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block: count BLK_OPEN_RESTRICT_WRITES openers

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The original changes in v6.8 do allow for a block device to be reopened
with BLK_OPEN_RESTRICT_WRITES provided the same holder is used as per
bdev_may_open(). I think this has a bug.

The first opener @f1 of that block device will set bdev->bd_writers to
-1. The second opener @f2 using the same holder will pass the check in
bdev_may_open() that bdev->bd_writers must not be greater than zero.

The first opener @f1 now closes the block device and in bdev_release()
will end up calling bdev_yield_write_access() which calls
bdev_writes_blocked() and sets bdev->bd_writers to 0 again.

Now @f2 holds a file to that block device which was opened with
exclusive write access but bdev->bd_writers has been reset to 0.

So now @f3 comes along and succeeds in opening the block device with
BLK_OPEN_WRITE betraying @f2's request to have exclusive write access.

This isn't a practical issue yet because afaict there's no codepath
inside the kernel that reopenes the same block device with
BLK_OPEN_RESTRICT_WRITES but it will be if there is.

Fix this by counting the number of BLK_OPEN_RESTRICT_WRITES openers. So
we only allow writes again once all BLK_OPEN_RESTRICT_WRITES openers are
done.

Link: https://lore.kernel.org/r/20240323-abtauchen-klauen-c2953810082d@brauner
Fixes: ed5cc702d3 ("block: Add config option to not allow writing to mounted devices")
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2024-03-23 17:11:20 +01:00
parent ddd65e19c6
commit 3ff56e285d
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
1 changed files with 3 additions and 3 deletions
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6
block/bdev.c
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@ -776,17 +776,17 @@ void blkdev_put_no_open(struct block_device *bdev)
static bool bdev_writes_blocked(struct block_device *bdev)
{
return bdev->bd_writers == -1;
return bdev->bd_writers < 0;
}
static void bdev_block_writes(struct block_device *bdev)
{
bdev->bd_writers = -1;
bdev->bd_writers--;
}
static void bdev_unblock_writes(struct block_device *bdev)
{
bdev->bd_writers = 0;
bdev->bd_writers++;
}
static bool bdev_may_open(struct block_device *bdev, blk_mode_t mode)