On Tue, Jul 04, 2023 at 02:56:49PM +0200, 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 writes to block devices open with > BLK_OPEN_BLOCK_WRITES flag are allowed. We will make filesystems use > this flag for used devices. > > 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> > --- > block/Kconfig | 16 ++++++++++ > block/bdev.c | 63 ++++++++++++++++++++++++++++++++++++++- > include/linux/blk_types.h | 1 + > include/linux/blkdev.h | 3 ++ > 4 files changed, 82 insertions(+), 1 deletion(-) > > diff --git a/block/Kconfig b/block/Kconfig > index 86122e459fe0..8b4fa105b854 100644 > --- a/block/Kconfig > +++ b/block/Kconfig > @@ -77,6 +77,22 @@ 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 > + help > + When a block device is mounted, writing to its buffer cache very likely > + going to cause filesystem corruption. It is also rather easy to crash > + the kernel in this way since the filesystem has no practical way of > + detecting these writes to buffer cache and verifying its metadata > + integrity. However there are some setups that need this capability > + like running fsck on read-only mounted root device, modifying some > + features on mounted ext4 filesystem, and similar. If you say N, the > + kernel will prevent processes from writing to block devices that are > + mounted by filesystems which provides some more protection from runaway > + priviledged processes. If in doubt, say Y. The configuration can be > + overridden with bdev_allow_write_mounted boot option. > + > config BLK_DEV_ZONED > bool "Zoned block device support" > select MQ_IOSCHED_DEADLINE > diff --git a/block/bdev.c b/block/bdev.c > index 523ea7289834..346e68dbf0bf 100644 > --- a/block/bdev.c > +++ b/block/bdev.c > @@ -30,6 +30,9 @@ > #include "../fs/internal.h" > #include "blk.h" > > +/* Should we allow writing to mounted block devices? */ > +static bool bdev_allow_write_mounted = IS_ENABLED(CONFIG_BLK_DEV_WRITE_MOUNTED); This might be premature at this point, but I wonder if you've given any consideration to adding a lockdown prohibition as well? e.g. static inline bool bdev_allow_write_mounted(void) { if (security_locked_down(LOCKDOWN_MOUNTED_BDEV) != 0) return false; return __bdev_allow_write_mounted; } --D > struct bdev_inode { > struct block_device bdev; > struct inode vfs_inode; > @@ -744,7 +747,34 @@ void blkdev_put_no_open(struct block_device *bdev) > { > put_device(&bdev->bd_device); > } > - > + > +static bool bdev_writes_blocked(struct block_device *bdev) > +{ > + return bdev->bd_writers == -1; > +} > + > +static void bdev_block_writes(struct block_device *bdev) > +{ > + bdev->bd_writers = -1; > +} > + > +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) > +{ > + if (!bdev_allow_write_mounted) { > + /* Writes blocked? */ > + if (mode & BLK_OPEN_WRITE && bdev_writes_blocked(bdev)) > + return false; > + if (mode & BLK_OPEN_BLOCK_WRITES && bdev->bd_writers > 0) > + return false; > + } > + return true; > +} > + > /** > * blkdev_get_by_dev - open a block device by device number > * @dev: device number of block device to open > @@ -787,6 +817,10 @@ struct bdev_handle *blkdev_get_by_dev(dev_t dev, blk_mode_t mode, void *holder, > if (ret) > goto free_handle; > > + /* Blocking writes requires exclusive opener */ > + if (mode & BLK_OPEN_BLOCK_WRITES && !holder) > + return ERR_PTR(-EINVAL); > + > bdev = blkdev_get_no_open(dev); > if (!bdev) { > ret = -ENXIO; > @@ -814,12 +848,21 @@ struct bdev_handle *blkdev_get_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_BLOCK_WRITES) > + bdev_block_writes(bdev); > + else if (mode & BLK_OPEN_WRITE) > + bdev->bd_writers++; > + } > if (holder) { > bd_finish_claiming(bdev, holder, hops); > > @@ -842,6 +885,7 @@ struct bdev_handle *blkdev_get_by_dev(dev_t dev, blk_mode_t mode, void *holder, > disk_unblock_events(disk); > handle->bdev = bdev; > handle->holder = holder; > + handle->mode = mode; > return handle; > put_module: > module_put(disk->fops->owner); > @@ -914,6 +958,14 @@ void blkdev_put(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_BLOCK_WRITES) > + bdev_unblock_writes(bdev); > + else if (handle->mode & BLK_OPEN_WRITE) > + bdev->bd_writers--; > + } > + > if (handle->holder) > bd_end_claim(bdev, handle->holder); > > @@ -1070,3 +1122,12 @@ void bdev_statx_dioalign(struct inode *inode, struct kstat *stat) > > blkdev_put_no_open(bdev); > } > + > +static int __init setup_bdev_allow_write_mounted(char *str) > +{ > + if (kstrtobool(str, &bdev_allow_write_mounted)) > + pr_warn("Invalid option string for bdev_allow_write_mounted:" > + " '%s'\n", str); > + return 1; > +} > +__setup("bdev_allow_write_mounted=", setup_bdev_allow_write_mounted); > diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h > index 0bad62cca3d0..5bf0d2d458fd 100644 > --- a/include/linux/blk_types.h > +++ b/include/linux/blk_types.h > @@ -70,6 +70,7 @@ struct block_device { > #ifdef CONFIG_FAIL_MAKE_REQUEST > bool bd_make_it_fail; > #endif > + int bd_writers; > /* > * keep this out-of-line as it's both big and not needed in the fast > * path > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h > index 4ae3647a0322..ca467525e6e4 100644 > --- a/include/linux/blkdev.h > +++ b/include/linux/blkdev.h > @@ -124,6 +124,8 @@ typedef unsigned int __bitwise blk_mode_t; > #define BLK_OPEN_NDELAY ((__force blk_mode_t)(1 << 3)) > /* open for "writes" only for ioctls (specialy hack for floppy.c) */ > #define BLK_OPEN_WRITE_IOCTL ((__force blk_mode_t)(1 << 4)) > +/* open is exclusive wrt all other BLK_OPEN_WRITE opens to the device */ > +#define BLK_OPEN_BLOCK_WRITES ((__force blk_mode_t)(1 << 5)) > > struct gendisk { > /* > @@ -1474,6 +1476,7 @@ struct blk_holder_ops { > struct bdev_handle { > struct block_device *bdev; > void *holder; > + blk_mode_t mode; > }; > > struct bdev_handle *blkdev_get_by_dev(dev_t dev, blk_mode_t mode, void *holder, > -- > 2.35.3 >
