linux-kernel - Re: [PATCH] zram: unsafe zram_get_element call in zram_read

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Date:   Tue, 7 Nov 2023 19:40:41 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Vasily Averin <vasily.averin@...ux.dev>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH] zram: unsafe zram_get_element call in zram_read_page()

On (23/11/07 16:39), Sergey Senozhatsky wrote:
> On (23/11/06 22:54), Vasily Averin wrote:
> > @@ -1362,14 +1362,14 @@ static int zram_read_page(struct zram *zram, struct page *page, u32 index,
> >  		ret = zram_read_from_zspool(zram, page, index);
> >  		zram_slot_unlock(zram, index);
> >  	} else {
> > +		unsigned long entry = zram_get_element(zram, index);
> >  		/*
> >  		 * The slot should be unlocked before reading from the backing
> >  		 * device.
> >  		 */
> >  		zram_slot_unlock(zram, index);
> >  
> > -		ret = read_from_bdev(zram, page, zram_get_element(zram, index),
> > -				     parent);
> > +		ret = read_from_bdev(zram, page, entry, parent);
> 
> Hmmm,
> We may want to do more here. Basically, we probably need to re-confirm
> after read_from_bdev() that the entry at index still has ZRAM_WB set
> and, if so, that it points to the same blk_idx. IOW, check that it has
> not been free-ed and re-used under us.
> 
> Minchan, what do you think? Is that scenario possible?

Basically

---

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index f5e3756fc21a..2d69f6efcee3 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1364,14 +1364,21 @@ static int zram_read_page(struct zram *zram, struct page *page, u32 index,
 		ret = zram_read_from_zspool(zram, page, index);
 		zram_slot_unlock(zram, index);
 	} else {
+		unsigned long idx = zram_get_element(zram, index);
 		/*
 		 * The slot should be unlocked before reading from the backing
 		 * device.
 		 */
 		zram_slot_unlock(zram, index);
 
-		ret = read_from_bdev(zram, page, zram_get_element(zram, index),
-				     parent);
+		ret = read_from_bdev(zram, page, idx, parent);
+		if (ret == 0) {
+			zram_slot_lock(zram, index);
+			if (!zram_test_flag(zram, index, ZRAM_WB) ||
+			    idx != zram_get_element(zram, index))
+				ret = -EINVAL;
+			zram_slot_unlock(zram, index);
+		}
 	}
 
 	/* Should NEVER happen. Return bio error if it does. */