[PATCH v2] zram: Fix the issue that the write - back limits might overflow

[PATCH v2] zram: Fix the issue that the write - back limits might overflow

[Yuwen Chen]

Since the value of bd_wb_limit is an unsigned number, when the
page size is larger than 4 KB, it may cause an out-of-bounds situation.

This patch fixes the issue by limiting bd_wb_limit to be an
integer multiple of PAGE_SIZE / 4096.

Fixes: 1d69a3f8ae77e ("zram: idle writeback fixes and cleanup")
Signed-off-by: Yuwen Chen <ywen.chen@foxmail.com>
---
Changes in v2:
  - Rebase the patch to adapt to the latest version.

 drivers/block/zram/zram_drv.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 8a13729..5780604 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -579,6 +579,7 @@ static ssize_t writeback_limit_store(struct device *dev,
 	if (kstrtoull(buf, 10, &val))
 		return ret;
 
+	val = val & (~((1UL << (PAGE_SHIFT - 12)) - 1));
 	down_write(&zram->init_lock);
 	zram->bd_wb_limit = val;
 	up_write(&zram->init_lock);
-- 
2.34.1

Re: [PATCH v2] zram: Fix the issue that the write - back limits might overflow

[Andrew Morton]

On Tue, 18 Nov 2025 16:15:50 +0800 Yuwen Chen <ywen.chen@foxmail.com> wrote:

> Since the value of bd_wb_limit is an unsigned number, when the
> page size is larger than 4 KB, it may cause an out-of-bounds situation.

Please update the changelog to fully describe this out-of-bounds
situation.  Where does it occur, why, how, with what effects?

> This patch fixes the issue by limiting bd_wb_limit to be an
> integer multiple of PAGE_SIZE / 4096.

I don't see at all how clearing the lower 11 bits is related to "page
size is larger than 4 KB".

All quite confusing!

> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -579,6 +579,7 @@ static ssize_t writeback_limit_store(struct device *dev,
>  	if (kstrtoull(buf, 10, &val))
>  		return ret;
>  
> +	val = val & (~((1UL << (PAGE_SHIFT - 12)) - 1));

Needs a little comment, I suggest.

>  	down_write(&zram->init_lock);
>  	zram->bd_wb_limit = val;
>  	up_write(&zram->init_lock);
> -- 
> 2.34.1