linux-kernel - Re: [PATCH v2] mm/slub: Fix incorrect interpretation of s->offset

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Message-ID: <20200427142204.GG29705@bombadil.infradead.org>
Date:   Mon, 27 Apr 2020 07:22:04 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Changbin Du <changbin.du@...il.com>
Subject: Re: [PATCH v2] mm/slub: Fix incorrect interpretation of s->offset

On Mon, Apr 27, 2020 at 10:08:22AM -0400, Waiman Long wrote:
> In a couple of places in the slub memory allocator, the code uses
> "s->offset" as a check to see if the free pointer is put right after the
> object. That check is no longer true with commit 3202fa62fb43 ("slub:
> relocate freelist pointer to middle of object").
> 
> As a result, echoing "1" into the validate sysfs file, e.g. of dentry,
> may cause a bunch of "Freepointer corrupt" error reports like the
> following to appear with the system in panic afterwards.
> 
> [   38.579769] =============================================================================
> [   38.580845] BUG dentry(666:pmcd.service) (Tainted: G    B): Freepointer corrupt
> [   38.581948] -----------------------------------------------------------------------------
> 
> To fix it, use the check "s->offset == s->inuse" in the new helper
> function freeptr_after_object() instead. Also add another helper function
> get_info_end() to return the end of info block (inuse + free pointer
> if not overlapping with object).
> 
> Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
> Signed-off-by: Waiman Long <longman@...hat.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>