Re: [PATCH] nilfs2: reject CLEAN_SEGMENTS ioctl with out-of-range segment numbers

[Viacheslav Dubeyko <vdubeyko@redhat.com>]

On Tue, 2026-04-28 at 09:32 +0530, Deepanshu Kartikey wrote:
> Syzbot reported a hung task in nilfs_transaction_begin() where multiple
> tasks performing chmod() on a nilfs2 mount blocked for over 143 seconds
> waiting to acquire ns_segctor_sem for read:
> 
>   INFO: task syz.0.17:5918 blocked for more than 143 seconds.
>   Call Trace:
>    schedule+0x164/0x360
>    rwsem_down_read_slowpath+0x6d9/0x940
>    down_read+0x99/0x2e0
>    nilfs_transaction_begin+0x364/0x710 fs/nilfs2/segment.c:221
>    nilfs_setattr+0x124/0x2c0 fs/nilfs2/inode.c:921
>    notify_change+0xc1a/0xf40
>    chmod_common+0x273/0x4a0
>    do_fchmodat+0x12d/0x230
> 
> The writer holding ns_segctor_sem was a concurrent NILFS_IOCTL_CLEAN_SEGMENTS
> caller, stuck inside printk while emitting per-element warnings from
> nilfs_sufile_updatev():
> 
>    __nilfs_msg+0x373/0x450 fs/nilfs2/super.c:78
>    nilfs_sufile_updatev+0x21c/0x6d0 fs/nilfs2/sufile.c:186
>    nilfs_sufile_freev fs/nilfs2/sufile.h:93 [inline]
>    nilfs_free_segments fs/nilfs2/segment.c:1140 [inline]
>    nilfs_segctor_collect_blocks fs/nilfs2/segment.c:1261 [inline]
>    nilfs_segctor_do_construct+0x1f55/0x76c0
>    nilfs_clean_segments+0x3bd/0xa50
>    nilfs_ioctl_clean_segments fs/nilfs2/ioctl.c:922 [inline]
>    nilfs_ioctl+0x261f/0x2780
> 
> The root cause is that nilfs_ioctl_clean_segments() does not validate
> the user-supplied segment numbers in kbufs[4] before calling
> nilfs_clean_segments(), which acquires ns_segctor_sem for write.  The
> range check on each segnum is performed deep inside the call chain by
> nilfs_sufile_updatev(), which emits a nilfs_warn() per invalid entry
> while still under the segctor lock and the sufile mi_sem.  Under load
> (repeated invocations across multiple mounts saturating the global
> printk path), the cumulative printk latency keeps ns_segctor_sem held
> long enough to trip the hung_task watchdog, blocking concurrent
> operations such as chmod() that need ns_segctor_sem for read.
> 
> Fix by validating the contents of kbufs[4] in the ioctl entry path,
> before any FS-wide lock is acquired.  Out-of-range segment numbers are
> rejected with -EINVAL synchronously, with no work performed under
> ns_segctor_sem.
> 
> Reported-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=62f0f99d2f2bb8e3bbd7
> Fixes: 4f6b828837b4 ("nilfs2: fix lock order reversal in nilfs_clean_segments ioctl")
> Tested-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> ---
>  fs/nilfs2/ioctl.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
> index e0a606643e87..38822dce1839 100644
> --- a/fs/nilfs2/ioctl.c
> +++ b/fs/nilfs2/ioctl.c
> @@ -846,6 +846,7 @@ static int nilfs_ioctl_clean_segments(struct inode *inode, struct file *filp,
>  	struct the_nilfs *nilfs;
>  	size_t len, nsegs;
>  	int n, ret;
> +	size_t i;

What about re-using the n variable? Does it make sense to introduce new one?

>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
> @@ -876,6 +877,21 @@ static int nilfs_ioctl_clean_segments(struct inode *inode, struct file *filp,
>  	}
>  	nilfs = inode->i_sb->s_fs_info;
>  
> +	/*
> +	 * Validate segment numbers against the filesystem's segment count
> +	 * before entering nilfs_clean_segments(), which acquires
> +	 * ns_segctor_sem for write.  Catching invalid segnums here avoids
> +	 * holding that lock while emitting per-element diagnostics under
> +	 * the segment constructor.
> +	 */
> +	for (i = 0; i < nsegs; i++) {
> +		if (((__u64 *)kbufs[4])[i] >= nilfs->ns_nsegments) {
> +			ret = -EINVAL;
> +			kfree(kbufs[4]);
> +			goto out;

Are you sure that you need to free buffer here and go to out? Maybe, we can
introduce another label and to jump to kfree(kbufs[4]) at the end of method?

Thanks,
Slava.

> +		}
> +	}
> +
>  	for (n = 0; n < 4; n++) {
>  		ret = -EINVAL;
>  		if (argv[n].v_size != argsz[n])