On Sun, 2024-11-17 at 21:44 +0300, Fedor Pchelkin wrote:

The symlink body (->target) should be freed at the same time as the inode itself per commit 4fdcfab5b553 ("jffs2: fix use-after-free on symlink traversal"). It is a filesystem-specific field but there exist several error paths during generic inode allocation when ->free_inode(), namely jffs2_free_inode(), is called with still uninitialized private info.

The calltrace looks like: alloc_inode inode_init_always // fails i_callback free_inode jffs2_free_inode // touches uninit ->target field

Commit af9a8730ddb6 ("jffs2: Fix potential illegal address access in jffs2_free_inode") approached the observed problem but fixed it only partially. Our local Syzkaller instance is still hitting these kinds of failures.

The thing is that jffs2_i_init_once(), where the initialization of f->target has been moved, is called once per slab allocation so it won't be called for the object structure possibly retrieved later from the slab cache for reuse.

The practice followed by many other filesystems is to initialize filesystem-private inode contents in the corresponding ->alloc_inode() callbacks. This also allows to drop initialization from jffs2_iget() and jffs2_new_inode() as ->alloc_inode() is called in those places.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 4fdcfab5b553 ("jffs2: fix use-after-free on symlink traversal") Cc: stable@vger.kernel.org Signed-off-by: Fedor Pchelkin pchelkin@ispras.ru

---

fs/jffs2/fs.c | 2 -- fs/jffs2/super.c | 3 ++- 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/jffs2/fs.c b/fs/jffs2/fs.c index d175cccb7c55..85c4b273918f 100644 --- a/fs/jffs2/fs.c +++ b/fs/jffs2/fs.c @@ -271,7 +271,6 @@ struct inode *jffs2_iget(struct super_block *sb, unsigned long ino) f = JFFS2_INODE_INFO(inode); c = JFFS2_SB_INFO(inode->i_sb);

• jffs2_init_inode_info(f); mutex_lock(&f->sem);

ret = jffs2_do_read_inode(c, f, inode->i_ino, &latest_node); @@ -439,7 +438,6 @@ struct inode *jffs2_new_inode (struct inode *dir_i, umode_t mode, struct jffs2_r return ERR_PTR(-ENOMEM); f = JFFS2_INODE_INFO(inode);

• jffs2_init_inode_info(f); mutex_lock(&f->sem);

memset(ri, 0, sizeof(*ri)); diff --git a/fs/jffs2/super.c b/fs/jffs2/super.c index 4545f885c41e..b56ff63357f3 100644 --- a/fs/jffs2/super.c +++ b/fs/jffs2/super.c @@ -42,6 +42,8 @@ static struct inode *jffs2_alloc_inode(struct super_block *sb) f = alloc_inode_sb(sb, jffs2_inode_cachep, GFP_KERNEL); if (!f) return NULL;

• jffs2_init_inode_info(f); return &f->vfs_inode;

} @@ -58,7 +60,6 @@ static void jffs2_i_init_once(void *foo) struct jffs2_inode_info *f = foo; mutex_init(&f->sem);

• f->target = NULL; inode_init_once(&f->vfs_inode);

}

Has this one been lost?