cc93a6
> > > I see no explanation for why this is the correct fix, and I doubt it is.
> > > What's the real reason for the NULL lru_list_one, and why doesn't this
> > > come up on other filesystems?
> > We can break it down into two questions (independent of each other):
> > 1) Error handling is necessary when l (lru_list_one) is NULL here.
> No, you're just hiding the actual bug - since I wasn't clear, I'm naking
> this patch.
We should use kmem_cache_alloc_lru() instead of kmem_cache_alloc(),
similar to the [1] modification.

Apply the following patch to fix the problem:

diff --git a/fs/bcachefs/fs.c b/fs/bcachefs/fs.c
index f9c9a95d7d4c..79a580dfb5e1 100644
--- a/fs/bcachefs/fs.c
+++ b/fs/bcachefs/fs.c
@@ -227,7 +227,8 @@ static struct inode *bch2_alloc_inode(struct 
super_block *sb)

 static struct bch_inode_info *__bch2_new_inode(struct bch_fs *c)
 {
-       struct bch_inode_info *inode = 
kmem_cache_alloc(bch2_inode_cache, GFP_NOFS);
+       struct bch_inode_info *inode = alloc_inode_sb(c->vfs_sb, 
bch2_inode_cache, GFP_NOFS);
        if (!inode)
                return NULL;

Link [1]: 
https://lwn.net/ml/linux-kernel/20220228122126.37293-5-songmuchun@xxxxxxxxxxxxx/

Thanks,
Youling.