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Date: Mon, 16 May 2022 15:29:33 +0800 From: xiujianfeng <xiujianfeng@...wei.com> To: Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, <benh@...nel.crashing.org>, <christophe.leroy@...roup.eu>, <mark.rutland@....com>, <paulus@...ba.org>, <tglx@...utronix.de> CC: <linux-hardening@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org> Subject: Re: [PATCH -next] powerpc: add support for syscall stack randomization 在 2022/5/12 21:17, Michael Ellerman 写道: > xiujianfeng <xiujianfeng@...wei.com> writes: >> 在 2022/5/10 17:23, Nicholas Piggin 写道: >>> Excerpts from Xiu Jianfeng's message of May 5, 2022 9:19 pm: >>>> Add support for adding a random offset to the stack while handling >>>> syscalls. This patch uses mftb() instead of get_random_int() for better >>>> performance. > ... >>>> @@ -405,6 +407,7 @@ interrupt_exit_user_prepare_main(unsigned long ret, struct pt_regs *regs) >>>> >>>> /* Restore user access locks last */ >>>> kuap_user_restore(regs); >>>> + choose_random_kstack_offset(mftb() & 0xFF); >>>> >>>> return ret; >>>> } >>> So this seems to be what x86 and s390 do, but why are we choosing a >>> new offset for every interrupt when it's only used on a syscall? >>> I would rather you do what arm64 does and just choose the offset >>> at the end of system_call_exception. >> thanks for you suggestion, will do in v2. >>> I wonder why the choose is separated from the add? I guess it's to >>> avoid a data dependency for stack access on an expensive random >>> function, so that makes sense (a comment would be nice in the >>> generic code). >>> >>> I don't actually know if mftb() is cheaper here than a RNG. It >>> may not be conditioned all that well either. I would be tempted >> #if defined(__powerpc64__) && (defined(CONFIG_PPC_CELL) || >> defined(CONFIG_E500)) >> #define mftb() ({unsigned long rval; \ >> asm volatile( \ >> "90: mfspr %0, %2;\n" \ >> ASM_FTR_IFSET( \ >> "97: cmpwi %0,0;\n" \ >> " beq- 90b;\n", "", %1) \ >> : "=r" (rval) \ >> : "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL) : >> "cr0"); \ >> rval;}) >> #elif defined(CONFIG_PPC_8xx) >> #define mftb() ({unsigned long rval; \ >> asm volatile("mftbl %0" : "=r" (rval)); rval;}) >> #else >> #define mftb() ({unsigned long rval; \ >> asm volatile("mfspr %0, %1" : \ >> "=r" (rval) : "i" (SPRN_TBRL)); >> rval;}) >> #endif /* !CONFIG_PPC_CELL */ >> >> there are 3 implementations of mftb() in >> arch/powerpc/include/asm/vdso/timebase.h, >> >> the last two cases have only one instruction, It's obviously cheaper >> than get_random_int, > Just because it's one instruction doesn't mean it's obviously cheaper. > On some CPUs mftb takes 10s of cycles, and can also stall the pipeline. > > But looking at get_random_u32() it does look pretty complicated, it > takes a lock and so on. It's also silly to call get_random_u32() for > 4-bits of randomness. > > My initial impression was that mftb() is too predictable to be useful > against a determined attacker. But looking closer I see that > choose_random_kstack_offset() xor's the value we pass with the existing > value. So that makes me less worried about using mftb(). > > We could additionally call choose_random_kstack_offset(get_random_int()) > less regularly, eg. during context switch. But I guess that's too > infrequent to actually make any difference. > > But limiting it to 4-bits of randomness seems insufficient. It seems > like we should allow the full 6 (10) bits, and anyone turning this > option on should probably also consider increasing their stack size. > > Also did you check the help text about stack-protector under > HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET? thanks for your reminder, will disable stack-protector for interrupt.c in v2, just like arm64 do. > > cheers
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