Re: [syzbot] [ext4?] WARNING in mb_cache_destroy

[Baokun Li]

Hi Honza,

On 2024/5/2 18:33, Jan Kara wrote:
> On Tue 30-04-24 08:04:03, syzbot wrote:
> > syzbot has bisected this issue to:
> >
> > commit 67d7d8ad99beccd9fe92d585b87f1760dc9018e3
> > Author: Baokun Li <libaokun1@xxxxxxxxxx>
> > Date:   Thu Jun 16 02:13:56 2022 +0000
> >
> >      ext4: fix use-after-free in ext4_xattr_set_entry
> So I'm not sure the bisect is correct since the change is looking harmless.
Yes, the root cause of the problem has nothing to do with this patch,
and please see the detailed analysis below.
> But it is sufficiently related that there indeed may be some relationship.
> Anyway, the kernel log has:
>
> [   44.932900][ T1063] EXT4-fs warning (device loop0): ext4_evict_inode:297: xattr delete (err -12)
> [   44.943316][ T1063] EXT4-fs (loop0): unmounting filesystem.
> [   44.949531][ T1063] ------------[ cut here ]------------
> [   44.955050][ T1063] WARNING: CPU: 0 PID: 1063 at fs/mbcache.c:409 mb_cache_destroy+0xda/0x110
>
> So ext4_xattr_delete_inode() called when removing inode has failed with
> ENOMEM and later mb_cache_destroy() was eventually complaining about having
> mbcache entry with increased refcount. So likely some error cleanup path is
> forgetting to drop mbcache entry reference somewhere but at this point I
> cannot find where. We'll likely need to play with the reproducer to debug
> that. Baokun, any chance for looking into this?
>
> 								Honza
As you guessed, when -ENOMEM is returned in ext4_sb_bread(),
the reference count of ce is not properly released, as follows.

ext4_create
 __ext4_new_inode
  security_inode_init_security
   ext4_initxattrs
    ext4_xattr_set_handle
     ext4_xattr_block_find
     ext4_xattr_block_set
      ext4_xattr_block_cache_find
        ce = mb_cache_entry_find_first
            __entry_find
            atomic_inc_not_zero(&entry->e_refcnt)
        bh = ext4_sb_bread(inode->i_sb, ce->e_value, REQ_PRIO);
        if (PTR_ERR(bh) == -ENOMEM)
            return NULL;

Before merging into commit 67d7d8ad99be("ext4: fix use-after-free
in ext4_xattr_set_entry"), it will not return early in 
ext4_xattr_ibody_find(),
so it tries to find it in iboy, fails the check in xattr_check_inode() and
returns without executing ext4_xattr_block_find(). Thus it will bisect
the patch, but actually has nothing to do with it.

ext4_xattr_ibody_get
 xattr_check_inode
  __xattr_check_inode
   check_xattrs
    if (end - (void *)header < sizeof(*header) + sizeof(u32))
      "in-inode xattr block too small"

Here's the patch in testing, I'll send it out officially after it is tested.
(PS:  I'm not sure if propagating the ext4_xattr_block_cache_find() 
errors would be better.)

Regards,
Baokun


From: Baokun Li <libaokun1@xxxxxxxxxx>
Date: Fri, 3 May 2024 16:51:43 +0800
Subject: [PATCH] ext4: fix mb_cache_entry's e_refcnt leak in
 ext4_xattr_block_cache_find()

Syzbot reports a warning as follows:

============================================
WARNING: CPU: 0 PID: 5075 at fs/mbcache.c:419 mb_cache_destroy+0x224/0x290
Modules linked in:
CPU: 0 PID: 5075 Comm: syz-executor199 Not tainted 6.9.0-rc6-gb947cc5bf6d7
RIP: 0010:mb_cache_destroy+0x224/0x290 fs/mbcache.c:419
Call Trace:
 <TASK>
 ext4_put_super+0x6d4/0xcd0 fs/ext4/super.c:1375
 generic_shutdown_super+0x136/0x2d0 fs/super.c:641
 kill_block_super+0x44/0x90 fs/super.c:1675
 ext4_kill_sb+0x68/0xa0 fs/ext4/super.c:7327
[...]
============================================

This is because when finding an entry in ext4_xattr_block_cache_find(), if
ext4_sb_bread() returns -ENOMEM, the ce's e_refcnt, which has already grown
in the __entry_find(), won't be put away, and eventually trigger the above
issue in mb_cache_destroy() due to reference count leakage. So correct the
handling of the -ENOMEM error branch to avoid the above issue.

Reported-by: syzbot+dd43bd0f7474512edc47@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=dd43bd0f7474512edc47
Fixes: fb265c9cb49e ("ext4: add ext4_sb_bread() to disambiguate ENOMEM 
cases")
Cc: stable@xxxxxxxxxx # v5.0-rc1
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
 fs/ext4/xattr.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index b67a176bfcf9..5c9e751915fd 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -3113,11 +3113,10 @@ ext4_xattr_block_cache_find(struct inode *inode,

         bh = ext4_sb_bread(inode->i_sb, ce->e_value, REQ_PRIO);
         if (IS_ERR(bh)) {
-            if (PTR_ERR(bh) == -ENOMEM)
-                return NULL;
+            if (PTR_ERR(bh) != -ENOMEM)
+                EXT4_ERROR_INODE(inode, "block %lu read error",
+                         (unsigned long)ce->e_value);
             bh = NULL;
-            EXT4_ERROR_INODE(inode, "block %lu read error",
-                     (unsigned long)ce->e_value);
         } else if (ext4_xattr_cmp(header, BHDR(bh)) == 0) {
             *pce = ce;
             return bh;
--
2.39.2