On Wed, Nov 01, 2023 at 06:43:08PM +0100, Jan Kara wrote: > Writing to mounted devices is dangerous and can lead to filesystem > corruption as well as crashes. Furthermore syzbot comes with more and > more involved examples how to corrupt block device under a mounted > filesystem leading to kernel crashes and reports we can do nothing > about. Add tracking of writers to each block device and a kernel cmdline > argument which controls whether other writeable opens to block devices > open with BLK_OPEN_RESTRICT_WRITES flag are allowed. We will make > filesystems use this flag for used devices. > > Note that this effectively only prevents modification of the particular > block device's page cache by other writers. The actual device content > can still be modified by other means - e.g. by issuing direct scsi > commands, by doing writes through devices lower in the storage stack > (e.g. in case loop devices, DM, or MD are involved) etc. But blocking > direct modifications of the block device page cache is enough to give > filesystems a chance to perform data validation when loading data from > the underlying storage and thus prevent kernel crashes. > > Syzbot can use this cmdline argument option to avoid uninteresting > crashes. Also users whose userspace setup does not need writing to > mounted block devices can set this option for hardening. > > Link: https://lore.kernel.org/all/60788e5d-5c7c-1142-e554-c21d709acfd9@xxxxxxxxxx > Signed-off-by: Jan Kara <jack@xxxxxxx> > --- A few minor tweaks I would do in-tree. Please see below. I know it's mostly stylistic that's why I would do it so there's no resend dance for non-technical reasons. > block/Kconfig | 20 +++++++++++++ > block/bdev.c | 62 ++++++++++++++++++++++++++++++++++++++- > include/linux/blk_types.h | 1 + > include/linux/blkdev.h | 2 ++ > 4 files changed, 84 insertions(+), 1 deletion(-) > > diff --git a/block/Kconfig b/block/Kconfig > index f1364d1c0d93..ca04b657e058 100644 > --- a/block/Kconfig > +++ b/block/Kconfig > @@ -78,6 +78,26 @@ config BLK_DEV_INTEGRITY_T10 > select CRC_T10DIF > select CRC64_ROCKSOFT > > +config BLK_DEV_WRITE_MOUNTED > + bool "Allow writing to mounted block devices" > + default y Let's hope that this can become the default one day. > +static void bdev_unblock_writes(struct block_device *bdev) > +{ > + bdev->bd_writers = 0; > +} > + > +static bool blkdev_open_compatible(struct block_device *bdev, blk_mode_t mode) I would like to mirror our may_{open,create}() routines here and call this: bdev_may_open() This is a well-known vfs pattern and also easy to understand for block devs as well. > @@ -800,12 +834,21 @@ struct bdev_handle *bdev_open_by_dev(dev_t dev, blk_mode_t mode, void *holder, > goto abort_claiming; > if (!try_module_get(disk->fops->owner)) > goto abort_claiming; > + ret = -EBUSY; > + if (!blkdev_open_compatible(bdev, mode)) > + goto abort_claiming; > if (bdev_is_partition(bdev)) > ret = blkdev_get_part(bdev, mode); > else > ret = blkdev_get_whole(bdev, mode); > if (ret) > goto put_module; > + if (!bdev_allow_write_mounted) { > + if (mode & BLK_OPEN_RESTRICT_WRITES) > + bdev_block_writes(bdev); > + else if (mode & BLK_OPEN_WRITE) > + bdev->bd_writers++; > + } I would like to move this to a tiny helper for clarity: static void bdev_claim_write_access(struct block_device *bdev) { if (!bdev_allow_write_mounted) return; /* Claim exclusive or shared write access to the block device. */ if (mode & BLK_OPEN_RESTRICT_WRITES) bdev_block_writes(bdev); else if (mode & BLK_OPEN_WRITE) bdev->bd_writers++; } > if (holder) { > bd_finish_claiming(bdev, holder, hops); > > @@ -901,6 +944,14 @@ void bdev_release(struct bdev_handle *handle) > sync_blockdev(bdev); > > mutex_lock(&disk->open_mutex); > + if (!bdev_allow_write_mounted) { > + /* The exclusive opener was blocking writes? Unblock them. */ > + if (handle->mode & BLK_OPEN_RESTRICT_WRITES) > + bdev_unblock_writes(bdev); > + else if (handle->mode & BLK_OPEN_WRITE) > + bdev->bd_writers--; > + } static void bdev_yield_write_access(struct block_device *bdev) { if (!bdev_allow_write_mounted) return; /* Yield exclusive or shared write access. */ if (handle->mode & BLK_OPEN_RESTRICT_WRITES) bdev_unblock_writes(bdev); else if (handle->mode & BLK_OPEN_WRITE) bdev->bd_writers--; }
