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: <aIPZXSnkDF5r-PR5@casper.infradead.org>
Date: Fri, 25 Jul 2025 20:22:05 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: 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>,
	Harry Yoo <harry.yoo@...cle.com>, 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 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)

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.