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b=F/F6IJ0WYU2227XVUTbyUysSDfsSjyALh6JLpmO0SamjLFjFamrQeKIeAfgvKxbXFd/xLE lf90zFTm+QAxUslquEYdgpXgxEWiE4Gb46fjqUfWu9TtPBwpNGIcjnVv4jVX85eZmyRb8t A1zdXhqBcW45cY6PE3ySE6iZrOp6DqPKuQVW2YR+TebVXhhzgJTjE48AIx8fRBdba3EVmB j7rV75cne6jGxMmApyzzSOPG/pnF1JCi8j6xA302obAQL42UfzTn2nKWEA9CquFy/VSnlc wR8pIYktLrqXYrz0vzSwuyA7VH2Y6Xdfw04MidbQnnbfA08us7r6oCeb0inTeQ== Message-ID: Date: Mon, 29 Apr 2024 11:09:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] slub: Fixes freepointer encoding for single free To: Vlastimil Babka , 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> Content-Language: en-US From: Nicolas Bouchinet In-Reply-To: <6f977874-2a18-44ef-b207-9eb0baad9d66@suse.cz> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-GND-Sasl: nicolas.bouchinet@clip-os.org X-Stat-Signature: qrrmz7fw3yem48zb56m1yehuux66ur1k X-Rspamd-Queue-Id: B7D1F120003 X-Rspamd-Server: rspam10 X-Rspam-User: X-HE-Tag: 1714381796-195972 X-HE-Meta: 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 eqxcKyUj rieClxQarfxRF2rBtewKWaz4dVpR8x79qc+w+u8TRE2QwjtbLMLCa6iPQ/Zofek3obt5BRpZtUrTXl0jpJGWRunS9Luos8ubL7E/U8MTCnuNOU4r50sX4lhI5fHutYWwGYSTOXMVaWSrDwzzsGp/k+RWE97tXad5PpWwxuqjiGHCYCj+Ffu5zJn7WTzO+8NGiIcoEFkUY+C0l+f0ox1VAlMOF1nLYJFPjAQRtpwF0+A9+d48J93JlmSIJPPXzcS07iKhG1LAPxP6nIqLV2/gHMRNDUA== 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: Hi Vlastimil, thanks for your review and your proposal. On 4/29/24 10:52, Vlastimil Babka wrote: > On 4/25/24 5:14 PM, Chengming Zhou wrote: >> On 2024/4/25 23:02, Nicolas Bouchinet wrote: > Thanks for finding the bug and the fix! > >>> Hy, >>> >>> First of all, thanks a lot for your time. >>> >>> On 4/25/24 10:36, Chengming Zhou wrote: >>>> On 2024/4/24 20:47, Nicolas Bouchinet wrote: >>>>> From: Nicolas Bouchinet >>>>> >>>>> Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing >>>>> separately") splits single and bulk object freeing in two functions >>>>> slab_free() and slab_free_bulk() which leads slab_free() to call >>>>> slab_free_hook() directly instead of slab_free_freelist_hook(). >>>> Right. >>>> y not suitable for a stable-candidate fix we need >>>>> If `init_on_free` is set, slab_free_hook() zeroes the object. >>>>> Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are >>>>> set, the do_slab_free() slowpath executes freelist consistency >>>>> checks and try to decode a zeroed freepointer which leads to a >>>>> "Freepointer corrupt" detection in check_object(). >>>> IIUC, the "freepointer" can be checked on the free path only when >>>> it's outside the object memory. Here slab_free_hook() zeroed the >>>> freepointer and caused the problem. >>>> >>>> But why we should zero the memory outside the object_size? It seems >>>> more reasonable to only zero the object_size when init_on_free is set? >>> The original purpose was to avoid leaking information through the object and its metadata / tracking information as described in init_on_free initial Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options"). >>> >>> I have to admit I didn't read the entire lore about the original patchset yet, though it could be interesting to know a bit more the threat models, specifically regarding the object metadata init. >> Thank you for the reference! I also don't get why it needs to zero >> the metadata and tracking information. > Hmm taking a step back, it seems really suboptimal to initialize the > outside-object freepointer as part of init_on_free: > > - the freeing itself will always set it one way or another, in this case > free_to_partial_list() will do set_freepointer() after free_debug_processing() > > - we lose the ability to detect if the allocated slab object's user wrote to > it, which is a buffer overflow > > So the best option to me would be to adjust the init in slab_free_hook() to > avoid the outside-object freepointer similarly to how it avoids the red zone. > > We'll still not have the buffer overflow detection ability for bulk free > where slab_free_freelist_hook() will set the free pointer before we reach > the checks, but changing that is most likely not worth the trouble, and > especially not suitable for a stable-candidate fix we need here. It seems like a good alternative to me, I'll push a V2 patch with those changes. 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. > >>> The patch could also be optimized a bit by restricting set_freepointer() call to the `CONFIG_SLAB_FREELIST_HARDENED` option value. >>> >> Yeah. Maybe memcg_alloc_abort_single() needs this too. >> >> Thanks. >> >>> Thanks again, Nicolas >>> >>>> Thanks. >>>> >>>>> Object's freepointer thus needs to be properly set using >>>>> set_freepointer() after init_on_free. >>>>> >>>>> To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the >>>>> command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`. >>>>> >>>>> dmesg sample log: >>>>> [   10.708715] ============================================================================= >>>>> [   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G    B           T ): Freepointer corrupt >>>>> [   10.712695] ----------------------------------------------------------------------------- >>>>> [   10.712695] >>>>> [   10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2) >>>>> [   10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c >>>>> [   10.716698] >>>>> [   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................ >>>>> [   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................ >>>>> [   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................ >>>>> [   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................ >>>>> [   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                                      .... >>>>> [   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed >>>>> >>>>> Signed-off-by: Nicolas Bouchinet >>>>> --- >>>>>   mm/slub.c | 8 +++++++- >>>>>   1 file changed, 7 insertions(+), 1 deletion(-) >>>>> >>>>> diff --git a/mm/slub.c b/mm/slub.c >>>>> index 3aa12b9b323d9..71dbff9ad8f17 100644 >>>>> --- a/mm/slub.c >>>>> +++ b/mm/slub.c >>>>> @@ -4342,10 +4342,16 @@ static __fastpath_inline >>>>>   void slab_free(struct kmem_cache *s, struct slab *slab, void *object, >>>>>              unsigned long addr) >>>>>   { >>>>> +    bool init = false; >>>>> + >>>>>       memcg_slab_free_hook(s, slab, &object, 1); >>>>> +    init = slab_want_init_on_free(s); >>>>>   -    if (likely(slab_free_hook(s, object, slab_want_init_on_free(s)))) >>>>> +    if (likely(slab_free_hook(s, object, init))) { >>>>> +        if (init) >>>>> +            set_freepointer(s, object, NULL); >>>>>           do_slab_free(s, slab, object, object, 1, addr); >>>>> +    } >>>>>   } >>>>>     static __fastpath_inline Thanks again for your review, Nicolas