Date: Mon, 4 Apr 2016 16:56:58 -0700 From: Kees Cook <keescook@...omium.org> To: Ralf Baechle <ralf@...ux-mips.org> Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Linux MIPS Mailing List <linux-mips@...ux-mips.org>, Matt Redfearn <matt.redfearn@...tec.com>, Aaro Koskinen <aaro.koskinen@...ia.com>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Alexander Sverdlin <alexander.sverdlin@...il.com>, LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, David Daney <ddaney@...iumnetworks.com>, Jaedon Shin <jaedon.shin@...il.com>, James Hogan <james.hogan@...tec.com>, Jonas Gorski <jogo@...nwrt.org>, Paul Burton <paul.burton@...tec.com> Subject: Re: [PATCH v2 00/11] MIPS relocatable kernel & KASLR On Mon, Apr 4, 2016 at 4:37 PM, Ralf Baechle <ralf@...ux-mips.org> wrote: > On Mon, Apr 04, 2016 at 12:46:29PM -0700, Kees Cook wrote: > >> This is great! Thanks for working on this! :) >> >> Without actually reading the code yet, I wonder if the x86 and MIPS >> relocs tool could be merged at all? Sounds like it might be more >> difficult though -- the relocation output is different and its storage >> location is different... >> >> > Restrictions: >> > * The new kernel is not allowed to overlap the old kernel, such that >> > the original kernel can still be booted if relocation fails. >> >> This sounds like physical-only relocation then? Is the virtual offset >> randomized as well (like arm64) or just physical (like x86 currently >> -- though there is a series to fix this). > > On MIPS we normally place the kernel in KSEG0 or XKPHYS which address > segments which are not mapped through the TLB so the difference is > kinda moot. Ah-ha, excellent. Does this mean that MIPS is effectively doing memory segmentation between userspace and kernel space (or some version of x86's SMEP/SMAP or ARM's PXN/PAN)? I don't know much about the MIPS architecture yet. What do I need to fill in on these tables for MIPS? http://kernsec.org/wiki/index.php/Exploit_Methods/Userspace_execution http://kernsec.org/wiki/index.php/Exploit_Methods/Userspace_data_usage > >> > * Relocation is supported only by multiples of 64k bytes. This >> > eliminates the need to handle R_MIPS_LO16 relocations as the bottom >> > 16bits will remain the same at the relocated address. >> >> IIUC, that's actually better than x86, which needs to be 2MB aligned. > > On MIPS a key concern was maintaining a reasonable size for the final > kernel image. The R_MIPS_LO16 relocatio records make a significant > portion of the relocations in a relocatable .o file, so we wanted to > get rid of them. This results in a relocation granularity of 64kB. > If we were truely, truely stingy we could come up with a relocation format > to save a few more bits but I doubt that'd make any sense. > >> > * In 64 bit kernels, relocation is supported only within the same 4Gb >> > memory segment as the kernel link address (CONFIG_PHYSICAL_START). >> > This eliminates the need to handle R_MIPS_HIGHEST and R_MIPS_HIGHER >> > relocations as the top 32bits will remain the same at the relocated >> > address. >> >> Interesting. Could the relocation code be updated in the future to >> bump the high addresses too? > > It could but yet again, the idea was to keep the size of the final > generated file under control. The R_MIPS_HIGHER and R_MIPS_HIGHEST > relocations can be discarded if we constrain the addresses to be in > a single 4GB segment. Removing this constraint would make a kernel > image much bigger so I suggested to add this restriction at least for > this initial version. Awesome, thanks for the details. -Kees -- Kees Cook Chrome OS & Brillo Security
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