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: <aIO6m2C8K4SrJ6mp@casper.infradead.org>
Date: Fri, 25 Jul 2025 18:10:51 +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 06:47:01PM +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)

> >  
> > +/*
> > + * object - should be a valid object.
> > + * check_valid_pointer(s, slab, object) should be true.
> > + */

This comment is very confusing.  It tries to ape kernel-doc style,
but if it were kernel-doc, the word before the hyphen should be the name
of the function, and it isn't.  If we did use kernel-doc for this, we'd
use @object to denote that we're documenting the argument.

But I don't see the need to pretend this is related to kernel-doc.  This
would be better:

/*
 * 'object' must be a valid pointer into this slab.  ie
 * check_valid_pointer() would return true
 */

I'm sure better wording for that is possible ...

> >  	if (!check_valid_pointer(s, slab, object)) {
> > -		object_err(s, slab, object, "Freelist Pointer check fails");
> > +		slab_err(s, slab, "Invalid object pointer 0x%p", object);
> >  		return 0;

No, the error message is now wrong.  It's not an object, it's the
freelist pointer.

		slab_err(s, slab, "Invalid freelist pointer %p", object);

(the 0x%p is wrong because it will print 0x twice)

But I think there are even more things wrong here.  Like slab_err() is
not nerely as severe as slab_bug(), which is what used to be called.
And object_err() adds a taint, which this skips.

Altogether, this is a poorly thought out patch and should be dropped.