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From:   Christophe JAILLET <christophe.jaillet@wanadoo.fr>
To:     Jens Axboe <axboe@kernel.dk>, Hannes Reinecke <hare@suse.de>,
        Jan Kara <jack@suse.cz>
Cc:     linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        linux-block@vger.kernel.org
Subject: [PATCH] block: Fix the maximum minor value is blk_alloc_ext_minor()
Date:   Sat, 26 Mar 2022 15:50:46 +0100
Message-Id: <cc17199798312406b90834e433d2cefe8266823d.1648306232.git.christophe.jaillet@wanadoo.fr>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

ida_alloc_range(..., min, max, ...) returns values from min to max,
inclusive.

So, NR_EXT_DEVT is a valid idx returned by blk_alloc_ext_minor().

This is an issue because in device_add_disk(), this value is used in:
   ddev->devt = MKDEV(disk->major, disk->first_minor);
and NR_EXT_DEVT is '(1 << MINORBITS)'.

So, should 'disk->first_minor' be NR_EXT_DEVT, it would overflow.

Fixes: 22ae8ce8b892 ("block: simplify bdev/disk lookup in blkdev_get")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
#define MKDEV(ma,mi)	(((ma) << MINORBITS) | (mi))

This patch is completely speculative, but it seems that idr_alloc() and
ida_alloc_range() don't have the same semantic regarding the upper bound.
idr_alloc() looks exclusive, while ida_alloc_range() is inclusive.

We changed from the first one to the other one in the commit in Fixes:.
---
 block/genhd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/genhd.c b/block/genhd.c
index c9a4fc90d3e9..b8b6759d670f 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -335,7 +335,7 @@ int blk_alloc_ext_minor(void)
 {
 	int idx;
 
-	idx = ida_alloc_range(&ext_devt_ida, 0, NR_EXT_DEVT, GFP_KERNEL);
+	idx = ida_alloc_range(&ext_devt_ida, 0, NR_EXT_DEVT - 1, GFP_KERNEL);
 	if (idx == -ENOSPC)
 		return -EBUSY;
 	return idx;
-- 
2.32.0