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[88.114.211.119]) by smtp.gmail.com with ESMTPSA id s16sm1076712ljj.35.2020.10.08.11.10.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 Oct 2020 11:10:17 -0700 (PDT) Subject: Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap() To: Jann Horn Cc: linux-hardening@vger.kernel.org, Andrew Morton , Linux-MM , kernel list References: <20201008165408.38228-1-toiwoton@gmail.com> From: Topi Miettinen Message-ID: <3413d0c8-17c7-fbae-e5fa-74a918e61239@gmail.com> Date: Thu, 8 Oct 2020 21:10:12 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8.10.2020 20.13, Jann Horn wrote: > On Thu, Oct 8, 2020 at 6:54 PM Topi Miettinen wrote: >> Writing a new value of 3 to /proc/sys/kernel/randomize_va_space >> enables full randomization of memory mappings created with mmap(NULL, >> ...). With 2, the base of the VMA used for such mappings is random, >> but the mappings are created in predictable places within the VMA and >> in sequential order. With 3, new VMAs are created to fully randomize >> the mappings. Also mremap(..., MREMAP_MAYMOVE) will move the mappings >> even if not necessary. > [...] >> + if ((flags & MREMAP_MAYMOVE) && randomize_va_space >= 3) { >> + /* >> + * Caller is happy with a different address, so let's >> + * move even if not necessary! >> + */ >> + new_addr = arch_mmap_rnd(); >> + >> + ret = mremap_to(addr, old_len, new_addr, new_len, >> + &locked, flags, &uf, &uf_unmap_early, >> + &uf_unmap); >> + goto out; >> + } > > You just pick a random number as the address, and try to place the > mapping there? Won't this fail if e.g. the old address range overlaps > with the new one, causing mremap_to() to bail out at "if (addr + > old_len > new_addr && new_addr + new_len > addr)"? Thanks for the review. I think overlap would be OK in this case and the check should be skipped. > Also, on Linux, the main program stack is (currently) an expanding > memory mapping that starts out being something like a couple hundred > kilobytes in size. If you allocate memory too close to the main > program stack, and someone then recurses deep enough to need more > memory, the program will crash. It sounds like your patch will > randomly make such programs crash. Right, especially on 32 bit systems this could be a real problem. I have limited the stack for tasks in the whole system to 2MB without problems (most use only 128kB) and on 48 bit virtual address systems the collision to 2MB area could be roughly 1/2^(48-21) which is a very small number. But perhaps this should be still be avoided by not picking an address too close to bottom of stack, say 64MB to be sure. It may also make this more useful also for 32 bit systems but overall I'm not so optimistic due to increased fragmentation. > Also, what's your strategy in general with regards to collisions with > existing mappings? Is your intention to just fall back to the classic > algorithm in that case? Maybe a different address could be tried (but not infinitely, say 5 times) and then fall back to classics. This would not be good for the ASLR but I haven't seen mremap() to be used much in my tests. > You may want to consider whether it would be better to store > information about free memory per subtree in the VMA tree, together > with the maximum gap size that is already stored in each node, and > then walk down the tree randomly, with the randomness weighted by free > memory in the subtrees, but ignoring subtrees whose gaps are too > small. And for expanding stacks, it might be a good idea for other > reasons as well (locking consistency) to refactor them such that the > size in the VMA tree corresponds to the maximum expansion of the stack > (and if an allocation is about to fail, shrink such stack mappings). This would reduce the randomization which I want to avoid. I think the extra overhead should be OK: if this is unacceptable for a workload or system constraints, don't use mode '3' but '2'. Instead of single global sysctl, this could be implemented as a new personality (or make this model the default and add a compatibility personality with no or less randomization), so it could be applied only for some tasks but not all. -Topi