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[195.135.220.15]) by mx.google.com with ESMTPS id k29si1330378edb.395.2019.04.16.04.41.07 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Apr 2019 04:41:07 -0700 (PDT) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 712CEAF8F; Tue, 16 Apr 2019 11:41:07 +0000 (UTC) Subject: Re: [patch V4 01/32] mm/slab: Fix broken stack trace storage To: Thomas Gleixner , Andy Lutomirski Cc: Josh Poimboeuf , LKML , X86 ML , Sean Christopherson , Andrew Morton , Pekka Enberg , Linux-MM , David Rientjes References: <20190414155936.679808307@linutronix.de> <20190414160143.591255977@linutronix.de> <20190415132339.wiqyzygqklliyml7@treble> <20190415161657.2zwboghblj5ducux@treble> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 4/15/19 11:22 PM, Thomas Gleixner wrote: > On Mon, 15 Apr 2019, Andy Lutomirski wrote: >> On Mon, Apr 15, 2019 at 9:17 AM Josh Poimboeuf wrote: >>> On Mon, Apr 15, 2019 at 06:07:44PM +0200, Thomas Gleixner wrote: >>>>> Looks like stack_trace.nr_entries isn't initialized? (though this code >>>>> gets eventually replaced by a later patch) >>>> >>>> struct initializer initialized the non mentioned fields to 0, if I'm not >>>> totally mistaken. >>> >>> Hm, it seems you are correct. And I thought I knew C. >>> >>>>> Who actually reads this stack trace? I couldn't find a consumer. >>>> >>>> It's stored directly in the memory pointed to by @addr and that's the freed >>>> cache memory. If that is used later (UAF) then the stack trace can be >>>> printed to see where it was freed. >>> >>> Right... but who reads it? >> >> That seems like a reasonable question. After some grepping and some >> git searching, it looks like there might not be any users. I found > > Anymore. There was something 10y+ ago. In theory it can be useful in a crash dump. But I don't see any related debugging check that would trigger a panic, in order to get one. >> SLAB_STORE_USER, but that seems to be independent. >> >> So maybe the whole mess should just be deleted. If anyone ever >> notices, they can re-add it better. > > No objections from my side, but the mm people might have opinions. Anyone who wants to debug wrong slab usage probably uses SLUB anyway, so I don't think it's a problem to remove broken SLAB debugging. Perhaps even SLAB itself will be removed soon if there's performance data supporting it [1]. [1] https://lore.kernel.org/linux-mm/20190412112816.GD18914@techsingularity.net/T/#u > Thanks, > > tglx >