Date: Sun, 4 Nov 2018 15:36:53 -0500 (EST) From: Nicolas Pitre <nicolas.pitre@...aro.org> To: Ard Biesheuvel <ard.biesheuvel@...aro.org> cc: linux-arm-kernel@...ts.infradead.org, Russell King - ARM Linux <linux@...linux.org.uk>, keescook@...omium.org, Arnd Bergmann <arnd@...db.de>, re.emese@...il.com, kernel-hardening@...ts.openwall.com, labbott@...hat.com, marc.zyngier@....com Subject: Re: [RFC/RFT PATCH] ARM: smp: add support for per-task stack canaries On Sun, 4 Nov 2018, Ard Biesheuvel wrote: > On ARM, we currently only change the value of the stack canary when > switching tasks if the kernel was built for UP. On SMP kernels, this > is impossible since the stack canary value is obtained via a global > symbol reference, which means > a) all running tasks on all CPUs must use the same value > b) we can only modify the value when no kernel stack frames are live, > which is effectively never. > > So instead, use a GCC plugin to add a RTL pass that replaces each > reference to the address of the __stack_chk_guard symbol with an > expression that produces the address of the 'stack_canary' field > that is added to struct thread_info. This way, each task will use > its own randomized value. > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org> > --- > Known issues: > - The plugin requires the values of THREAD_SIZE and the offset of > stack_canary in struct thread_info at a really early stage in the > build, so I had to redefine them in the plugin source. Can't you rely on the asm-offsets.c mechanism before the plugin is used? > Questions: > - Are there any execution contexts except for the decompressor and the > EFI stub where we should disable this? KVM perhaps? Most certainly head-inflate-data.c... who knows if someone will use an XIP kernel on an SMP system someday. [...] > +#ifdef CONFIG_STACKPROTECTOR_PER_TASK > + BUILD_BUG_ON(offsetof(struct thread_info, stack_canary) != 100); > + BUILD_BUG_ON(THREAD_SIZE != 0x2000); You should at least define those values in a header file that gets included by both this code and the plugin source. But asm-offsets.c would be best. Nicolas
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