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

On Sun, 2026-05-03 at 13:33 +0900, Ryusuke Konishi wrote:
> From: Deepanshu Kartikey <[email protected]>
> 
> 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 user-supplied segment numbers are not validated
> before nilfs_clean_segments() begins doing work; 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 holding 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 nilfs_clean_segments()
> immediately after acquiring ns_segctor_sem via nilfs_transaction_lock().
> Holding ns_segctor_sem serializes the check against
> nilfs_ioctl_resize(), which can modify ns_nsegments, so the validation
> uses a consistent value.  Out-of-range segment numbers are rejected
> with -EINVAL before any segment-cleaning work begins, so the bad
> entries never reach the per-element diagnostic path inside
> nilfs_sufile_updatev().
> 
> Reported-by: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=62f0f99d2f2bb8e3bbd7
> Tested-by: [email protected]
> Cc: [email protected]
> Signed-off-by: Deepanshu Kartikey <[email protected]>
> Fixes: 071cb4b81987 ("nilfs2: eliminate removal list of segments")
> Signed-off-by: Ryusuke Konishi <[email protected]>
> ---
> Hi Viacheslav,
> 
> Please queue this patch.
> 
> This is a fix by Deepanshu that addresses the problem recently
> detected by syzbot, a hang-up that can occur when GC ioctl parameters
> are invalid (this time, when a segment number to be freed is
> out-of-range).
> 
> Thanks,
> Ryusuke Konishi
> 
>  fs/nilfs2/segment.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
> index 1491a4d4b1e1..9332f5ac6083 100644
> --- a/fs/nilfs2/segment.c
> +++ b/fs/nilfs2/segment.c
> @@ -2512,12 +2512,33 @@ int nilfs_clean_segments(struct super_block *sb, struct nilfs_argv *argv,
>  	struct nilfs_sc_info *sci = nilfs->ns_writer;
>  	struct nilfs_transaction_info ti;
>  	int err;
> +	size_t i, nfreesegs = argv[4].v_nmembs;
> +	__u64 *segnumv = kbufs[4];
>  
>  	if (unlikely(!sci))
>  		return -EROFS;
>  
>  	nilfs_transaction_lock(sb, &ti, 1);
>  
> +	/*
> +	 * Validate segment numbers under ns_segctor_sem (held for write
> +	 * by nilfs_transaction_lock above) so the check is serialized
> +	 * against nilfs_ioctl_resize(), which can modify ns_nsegments.
> +	 * Rejecting bad input here, before any segment-cleaning work
> +	 * begins, avoids the per-element diagnostic path inside
> +	 * nilfs_sufile_updatev() that would otherwise run under this
> +	 * same lock and stall concurrent readers.
> +	 */
> +	for (i = 0; i < nfreesegs; i++) {
> +		if (segnumv[i] >= nilfs->ns_nsegments) {
> +			nilfs_err(sb,
> +				 "Segment number %llu to be freed is out of range",
> +				 (unsigned long long)segnumv[i]);
> +			err = -EINVAL;
> +			goto bail_unlock;
> +		}
> +	}
> +
>  	err = nilfs_mdt_save_to_shadow_map(nilfs->ns_dat);
>  	if (unlikely(err))
>  		goto out_unlock;
> @@ -2558,6 +2579,7 @@ int nilfs_clean_segments(struct super_block *sb, struct nilfs_argv *argv,
>  	sci->sc_freesegs = NULL;
>  	sci->sc_nfreesegs = 0;
>  	nilfs_mdt_clear_shadow_map(nilfs->ns_dat);
> + bail_unlock:
>  	nilfs_transaction_unlock(sb);
>  	return err;
>  }

Applied.

Thanks,
Slava.