2022-03-26 20:19:57

by Christophe JAILLET

Subject: [PATCH] block: Fix the maximum minor value is blk_alloc_ext_minor()

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 <[email protected]>
---
#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

2022-03-28 10:24:54

by Jan Kara

Subject: Re: [PATCH] block: Fix the maximum minor value is blk_alloc_ext_minor()

On Sat 26-03-22 15:50:46, Christophe JAILLET wrote:
> 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 <[email protected]>

Indeed. The patch looks good to me so feel free to add:

Reviewed-by: Jan Kara <[email protected]>

> ---
> #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:.

Yes, this difference is really a landmine. Matthew, why is the semantics of
max parameter for idr_alloc() different from ida_alloc_range() or say
idr_alloc_u32()? It is really easy to introduce subtle bugs with this...

Honza

> ---
> 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
>
--
Jan Kara <[email protected]>
SUSE Labs, CR

2022-03-28 22:39:32

by Jens Axboe

Subject: Re: [PATCH] block: Fix the maximum minor value is blk_alloc_ext_minor()

On Sat, 26 Mar 2022 15:50:46 +0100, Christophe JAILLET wrote:
> 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)'.
>
> [...]

Applied, thanks!

[1/1] block: Fix the maximum minor value is blk_alloc_ext_minor()
commit: d1868328dec5ae2cf210111025fcbc71f78dd5ca

Best regards,
--
Jens Axboe