Re: [PATCH 1/6] block: Add config option to not allow writing to mounted devices

[Jan Kara <jack@suse.cz>]

On Tue 04-07-23 11:56:44, Colin Walters wrote:
> On Tue, Jul 4, 2023, at 8:56 AM, 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@linaro.org
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  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
> 
> s/very/is very/
> 
> > +	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
> 
> s/priviledged/privileged/
> 
> > +	overridden with bdev_allow_write_mounted boot option.
> 
> s/with/with the/

Thanks for the language fixes!

> > +/* open is exclusive wrt all other BLK_OPEN_WRITE opens to the device */
> > +#define BLK_OPEN_BLOCK_WRITES	((__force blk_mode_t)(1 << 5))
> 
> Bikeshed but: I think BLK and BLOCK "stutter" here.  The doc comment
> already uses the term "exclusive" so how about BLK_OPEN_EXCLUSIVE ?  

Well, we already have exclusive opens of block devices which are different
(they are exclusive only wrt other exclusive opens) so BLK_OPEN_EXCLUSIVE
will be really confusing. But BLK_OPEN_RESTRICT_WRITES sounds good to me.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR