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Mon, 29 Apr 2024 20:22:52 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id A0RqOZwBMGYweQAAD6G6ig (envelope-from ); Mon, 29 Apr 2024 20:22:52 +0000 Message-ID: Date: Mon, 29 Apr 2024 22:22:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] slub: Fixes freepointer encoding for single free Content-Language: en-US To: Nicolas Bouchinet , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: cl@linux.com, penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, roman.gushchin@linux.dev, 42.hyeyoo@gmail.com, Xiongwei Song References: <5c34b253-b476-494a-96c9-fe3c95b9b74d@linux.dev> <6f977874-2a18-44ef-b207-9eb0baad9d66@suse.cz> <7074c0b4-62d4-444d-8e59-e23bbbccf9b8@clip-os.org> <83fda406-0340-4b7b-9f02-e9eb41c77f0e@clip-os.org> <73f80886-e390-4320-84dd-68e7cd7e8c62@linux.dev> <10c9a07a-1c6d-4ea7-8c1d-03a7dc5b29d8@clip-os.org> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Action: no action X-Rspamd-Queue-Id: F061FC0018 X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: fi4cse4bzoi8hk87nqbn43wdupms8h87 X-HE-Tag: 1714422174-546009 X-HE-Meta: U2FsdGVkX1/wVp8JnpWGZE/Pmabg5EoDUF/uYH02YoMUaKwT+jJdB+SSEfFYLoVB6QQExhDEJKkCpK3Sd1Dxsumf2xcqtBlA+jTOxbGdjSzqkmBSxH7+YVaw856N/ZVY8xuux78Bf2T6wYWlkyhz5+1gq2KJqwmDl89tekSGl0WOecZI1+swDppgv5T/mCKFTzEiVuBdzr/f/GCMjic3ohWXGnVD0NDc2Gh0GwXkBJpsauFCY1T92tS1Rujv+SCQZl0kD9wcFDyc+VOIDGSRBgyBRAA/qwFaFijbDQMaQ2uWbYyghfH66Ck8xbPiyrNGMW4eKSq/uDNsynRlNFXUkncAXI7E0ldQYza6Bwwa31YcOhpU88AcICqZtK2NfasXaFV2cpVBDsn6gOxPSOfGc7rXqSysdgePXFXtEvnvEaub+kxbR+93q+ctbMwUPdVZlKZWkay0NvSSAv/CMhvnVJazq4KphO9P/7b0wEUBzKXEyRAtZicFKP9fEaqTb/aPsLzfWA5bM32gT6wJJpeqps7RBpYfSRm0tkH32VWoznV0/kMvHe0W+hI0Dr71ahFzKVCfeHo6qHqyMRArtC/0eTixYCPXuus4XUTT1LwqH9mDTMVAGduwZHLIeBPHqyJncIJhnItIg3lYmbXCsYWzU12OEpzkmeJa8P6QR5W+1u9Pd5UJ5MxA/K2HM7198LV60pqjvsbnAIkCdaT9vOlKSJt1H17rGI9U+qwOcehT2VwV9iTNdrinCwHC6VBWE9U1wBc8j1tBkaU0T0u3OERHoF83ut1pF48UCymF9+r5uIcjoyhMqRjfG9MY4n30PKVaKqcST2VjgPdEAvwl9nJgYfVBLWaBf9rbs8u2Xsw+NBki7YvGVrJ/q20f2tPoBF+qx0cvThoWqyCzkljqq4CSPfITD9tBw9h5oRgpTS1yTs/ymmHkMRHimzwQ+CTRMm9Z1m52inrekp9ZcdOKjP4 mRSWvf/7 roRXELzv3nbQRY/zkQ27nA42bbX3DwVNN0I30n9R0v5f0tYevLH25Xji6+3yfygqZDqrjNahbolBPLD5rk/6zog396aYJ8e9uPd+9Yd1fdgisUXFebk4z7tV+799Weqz8RitRaPLCHqi3ACCtWOYkbShTA/oyZsPlCth84JGNwAaJMslJUlhPafmbkCuMEk2zozvLNiQlVbwrDeTKZND6hpxVhiicE8gspkLsn48sW96UAImkqfODkDogEouhflTpbNmZefAQAyIQbdYHQDTZE4pR0+OcN8WhkkpcwAJXI7theAjenw6B3TLk7286m4cxqyvALkAtVS63v2OYtLe36qhwlV1afDrJjV3UjZJw4B7mATY= 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: List-Subscribe: List-Unsubscribe: On 4/29/24 6:16 PM, Nicolas Bouchinet wrote: > On 4/29/24 16:52, Chengming Zhou wrote: >> On 2024/4/29 22:32, Nicolas Bouchinet wrote: >>> On 4/29/24 15:35, Chengming Zhou wrote: >>>> On 2024/4/29 20:59, Nicolas Bouchinet wrote: >>>>>> >>>>>> I help maintaining the Linux-Hardened patchset in which we have a slab object canary feature that helps detecting overflows. It is located just after the object freepointer. >>>>> I've tried a patch where the freepointer is avoided but it results in the same bug. It seems that the commit 0f181f9fbea8bc7ea ("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk allocations") inits the freepointer on allocation if init_on_free is set in order to return a clean initialized object to the caller. >>>>> >>>> Good catch! You may need to change maybe_wipe_obj_freeptr() too, >>>> I haven't tested this, not sure whether it works for you. :) >>>> >>>> diff --git a/mm/slub.c b/mm/slub.c >>>> index 3e33ff900d35..3f250a167cb5 100644 >>>> --- a/mm/slub.c >>>> +++ b/mm/slub.c >>>> @@ -3796,7 +3796,8 @@ static void *__slab_alloc_node(struct kmem_cache *s, >>>>   static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s, >>>>                                                     void *obj) >>>>   { >>>> -       if (unlikely(slab_want_init_on_free(s)) && obj) >>>> +       if (unlikely(slab_want_init_on_free(s)) && obj && >>>> +           !freeptr_outside_object(s)) >>>>                  memset((void *)((char *)kasan_reset_tag(obj) + s->offset), >>>>                          0, sizeof(void *)); >>>>   } >>>> >>>> Thanks! >>> Indeed since check_object() avoids objects for which freepointer is in the object and since val is equal to SLUB_RED_ACTIVE in our specific case it should work. Do you want me to add you as Co-authored ? >>> >> Ok, it's great. Thanks! > > Now I think of it, doesn't it seems a bit odd to only properly > init_on_free object's freepointer only if it's inside the object ? IMHO > it is equally necessary to avoid information leaking about the > freepointer whether it is inside or outside the object. > I think it break the semantic of the commit 0f181f9fbea8bc7ea > ("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk > allocations") ? Hm, AFAIU, wiping inside object prevents misuse of some buggy kernel code that would allocate and accidentally leak prior content (including the in-object freepointer) somewhere the attacker can read. Now for wiping the freepointer outside the object to be useful it would have assume said leak-prone code to additionally be reading past the allocated object size, i.e. a read buffer overflow. That to me seems to be a much more rare combination, and also in that case such code could also likely read even further past the object, i.e. leak the next object's data? IOW I don't think it buys us much additional security protection in practice? > Thanks. >