[dm-devel] [PATCH] fix an integer overflow in __blkdev_sectors_to_bio_pages

[Mikulas Patocka]

On Mon, 14 Aug 2017, Damien Le Moal wrote:

> On Sun, 2017-08-13 at 22:47 -0400, Mikulas Patocka wrote:
> > 
> > On Wed, 9 Aug 2017, hch at lst.de wrote:
> > 
> > > Does commit 615d22a51c04856efe62af6e1d5b450aaf5cc2c0
> > > "block: Fix __blkdev_issue_zeroout loop" fix the issue for you?
> > > 
> > > --
> > > dm-devel mailing list
> > > dm-devel at redhat.com
> > > https://www.redhat.com/mailman/listinfo/dm-devel
> > 
> > I think that patch is incorrect. sector_t may be a 32-bit type and 
> > nr_sects << 9 may overflow.
> > 
> > static unsigned int __blkdev_sectors_to_bio_pages(sector_t nr_sects)
> > {
> >        sector_t bytes = (nr_sects << 9) + PAGE_SIZE - 1;
> > 
> >        return min(bytes >> PAGE_SHIFT, (sector_t)BIO_MAX_PAGES);
> > }
> > 
> > Mikulas
> 
> Mikulas,
> 
> Does the follwing patch fix the problem ?
> 
> From 947b3cf41e759b2b23f684e215e651d0c8037f88 Mon Sep 17 00:00:00 2001
> From: Damien Le Moal <damien.lemoal at wdc.com>
> Date: Mon, 14 Aug 2017 13:01:16 +0900
> Subject: [PATCH] block: Fix __blkdev_sectors_to_bio_pages()
> 
> On 32bit systems where sector_t is a 32bits type, the calculation of
> bytes may overflow. Use the u64 type for the local calculation to avoid
> overflows.
> 
> Signed-off-by: Damien Le Moal <damien.lemoal at wdc.com>
> ---
>  block/blk-lib.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 3fe0aec90597..ccf22dba21f0 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -269,9 +269,9 @@ static int __blkdev_issue_write_zeroes(struct block_device
> *bdev,
>   */
>  static unsigned int __blkdev_sectors_to_bio_pages(sector_t nr_sects)
>  {
> -	sector_t bytes = (nr_sects << 9) + PAGE_SIZE - 1;
> +	u64 bytes = ((u64)nr_sects << 9) + PAGE_SIZE - 1;
>  
> -	return min(bytes >> PAGE_SHIFT, (sector_t)BIO_MAX_PAGES);
> +	return min(bytes >> PAGE_SHIFT, (u64)BIO_MAX_PAGES);
>  }
>  

It's OK, but it is not needed to use 64-bit arithmetic here if all we need 
is to shift the value right. Here I submit a simplified patch, using the 
macro DIV_ROUND_UP_SECTOR_T (the macro gets optimized to just an addition 
and right shift).



From: Mikulas Patocka <mpatocka at redhat.com>

Fix possible integer overflow in __blkdev_sectors_to_bio_pages if sector_t 
is 32-bit.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Fixes: 615d22a51c04 ("block: Fix __blkdev_issue_zeroout loop")

---
 block/blk-lib.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux-2.6/block/blk-lib.c
===================================================================
--- linux-2.6.orig/block/blk-lib.c
+++ linux-2.6/block/blk-lib.c
@@ -269,9 +269,9 @@ static int __blkdev_issue_write_zeroes(s
  */
 static unsigned int __blkdev_sectors_to_bio_pages(sector_t nr_sects)
 {
-	sector_t bytes = (nr_sects << 9) + PAGE_SIZE - 1;
+	sector_t pages = DIV_ROUND_UP_SECTOR_T(nr_sects, PAGE_SIZE / 512);
 
-	return min(bytes >> PAGE_SHIFT, (sector_t)BIO_MAX_PAGES);
+	return min(pages, (sector_t)BIO_MAX_PAGES);
 }
 
 /**