A panic occurs in ocfs2_unlink due to WARN_ON(inode->i_nlink == 0) when
handling a corrupted inode with i_mode=0 and i_nlink=0 in memory.

This "zombie" inode is created because ocfs2_read_locked_inode proceeds
even after ocfs2_validate_inode_block successfully validates a block
that structurally looks okay (passes checksum, signature etc.) but
contains semantically invalid data (specifically i_mode=0). The current
validation function doesn't check for i_mode being zero.

This results in an in-memory inode with i_mode=0 being added to the VFS
cache, which later triggers the panic during unlink.

Prevent this by adding an explicit check for (i_mode == 0, i_nlink == 0, non-orphan)
within ocfs2_validate_inode_block. If the check is true, return -EFSCORRUPTED to signal
corruption. This causes the caller (ocfs2_read_locked_inode) to invoke
make_bad_inode(), correctly preventing the zombie inode from entering
the cache.

Reported-by: syzbot+55c40ae8a0e5f3659f2b@syzkaller.appspotmail.com
Fixes: https://syzkaller.appspot.com/bug?extid=55c40ae8a0e5f3659f2b
Co-developed-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Ahmet Eray Karadag <eraykrdg1@gmail.com>
Previous link: https://lore.kernel.org/all/20251022222752.46758-2-eraykrdg1@gmail.com/T/
---
v2:
 - Only checking either i_links_count == 0 or i_mode == 0
 - Not performing le16_to_cpu() anymore
 - Tested with ocfs2-test
---
v3:
 - Add checking both high and low bits of i_links_count
---
 fs/ocfs2/inode.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index 14bf440ea4df..c8b129db756e 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -1456,6 +1456,13 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 		goto bail;
 	}
 
+	if (!ocfs2_read_links_count(di) || !di->i_mode) {
+		mlog(ML_ERROR, "Invalid dinode #%llu: "
+			"Corrupt state (nlink=0 or mode=0,) detected!\n",
+		        (unsigned long long)bh->b_blocknr);
+		rc = -EFSCORRUPTED;
+		goto bail;
+	}
 	/*
 	 * Errors after here are fatal.
 	 */
-- 
2.43.0

On Thu, Nov 20, 2025 at 01:17:24AM +0300, Ahmet Eray Karadag wrote:
> A panic occurs in ocfs2_unlink due to WARN_ON(inode->i_nlink == 0) when
> handling a corrupted inode with i_mode=0 and i_nlink=0 in memory.
> 
> This "zombie" inode is created because ocfs2_read_locked_inode proceeds
> even after ocfs2_validate_inode_block successfully validates a block
> that structurally looks okay (passes checksum, signature etc.) but
> contains semantically invalid data (specifically i_mode=0). The current
> validation function doesn't check for i_mode being zero.
> 
> This results in an in-memory inode with i_mode=0 being added to the VFS
> cache, which later triggers the panic during unlink.
> 
> Prevent this by adding an explicit check for (i_mode == 0, i_nlink == 0, non-orphan)
> within ocfs2_validate_inode_block. If the check is true, return -EFSCORRUPTED to signal
> corruption. This causes the caller (ocfs2_read_locked_inode) to invoke
> make_bad_inode(), correctly preventing the zombie inode from entering
> the cache.
> 
> Reported-by: syzbot+55c40ae8a0e5f3659f2b@syzkaller.appspotmail.com
> Fixes: https://syzkaller.appspot.com/bug?extid=55c40ae8a0e5f3659f2b
> Co-developed-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
> Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
> Signed-off-by: Ahmet Eray Karadag <eraykrdg1@gmail.com>
> Previous link: https://lore.kernel.org/all/20251022222752.46758-2-eraykrdg1@gmail.com/T/
> ---
> v2:
>  - Only checking either i_links_count == 0 or i_mode == 0
>  - Not performing le16_to_cpu() anymore
>  - Tested with ocfs2-test
> ---
> v3:
>  - Add checking both high and low bits of i_links_count
> ---
>  fs/ocfs2/inode.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
> index 14bf440ea4df..c8b129db756e 100644
> --- a/fs/ocfs2/inode.c
> +++ b/fs/ocfs2/inode.c
> @@ -1456,6 +1456,13 @@ int ocfs2_validate_inode_block(struct super_block *sb,
>  		goto bail;
>  	}
>  
> +	if (!ocfs2_read_links_count(di) || !di->i_mode) {

the code logic looks good to me, but I prefer the following code to save a few
cpu cycles.

```
/* only check if the "link count" or i_mode is ZERO */
if (!(di->i_links_count | di->i_links_count_hi) || !di->i_mode)
```

@Joseph
which do you like?

