[PATCH 2/2] [RFC]: block: count BLK_OPEN_RESTRICT_WRITES openers

[Christian Brauner <brauner@kernel.org>]

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.

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@kernel.org>
---
 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