From: Jan Kara <jack@suse.cz>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Jan Kara <jack@suse.cz>,
linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
Christoph Hellwig <hch@infradead.org>,
Christian Brauner <brauner@kernel.org>,
Jens Axboe <axboe@kernel.dk>, Kees Cook <keescook@google.com>,
Ted Tso <tytso@mit.edu>, syzkaller <syzkaller@googlegroups.com>,
Alexander Popov <alex.popov@linux.com>,
linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org,
Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH 1/6] block: Add config option to not allow writing to mounted devices
Date: Wed, 5 Jul 2023 12:30:37 +0200 [thread overview]
Message-ID: <20230705103037.fvizcrecsjhswngn@quack3> (raw)
In-Reply-To: <20230704184416.GE1851@sol.localdomain>
On Tue 04-07-23 11:44:16, Eric Biggers wrote:
> 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@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
> > + 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.
>
> Does this prevent the underlying storage from being written to? Say if the
> mounted block device is /dev/sda1 and someone tries to write to /dev/sda in the
> region that contains sda1.
>
> I *think* the answer is no, writes to /dev/sda are still allowed since the goal
> is just to prevent writes to the buffer cache of mounted block devices, not
> writes to the underlying storage. That is really something that should be
> stated explicitly, though.
You are correct. The answer is "no" because as Ted says, there are many
ways to do that anyway and for a filesystem it is generally not much
different from just corrupted fs image. I'll explicitely mention it in the
config text, that's a good idea.
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2023-07-05 10:30 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-04 12:56 [PATCH RFC 0/6 v2] block: Add config option to not allow writing to mounted devices Jan Kara
2023-07-04 12:56 ` [PATCH 1/6] " Jan Kara
2023-07-04 15:56 ` Colin Walters
2023-07-04 16:52 ` Eric Biggers
2023-08-14 16:41 ` Jan Kara
2023-08-14 16:43 ` Jan Kara
2023-07-04 18:44 ` Eric Biggers
2023-07-04 20:55 ` Theodore Ts'o
2023-07-05 10:30 ` Jan Kara [this message]
2023-07-05 15:12 ` Darrick J. Wong
2023-08-22 5:35 ` Eric Biggers
2023-08-22 10:11 ` Jan Kara
2023-10-19 9:16 ` Aleksandr Nogikh
2023-10-24 11:10 ` Jan Kara
2023-10-27 12:06 ` Aleksandr Nogikh
2023-11-08 10:10 ` Jan Kara
2023-11-08 18:24 ` Aleksandr Nogikh
2023-07-04 12:56 ` [PATCH 2/6] fs: Block writes to mounted block devices Jan Kara
2023-07-04 12:56 ` [PATCH 3/6] xfs: Block writes to log device Jan Kara
2023-07-04 15:53 ` Darrick J. Wong
2023-07-05 10:31 ` Jan Kara
2023-07-04 12:56 ` [PATCH 4/6] ext4: Block writes to journal device Jan Kara
2023-07-04 12:56 ` [PATCH 5/6] btrfs: Block writes to seed devices Jan Kara
2023-07-12 14:33 ` David Sterba
2023-07-04 12:56 ` [PATCH 6/6] fs: Make bind mounts work with bdev_allow_write_mounted=n Jan Kara
2023-07-04 13:59 ` Christian Brauner
2023-07-05 13:00 ` Jan Kara
2023-07-05 13:46 ` Christian Brauner
2023-07-05 16:14 ` Jan Kara
2023-07-06 15:55 ` Christoph Hellwig
2023-07-06 16:12 ` Jan Kara
2023-07-07 7:39 ` Christian Brauner
2023-07-07 10:48 ` Jan Kara
2023-07-07 11:31 ` Christoph Hellwig
2023-07-07 12:28 ` Jan Kara
2023-07-07 11:30 ` Christoph Hellwig
2023-07-04 13:40 ` [PATCH RFC 0/6 v2] block: Add config option to not allow writing to mounted devices Christian Brauner
2023-07-05 12:27 ` Mike Fleetwood
2023-08-14 16:39 ` Jan Kara
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