- Heming
> +		mlog(ML_ERROR, "Invalid dinode #%llu: "
> +			"Corrupt state (nlink=0 or mode=0,) detected!\n",
> +		        (unsigned long long)bh->b_blocknr);
> +		rc = -EFSCORRUPTED;
> +		goto bail;
> +	}
>  	/*
>  	 * Errors after here are fatal.
>  	 */
> -- 
> 2.43.0
> 
>

On 2025/11/20 10:32, Heming Zhao wrote:
> On Thu, Nov 20, 2025 at 01:17:24AM +0300, Ahmet Eray Karadag wrote:
>> A panic occurs in ocfs2_unlink due to WARN_ON(inode->i_nlink == 0) when
>> handling a corrupted inode with i_mode=0 and i_nlink=0 in memory.
>>
>> This "zombie" inode is created because ocfs2_read_locked_inode proceeds
>> even after ocfs2_validate_inode_block successfully validates a block
>> that structurally looks okay (passes checksum, signature etc.) but
>> contains semantically invalid data (specifically i_mode=0). The current
>> validation function doesn't check for i_mode being zero.
>>
>> This results in an in-memory inode with i_mode=0 being added to the VFS
>> cache, which later triggers the panic during unlink.
>>
>> Prevent this by adding an explicit check for (i_mode == 0, i_nlink == 0, non-orphan)
>> within ocfs2_validate_inode_block. If the check is true, return -EFSCORRUPTED to signal
>> corruption. This causes the caller (ocfs2_read_locked_inode) to invoke
>> make_bad_inode(), correctly preventing the zombie inode from entering
>> the cache.
>>
>> Reported-by: syzbot+55c40ae8a0e5f3659f2b@syzkaller.appspotmail.com
>> Fixes: https://syzkaller.appspot.com/bug?extid=55c40ae8a0e5f3659f2b
>> Co-developed-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
>> Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
>> Signed-off-by: Ahmet Eray Karadag <eraykrdg1@gmail.com>
>> Previous link: https://lore.kernel.org/all/20251022222752.46758-2-eraykrdg1@gmail.com/T/
>> ---
>> v2:
>>  - Only checking either i_links_count == 0 or i_mode == 0
>>  - Not performing le16_to_cpu() anymore
>>  - Tested with ocfs2-test
>> ---
>> v3:
>>  - Add checking both high and low bits of i_links_count
>> ---
>>  fs/ocfs2/inode.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
>> index 14bf440ea4df..c8b129db756e 100644
>> --- a/fs/ocfs2/inode.c
>> +++ b/fs/ocfs2/inode.c
>> @@ -1456,6 +1456,13 @@ int ocfs2_validate_inode_block(struct super_block *sb,
>>  		goto bail;
>>  	}
>>  
>> +	if (!ocfs2_read_links_count(di) || !di->i_mode) {
> 
> the code logic looks good to me, but I prefer the following code to save a few
> cpu cycles.
> 
> ```
> /* only check if the "link count" or i_mode is ZERO */
> if (!(di->i_links_count | di->i_links_count_hi) || !di->i_mode)
> ```
> 
> @Joseph
> which do you like?
> 
Sorry to miss this thread discuss.
The above check looks wried since "(di->i_links_count|di->i_links_count_hi)"
is meaningless, though it may work well...
So I'd prefer the code readability first.

BTW, from the syzbot report, I haven't seen where to show i_mode=0 or
i_nlink=0. Am I missing something?

