linux-kernel - Re: [PATCH v2] mm: slub: avoid deref of free pointer in sanity checks if object is invalid

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Message-ID: <aIQJ8QQQbA2a-x2n@hyeyoo>
Date: Sat, 26 Jul 2025 07:49:21 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, Li Qiong <liqiong@...china.com>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm: slub: avoid deref of free pointer in sanity
 checks if object is invalid

On Fri, Jul 25, 2025 at 08:22:05PM +0100, Matthew Wilcox wrote:
> On Fri, Jul 25, 2025 at 06:10:51PM +0100, Matthew Wilcox wrote:
> > On Fri, Jul 25, 2025 at 06:47:01PM +0200, Vlastimil Babka wrote:
> > > On 7/25/25 08:49, Li Qiong wrote:
> > > > For debugging, object_err() prints free pointer of the object.
> > > > However, if check_valid_pointer() returns false for a object,
> > > > dereferncing `object + s->offset` can lead to a crash. Therefore,
> > > > print the object's address in such cases.
> > 
> > I don't know where this patch came from (was it cc'd to linux-mm? i
> > don't see it)

Oops, I missed this email when I was replying to the previous email.

> I've spent some more time thinking about this and I now believe that
> there are several calls to object_err() that can be passed a bad
> pointer:
> 
> freelist_corrupted()
> check_object()
> on_freelist()
> alloc_consistency_checks()
> free_consistency_checks()
> 
> so I think this line of attack is inappropriate. Instead, I think we
> need to make object_err() resilient against wild pointers.  Specifically,
> avoid doing risky things in print_trailer() if object is not within slab.

Making object_err() more resilient sounds good to me.

-- 
Cheers,
Harry / Hyeonggon