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

[PATCH] 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>
---
 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 4f2824a..4ecf2e7 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -562,6 +562,7 @@ static ssize_t writeback_limit_store(struct device *dev,
 	if (kstrtoull(buf, 10, &val))
 		return ret;
 
+	val = val & (~((1UL << (PAGE_SHIFT - 12)) - 1));
 	down_read(&zram->init_lock);
 	spin_lock(&zram->wb_limit_lock);
 	zram->bd_wb_limit = val;
-- 
2.34.1

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

[Sergey Senozhatsky]

On (25/11/18 15:39), Yuwen Chen 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.
> 
> 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>
> ---
>  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 4f2824a..4ecf2e7 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -562,6 +562,7 @@ static ssize_t writeback_limit_store(struct device *dev,
>  	if (kstrtoull(buf, 10, &val))
>  		return ret;
>  
> +	val = val & (~((1UL << (PAGE_SHIFT - 12)) - 1));
>  	down_read(&zram->init_lock);
>  	spin_lock(&zram->wb_limit_lock);
>  	zram->bd_wb_limit = val;

This patch is against unfixed writeback_limit_store() function,
it will need to be resubmitted once [1] lands.

[1] https://lore.kernel.org/linux-mm/20251118073000.1928107-1-senozhatsky@chromium.org

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

[Sergey Senozhatsky]

On (25/11/18 15:39), Yuwen Chen 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.
> 
> This patch fixes the issue by limiting bd_wb_limit to be an
> integer multiple of PAGE_SIZE / 4096.

I really wish we could just change wb code to use PAGE_SIZE not 4K
units [1], unfortunately it's probably too late to change that now.

[1] https://lore.kernel.org/linux-kernel/20251110052741.92031-1-senozhatsky@chromium.org

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

[Yuwen Chen]

On (25/11/18 15:39), Sergey Senozhatsky wrote:
> I really wish we could just change wb code to use PAGE_SIZE not 4K
> units [1], unfortunately it's probably too late to change that now.
>
> [1] https://lore.kernel.org/linux-kernel/20251110052741.92031-1-senozhatsky@chromium.org

I've taken a look at the patch you submitted before. This is indeed
a very frustrating thing. I'm not sure if we can make the change. If
we can change it to use PAGE_SIZE, it will reduce a lot of unnecessary
logic.