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

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On 2024/5/3 18:23, Jan Kara wrote:

On Fri 03-05-24 17:51:07, Baokun Li wrote:
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?

As you guessed, when -ENOMEM is returned in ext4_sb_bread(),
the reference count of ce is not properly released, as follows.

         ce = mb_cache_entry_find_first
         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
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.

     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.)
Great! Thanks for debugging this! Some comments to your fix below:

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;
So if we get the ENOMEM error, continuing the iteration seems to be
pointless as we'll likely get it for the following entries as well. I think
the original behavior of aborting the iteration in case of ENOMEM is
actually better. We just have to do mb_cache_entry_put(ea_block_cache, ce)
before returning...

Returning NULL here would normally attempt to allocate a new
xattr_block in ext4_xattr_block_set(), and when ext4_sb_bread() fails,
allocating the new block and inserting it would most likely fail as well,
so my initial thought was to propagate the error from ext4_sb_bread()
to also make ext4_xattr_block_set() fail when ext4_sb_bread() fails.

But I noticed that before Ted added the special handling for -ENOMEM,
EXT4_ERROR_INODE was called to set the ERROR_FS flag no matter
what error ext4_sb_bread() returned, and after we can distinguish
between -EIO and -ENOMEM, we don't have to set the ERROR_FS flag
in the case of -ENOMEM. So there's this conservative fix now.

In short, in my personal opinion, for -EIO and -ENOMEM, they should
be the same except whether or not the ERROR_FS flag is set.
Otherwise, I think adding mb_cache_entry_put() directly is the easiest
and most straightforward fix.  Honza, do you have any other thoughts?


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