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Date: Wed, 27 May 2026 09:24:14 +0200
From: Christoph Hellwig <hch@lst.de>
To: Cunlong Li <shenxiaogll@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Jens Axboe <axboe@kernel.dk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>, linux-block@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH v2 1/2] zram: fix use-after-free in
 zram_bvec_write_partial()
Message-ID: <20260527072414.GA17856@lst.de>
References: <20260527-zram-v2-0-2fb84b054b5c@gmail.com> <20260527-zram-v2-1-2fb84b054b5c@gmail.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Wed, May 27, 2026 at 12:49:24PM +0800, Cunlong Li wrote:
> zram_read_page() picks the sync or async backing device read path
> based on whether the parent bio is NULL.  zram_bvec_write_partial()
> passes its parent bio down, so for ZRAM_WB slots the read is
> dispatched asynchronously and zram_read_page() returns 0 while the
> bio is still in flight.  The caller then runs memcpy_from_bvec(),
> zram_write_page() and __free_page() on the buffer, leaving the
> async read to write into a freed page.
> 
> zram_bvec_read_partial() was switched to NULL in commit 4e3c87b9421d
> ("zram: fix synchronous reads") for the same reason; the
> write_partial counterpart was missed.
> 
> Fixes: 4e3c87b9421d ("zram: fix synchronous reads")

That's just the last patch touching the line.  This bio chaining goes
further back.  AFAICS all the way to introducing backing device support
in: 8e654f8fbff5 ("zram: read page from backing device")

The patch itself looks good, though:

Reviewed-by: Christoph Hellwig <hch@lst.de>