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Thu, 30 Apr 2026 11:11:45 -0700 (PDT) Received: from li-4c4c4544-0032-4210-804c-c3c04f423534.ibm.com ([2600:1700:6476:1430::29]) by smtp.gmail.com with ESMTPSA id 00721157ae682-7bd6683836asm556967b3.25.2026.04.30.11.11.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Apr 2026 11:11:44 -0700 (PDT) Message-ID: Subject: Re: [PATCH v3] nilfs2: reject CLEAN_SEGMENTS ioctl with out-of-range segment numbers From: Viacheslav Dubeyko To: Deepanshu Kartikey , konishi.ryusuke@gmail.com, slava@dubeyko.com Cc: linux-nilfs@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com, stable@vger.kernel.org Date: Thu, 30 Apr 2026 11:11:43 -0700 In-Reply-To: <20260430040704.113622-1-kartikey406@gmail.com> References: <20260430040704.113622-1-kartikey406@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.0 (3.60.0-1.fc44app2) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-04-30 at 09:37 +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: >=20 > 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 >=20 > The writer holding ns_segctor_sem was a concurrent=20 > NILFS_IOCTL_CLEAN_SEGMENTS caller, stuck inside printk while emitting=20 > per-element warnings from nilfs_sufile_updatev(): >=20 > __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 >=20 > 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. >=20 > 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(). >=20 > Reported-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=3D62f0f99d2f2bb8e3bbd7 > Tested-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com > Fixes: 4f6b828837b4 ("nilfs2: fix lock order reversal in nilfs_clean_segm= ents ioctl") > Cc: stable@vger.kernel.org > Signed-off-by: Deepanshu Kartikey > --- > Changes in v3: > - Move validation from nilfs_ioctl_clean_segments() into > nilfs_clean_segments(), under ns_segctor_sem held for write > by nilfs_transaction_lock(), to serialize against > nilfs_ioctl_resize() which can modify ns_nsegments > (Ryusuke Konishi) > - Introduce local variables segnumv and nfreesegs for readability, > rather than open-coding casts of kbufs[4] (Ryusuke Konishi) > - Emit nilfs_err() once on the first out-of-range segnum and bail > out, instead of nilfs_warn() per element (Ryusuke Konishi) > - Add bail_unlock label for the early-failure path, parallel to > the existing out_unlock structure (Ryusuke Konishi) >=20 > Changes in v2: > - Reuse existing 'n' loop variable instead of introducing a new > one (Slava Dubeyko) > - Add dedicated out_free_segnums label so the validation-failure > path falls through the existing cleanup ladder rather than > duplicating kfree(kbufs[4]) inline (Slava Dubeyko) > --- > fs/nilfs2/segment.c | 22 ++++++++++++++++++++++ > 1 file changed, 22 insertions(+) >=20 > diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c > index 1491a4d4b1e1..dc54643866ce 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 =3D nilfs->ns_writer; > struct nilfs_transaction_info ti; > int err; Usually, I prefer to keep the err variable at the end of declarations. Beca= use, it is the ending state of the function. And I am feeling that something is = wrong every time when likewise variable is hidden inside of declaration list. :) = There is nothing critical in my remark. But anyway... :) The path looks good to me. Thanks, Slava. > + size_t i, nfreesegs =3D argv[4].v_nmembs; > + __u64 *segnumv =3D kbufs[4]; > =20 > if (unlikely(!sci)) > return -EROFS; > =20 > nilfs_transaction_lock(sb, &ti, 1); > =20 > + /* > + * 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 =3D 0; i < nfreesegs; i++) { > + if (segnumv[i] >=3D nilfs->ns_nsegments) { > + nilfs_err(sb, > + "Segment number %llu to be freed is out of range", > + (unsigned long long)segnumv[i]); > + err =3D -EINVAL; > + goto bail_unlock; > + } > + } > + > err =3D 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, st= ruct nilfs_argv *argv, > sci->sc_freesegs =3D NULL; > sci->sc_nfreesegs =3D 0; > nilfs_mdt_clear_shadow_map(nilfs->ns_dat); > + bail_unlock: > nilfs_transaction_unlock(sb); > return err; > }