linux-kernel - Re: [PATCH 3/3] dm-pcache: avoid leaking invalid metadata in pcache_meta_find

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Message-ID: <19a3f8ae9db.8a18892d3330798.1127794710232272337@linux.beauty>
Date: Sat, 01 Nov 2025 21:10:59 +0800
From: Li Chen <me@...ux.beauty>
To: "Zheng Gu" <cengku@...il.com>
Cc: "dm-devel" <dm-devel@...ts.linux.dev>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"Dongsheng Yang" <dongsheng.yang@...ux.dev>
Subject: Re: [PATCH 3/3] dm-pcache: avoid leaking invalid metadata in
 pcache_meta_find_latest()

Hi Zheng,

 ---- On Fri, 31 Oct 2025 10:01:23 +0800  Zheng Gu <cengku@...il.com> wrote --- 
 >> On Thu, Oct 30, 2025 at 8:36 PM Li Chen <me@...ux.beauty> wrote:From: Li Chen <chenl311@...natelecom.cn>
 >> 
 >> Before this change pcache_meta_find_latest() was copying each
 >> slot directly into meta_ret while scanning. If no valid slot
 >> was found and the function returned NULL, meta_ret still held
 >> whatever was last copied (possibly CRC-bad). Later users
 >> (e.g. cache_segs_init) could mistakenly trust that data.
 > 
 > This functions is * __must_check*, users must check the return value first before touching the meta_ret, so it should not be a problem here.

Right now, the callers only check the return value with IS_ERR(). If the
function returns NULL instead of an error pointer, a caller like
cache_info_init() will assume that no valid cache_info was found because all cache_info are
corrupted. Instead, it will try to init a new one, and then return 0 (success), 
https://github.com/torvalds/linux/blob/master/drivers/md/dm-pcache/cache.c#L61

Later, cache_tail_init() will access cache->cache_info.flags. But in this
path all cache_info may have already been corrupted, and the CRCs are mismatched
(https://github.com/torvalds/linux/blob/ba36dd5ee6fd4643ebbf6ee6eefcecf0b07e35c7/drivers/md/dm-pcache/pcache_internal.h#L97),
so flags may contain garbage.

This commit fixes this issue by allocating a temp buffer with kvmalloc, so meta_ret would never
contain corrupted values.

Regards,

Li