linux-kernel - [PATCH 1/2] Fail I/O to thin pool devices

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Date:   Mon,  6 Feb 2023 20:18:48 -0500
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com
Cc:     Demi Marie Obenour <demi@...isiblethingslab.com>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] Fail I/O to thin pool devices

A thin pool device currently just passes all I/O to its origin device,
but this is a footgun: the user might not realize that tools that
operate on thin pool metadata must operate on the metadata volume.  This
could have security implications.

Fix this by failing all I/O to thin pool devices.

Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
---
 drivers/md/dm-thin.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index 64cfcf46881dc5d87d5dfdb5650ba9babd32cd31..d85fdbd782ae5426003c99a4b4bf53818cc85efa 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -3405,19 +3405,14 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
 
 static int pool_map(struct dm_target *ti, struct bio *bio)
 {
-	int r;
-	struct pool_c *pt = ti->private;
-	struct pool *pool = pt->pool;
-
 	/*
-	 * As this is a singleton target, ti->begin is always zero.
+	 * Previously, access to the pool was passed down to the origin device.
+	 * However, this turns out to be error-prone: if the user runs any of
+	 * the thin tools on the pool device, the tools could wind up parsing
+	 * potentially attacker-controlled data.  This mistake has actually
+	 * happened in practice.  Therefore, fail all I/O on the pool device.
 	 */
-	spin_lock_irq(&pool->lock);
-	bio_set_dev(bio, pt->data_dev->bdev);
-	r = DM_MAPIO_REMAPPED;
-	spin_unlock_irq(&pool->lock);
-
-	return r;
+	return -EIO;
 }
 
 static int maybe_resize_data_dev(struct dm_target *ti, bool *need_commit)
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab