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([2405:201:682f:389d:d8f3:146b:e2bb:b2f0]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-82ca8464576sm6075154b3a.14.2026.03.29.21.43.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 29 Mar 2026 21:43:41 -0700 (PDT) From: Deepanshu Kartikey To: axboe@kernel.dk, tytso@mit.edu, dvyukov@google.com Cc: adilger.kernel@dilger.ca, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org, Deepanshu Kartikey , syzbot+fb32afec111a7d61b939@syzkaller.appspotmail.com Subject: [PATCH] loop: block loop reconfiguration of offset/sizelimit on mounted device Date: Mon, 30 Mar 2026 10:13:34 +0530 Message-ID: <20260330044334.373480-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" LOOP_SET_STATUS{64} allows changing lo_offset and lo_sizelimit while a filesystem is mounted on the loop device. This effectively mutates the data visible to the mounted filesystem, which is equivalent to writing directly to the block device. When bdev_allow_write_mounted is false, direct writes to a mounted block device are blocked via bdev_writes_blocked(). However, LOOP_SET_STATUS{64} bypasses this protection because it modifies the loop configuration through an ioctl rather than opening the block device for writing. Fix this by checking bdev_writes_blocked() before allowing changes to lo_offset or lo_sizelimit. If the loop device has writes blocked (indicating a filesystem is mounted with write protection), return -EBUSY. Other loop status fields that do not affect the visible data can still be changed while mounted. Export bdev_writes_blocked() so it can be used from the loop driver. Suggested-by: Theodore Ts'o Reported-by: syzbot+fb32afec111a7d61b939@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3Dfb32afec111a7d61b939 Tested-by: syzbot+fb32afec111a7d61b939@syzkaller.appspotmail.com Signed-off-by: Deepanshu Kartikey --- block/bdev.c | 4 +++- drivers/block/loop.c | 12 ++++++++++++ include/linux/blkdev.h | 1 + 3 files changed, 16 insertions(+), 1 deletion(-) diff --git a/block/bdev.c b/block/bdev.c index ed022f8c48c7..96520fac7b2f 100644 --- a/block/bdev.c +++ b/block/bdev.c @@ -860,10 +860,12 @@ void blkdev_put_no_open(struct block_device *bdev) put_device(&bdev->bd_device); } =20 -static bool bdev_writes_blocked(struct block_device *bdev) +bool bdev_writes_blocked(struct block_device *bdev) { return bdev->bd_writers < 0; } +EXPORT_SYMBOL_GPL(bdev_writes_blocked); + =20 static void bdev_block_writes(struct block_device *bdev) { diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0000913f7efc..3f3a29abad1f 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -1239,6 +1239,18 @@ loop_set_status(struct loop_device *lo, const struct= loop_info64 *info) goto out_unlock; } =20 + /* + * Changing lo_offset or lo_sizelimit on a mounted device is + * equivalent to modifying the block device contents, block + * this if writes are blocked on the device. + */ + if ((lo->lo_offset !=3D info->lo_offset || + lo->lo_sizelimit !=3D info->lo_sizelimit) && + bdev_writes_blocked(lo->lo_device)) { + err =3D -EBUSY; + goto out_unlock; + } + if (lo->lo_offset !=3D info->lo_offset || lo->lo_sizelimit !=3D info->lo_sizelimit) { size_changed =3D true; diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index d463b9b5a0a5..6b908e9dd035 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -820,6 +820,7 @@ static inline bool bdev_read_only(struct block_device *= bdev) return bdev_test_flag(bdev, BD_READ_ONLY) || get_disk_ro(bdev->bd_disk); } =20 +bool bdev_writes_blocked(struct block_device *bdev); bool set_capacity_and_notify(struct gendisk *disk, sector_t size); void disk_force_media_change(struct gendisk *disk); void bdev_mark_dead(struct block_device *bdev, bool surprise); --=20 2.43.0