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block: count BLK_OPEN_RESTRICT_WRITES openers
[ Upstream commit 3ff56e2 ] 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: ed5cc70 ("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> Signed-off-by: Sasha Levin <sashal@kernel.org>
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