block: Add config option to not allow writing to mounted devices

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@linaro.org
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20231101174325.10596-3-jack@suse.cz
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 20 insertions, 0 deletions

diff --git a/block/Kconfig b/block/Kconfig
index 55ae2286a4de..1de4682d48cc 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
+	help
+	When a block device is mounted, writing to its buffer cache is 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
+	privileged processes and generally makes it much harder to crash
+	filesystem drivers. Note however that this does not prevent
+	underlying device(s) from being modified by other means, e.g. by
+	directly submitting SCSI commands or through access to lower layers of
+	storage stack. If in doubt, say Y. The configuration can be overridden
+	with the bdev_allow_write_mounted boot option.
+
 config BLK_DEV_ZONED
 	bool "Zoned block device support"
 	select MQ_IOSCHED_DEADLINE