On Mon 06-11-23 15:47:54, Christian Brauner wrote: > 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. Whatever works best for you. I agree with the changes but please see my comments below for some fixes. > > 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. Yes, I'd hope as well but we need some tooling work (util-linux, e2fsprogs) before that can happen. > > +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. Sure. > > @@ -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) This should be the other way around. > 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) And this as well. > 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--; > } Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