Thanks,
Joseph

On Tue, Dec 02, 2025 at 09:25:14AM +0800, Joseph Qi wrote:
> 
> 
> On 2025/11/20 10:32, Heming Zhao wrote:
> > On Thu, Nov 20, 2025 at 01:17:24AM +0300, Ahmet Eray Karadag wrote:
> >> A panic occurs in ocfs2_unlink due to WARN_ON(inode->i_nlink == 0) when
> >> handling a corrupted inode with i_mode=0 and i_nlink=0 in memory.
> >>
> >> This "zombie" inode is created because ocfs2_read_locked_inode proceeds
> >> even after ocfs2_validate_inode_block successfully validates a block
> >> that structurally looks okay (passes checksum, signature etc.) but
> >> contains semantically invalid data (specifically i_mode=0). The current
> >> validation function doesn't check for i_mode being zero.
> >>
> >> This results in an in-memory inode with i_mode=0 being added to the VFS
> >> cache, which later triggers the panic during unlink.
> >>
> >> Prevent this by adding an explicit check for (i_mode == 0, i_nlink == 0, non-orphan)
> >> within ocfs2_validate_inode_block. If the check is true, return -EFSCORRUPTED to signal
> >> corruption. This causes the caller (ocfs2_read_locked_inode) to invoke
> >> make_bad_inode(), correctly preventing the zombie inode from entering
> >> the cache.
> >>
> >> Reported-by: syzbot+55c40ae8a0e5f3659f2b@syzkaller.appspotmail.com
> >> Fixes: https://syzkaller.appspot.com/bug?extid=55c40ae8a0e5f3659f2b
> >> Co-developed-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
> >> Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
> >> Signed-off-by: Ahmet Eray Karadag <eraykrdg1@gmail.com>
> >> Previous link: https://lore.kernel.org/all/20251022222752.46758-2-eraykrdg1@gmail.com/T/
> >> ---
> >> v2:
> >>  - Only checking either i_links_count == 0 or i_mode == 0
> >>  - Not performing le16_to_cpu() anymore
> >>  - Tested with ocfs2-test
> >> ---
> >> v3:
> >>  - Add checking both high and low bits of i_links_count
> >> ---
> >>  fs/ocfs2/inode.c | 7 +++++++
> >>  1 file changed, 7 insertions(+)
> >>
> >> diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
> >> index 14bf440ea4df..c8b129db756e 100644
> >> --- a/fs/ocfs2/inode.c
> >> +++ b/fs/ocfs2/inode.c
> >> @@ -1456,6 +1456,13 @@ int ocfs2_validate_inode_block(struct super_block *sb,
> >>  		goto bail;
> >>  	}
> >>  
> >> +	if (!ocfs2_read_links_count(di) || !di->i_mode) {
> > 
> > the code logic looks good to me, but I prefer the following code to save a few
> > cpu cycles.
> > 
> > ```
> > /* only check if the "link count" or i_mode is ZERO */
> > if (!(di->i_links_count | di->i_links_count_hi) || !di->i_mode)
> > ```
> > 
> > @Joseph
> > which do you like?
> > 
> Sorry to miss this thread discuss.
> The above check looks wried since "(di->i_links_count|di->i_links_count_hi)"
> is meaningless, though it may work well...
> So I'd prefer the code readability first.
> 
> BTW, from the syzbot report, I haven't seen where to show i_mode=0 or
> i_nlink=0. Am I missing something?

You are right, the syzbot report itself doesn't explicitly dump the inode 
values. I reproduced the issue locally and debugged ocfs2_unlink() to 
inspect the inode attributes just before the crash.
The debug output confirmed that the inode had:
   i_ino = 17057
   i_nlink = 0
   i_mode = 0

It appears the inode block read succeeds (physically), but due to corruption, 
validation was insufficient, allowing a zeroed/corrupted inode struct to be 
initialized in the VFS cache.

> 
> Thanks,
> Joseph
>

Regarding the patch logic: We had previously discussed and seemed to agree 
on an approach in the thread linked below, but since I didn't get a 
final response there, I refreshed the patch here.

https://lore.kernel.org/all/20251022222752.46758-2-eraykrdg1@gmail.com/T/

Thanks,
Eray

Hi all,

Just a kind reminder regarding this patch.

Since We are participating in the Linux Kernel Mentorship Program and have
a nearing deadline, your feedback would be incredibly helpful for us to
move forward.

Thanks,
Ahmet Eray

On Fri, Nov 28, 2025 at 05:36:06PM +0300, Ahmet Eray Karadag wrote:
> Hi all,
> 
> Just a kind reminder regarding this patch.
> 
> Since We are participating in the Linux Kernel Mentorship Program and have
> a nearing deadline, your feedback would be incredibly helpful for us to
> move forward.
> 
> Thanks,
> Ahmet Eray

I have provided my comment on the v3 patch. The sanity check is expensive,
costing about six lines of asm code for ocfs2_read_links_count() on a big-endian
architecture.

Also, since I'm not the maintainer, if Joseph agrees with your patch, he can
provide his Reviewed-by tag.

PS: Kindly note that the kernel mailing list generally discourages separate
emails for patch review threads.