From: Deepanshu Kartikey <kartikey406@gmail.com>

nilfs_ioctl_mark_blocks_dirty() uses bd_oblocknr to detect dead blocks
by comparing it with the current block number bd_blocknr. If they differ,
the block is considered dead and skipped.

However, bd_oblocknr should never be 0 since block 0 typically stores the
primary superblock and is never a valid GC target block. A corrupted ioctl
request with bd_oblocknr set to 0 causes the comparison to incorrectly
match when the lookup returns -ENOENT and sets bd_blocknr to 0, bypassing
the dead block check and calling nilfs_bmap_mark() on a non-existent
block. This causes nilfs_btree_do_lookup() to return -ENOENT, triggering
the WARN_ON(ret == -ENOENT).

Fix this by rejecting ioctl requests with bd_oblocknr set to 0 at the
beginning of each iteration.

[ryusuke: slightly modified the commit message and comments for accuracy]

Fixes: 7942b919f732 ("nilfs2: ioctl operations")
Reported-by: syzbot+98a040252119df0506f8@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=98a040252119df0506f8
Suggested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
Reported-by: syzbot+466a45fcfb0562f5b9a0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=466a45fcfb0562f5b9a0
Cc: Junjie Cao <junjie.cao@linux.dev>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
Hi Viacheslav,

Please add this to the queue for the next cycle.  This fixes assertion
failures that can occur with broken GC ioctl calls recently discovered
by syzbot.

Thanks,
Ryusuke Konishi

 fs/nilfs2/ioctl.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
index e17b8da66491..e0a606643e87 100644
--- a/fs/nilfs2/ioctl.c
+++ b/fs/nilfs2/ioctl.c
@@ -736,6 +736,12 @@ static int nilfs_ioctl_mark_blocks_dirty(struct the_nilfs *nilfs,
 	int ret, i;
 
 	for (i = 0; i < nmembs; i++) {
+		/*
+		 * bd_oblocknr must never be 0 as block 0
+		 * is never a valid GC target block
+		 */
+		if (unlikely(!bdescs[i].bd_oblocknr))
+			return -EINVAL;
 		/* XXX: use macro or inline func to check liveness */
 		ret = nilfs_bmap_lookup_at_level(bmap,
 						 bdescs[i].bd_offset,
-- 
2.43.0

Hi Ryusuke,

On Wed, 2026-04-01 at 02:52 +0900, Ryusuke Konishi wrote:
> From: Deepanshu Kartikey <kartikey406@gmail.com>
> 
> nilfs_ioctl_mark_blocks_dirty() uses bd_oblocknr to detect dead blocks
> by comparing it with the current block number bd_blocknr. If they differ,
> the block is considered dead and skipped.
> 
> However, bd_oblocknr should never be 0 since block 0 typically stores the
> primary superblock and is never a valid GC target block. A corrupted ioctl
> request with bd_oblocknr set to 0 causes the comparison to incorrectly
> match when the lookup returns -ENOENT and sets bd_blocknr to 0, bypassing
> the dead block check and calling nilfs_bmap_mark() on a non-existent
> block. This causes nilfs_btree_do_lookup() to return -ENOENT, triggering
> the WARN_ON(ret == -ENOENT).
> 
> Fix this by rejecting ioctl requests with bd_oblocknr set to 0 at the
> beginning of each iteration.
> 
> [ryusuke: slightly modified the commit message and comments for accuracy]
> 
> Fixes: 7942b919f732 ("nilfs2: ioctl operations")
> Reported-by: syzbot+98a040252119df0506f8@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=98a040252119df0506f8
> Suggested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
> Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
> Reported-by: syzbot+466a45fcfb0562f5b9a0@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=466a45fcfb0562f5b9a0
> Cc: Junjie Cao <junjie.cao@linux.dev>
> Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
> ---
> Hi Viacheslav,
> 
> Please add this to the queue for the next cycle.  This fixes assertion
> failures that can occur with broken GC ioctl calls recently discovered
> by syzbot.
> 
> Thanks,
> Ryusuke Konishi
> 
>  fs/nilfs2/ioctl.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
> index e17b8da66491..e0a606643e87 100644
> --- a/fs/nilfs2/ioctl.c
> +++ b/fs/nilfs2/ioctl.c
> @@ -736,6 +736,12 @@ static int nilfs_ioctl_mark_blocks_dirty(struct the_nilfs *nilfs,
>  	int ret, i;
>  
>  	for (i = 0; i < nmembs; i++) {
> +		/*
> +		 * bd_oblocknr must never be 0 as block 0
> +		 * is never a valid GC target block
> +		 */
> +		if (unlikely(!bdescs[i].bd_oblocknr))
> +			return -EINVAL;
>  		/* XXX: use macro or inline func to check liveness */
>  		ret = nilfs_bmap_lookup_at_level(bmap,
>  						 bdescs[i].bd_offset,

Applied on for-next branch of NILFS2 git tree.

Thanks,
Slava.