On Sat 23-03-24 17:11:20, Christian Brauner wrote:
> 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 that may have a bug.
^^^^^^^^ is :)
Also AFAICT this can happen only the the reopen is O_RDONLY which would be
probably good to mention here.
> 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.
>
> If that's right then 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.
>
> Fixes: ed5cc702d311 ("block: Add config option to not allow writing to mounted devices")
> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
Otherwise looks good so feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> block/bdev.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/block/bdev.c b/block/bdev.c
> index f819f3086905..42f84692404c 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -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)
> --
> 2.43.0
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
