From mboxrd@z Thu Jan 1 00:00:00 1970 From: ard.biesheuvel@linaro.org (Ard Biesheuvel) Date: Fri, 5 Feb 2016 17:54:46 +0100 Subject: [PATCH v5sub2 1/8] arm64: add support for module PLTs In-Reply-To: <20160205164648.GH6076@e104818-lin.cambridge.arm.com> References: <1454332178-4414-1-git-send-email-ard.biesheuvel@linaro.org> <1454332178-4414-2-git-send-email-ard.biesheuvel@linaro.org> <20160204151303.GB6076@e104818-lin.cambridge.arm.com> <20160205154234.GF6076@e104818-lin.cambridge.arm.com> <20160205160054.GG6076@e104818-lin.cambridge.arm.com> <20160205164648.GH6076@e104818-lin.cambridge.arm.com> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 5 February 2016 at 17:46, Catalin Marinas wrote: > On Fri, Feb 05, 2016 at 05:20:14PM +0100, Ard Biesheuvel wrote: >> On 5 February 2016 at 17:00, Catalin Marinas wrote: >> > On Fri, Feb 05, 2016 at 04:53:10PM +0100, Ard Biesheuvel wrote: >> >> On 5 February 2016 at 16:42, Catalin Marinas wrote: >> >> > On Thu, Feb 04, 2016 at 04:31:59PM +0100, Ard Biesheuvel wrote: >> >> >> On 4 February 2016 at 16:13, Catalin Marinas wrote: >> >> >> > On Mon, Feb 01, 2016 at 02:09:31PM +0100, Ard Biesheuvel wrote: >> >> >> >> This adds support for emitting PLTs at module load time for relative >> >> >> >> branches that are out of range. This is a prerequisite for KASLR, which >> >> >> >> may place the kernel and the modules anywhere in the vmalloc area, >> >> >> >> making it more likely that branch target offsets exceed the maximum >> >> >> >> range of +/- 128 MB. >> >> >> > >> >> >> > Any downside to trying to keep the kernel+modules coupled together so >> >> >> > that we avoid the PLT? >> >> >> >> >> >> First of all, note that it is unlikely that the PLTs are ever required >> >> >> in practice, considering that either >> >> >> a) the kernel is loaded at the default location right at the base of >> >> >> the vmalloc range, and in this case, the module space is reserved for >> >> >> modules only, or >> >> >> b) the kernel is loaded at some random offset in the 240+ GB vmalloc >> >> >> space, and it is unlikely that all VMA space around the kernel happens >> >> >> to be given out to non-randomized vmalloc/ioremap allocations >> >> > >> >> > My worry is that we merge some code that's rarely tested. >> >> >> >> I understand. But unfortunately, having corner cases that are unlikely >> >> but not impossible comes with the territory of randomization. >> >> >> >> Alternatively, we could take the performance hit if KASLR is in effect >> >> and allocate each module completely randomly as well. This way, the >> >> code is always exercised (for now), and we can always backpedal later >> >> if the performance is measurably worse. >> > >> > I'm fine with this. You can post it as a separate patch that we can >> > easily revert/modify later (like turning it into a config option). >> >> OK, I will hack something up > > If it's simpler, you can just add a config option but defaulting to the > full vmalloc space for modules. > What would be the simplest is to randomize the 128 MB module region as a whole, and either put it close to the kernel (which is what I am doing now), or put it at a random offset inside the vmalloc space, in which case all branches will be resolved via PLTs. My suggestion to randomize each module_alloc() call separately is actually not that straight-forward. So what I propose now is to keep a single module_load_offset that randomizes the base of the region, and add a default-n config option that shrinks the interval it is chosen from so that PLTs are usually not needed. -- Ard.