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Date: Wed, 11 May 2022 16:34:13 +0800 From: xiujianfeng <xiujianfeng@...wei.com> To: Nicholas Piggin <npiggin@...il.com>, <benh@...nel.crashing.org>, <christophe.leroy@...roup.eu>, <mark.rutland@....com>, <mpe@...erman.id.au>, <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 Hi, 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. > Hey, very nice. > >> Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com> >> --- >> arch/powerpc/Kconfig | 1 + >> arch/powerpc/kernel/interrupt.c | 3 +++ >> 2 files changed, 4 insertions(+) >> >> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig >> index 5fc9153927ac..7e04c9f80cbc 100644 >> --- a/arch/powerpc/Kconfig >> +++ b/arch/powerpc/Kconfig >> @@ -192,6 +192,7 @@ config PPC >> select HAVE_ARCH_KASAN if PPC32 && PPC_PAGE_SHIFT <= 14 >> select HAVE_ARCH_KASAN_VMALLOC if PPC32 && PPC_PAGE_SHIFT <= 14 >> select HAVE_ARCH_KFENCE if PPC_BOOK3S_32 || PPC_8xx || 40x >> + select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET >> select HAVE_ARCH_KGDB >> select HAVE_ARCH_MMAP_RND_BITS >> select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT >> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c >> index 784ea3289c84..459385769721 100644 >> --- a/arch/powerpc/kernel/interrupt.c >> +++ b/arch/powerpc/kernel/interrupt.c >> @@ -4,6 +4,7 @@ >> #include <linux/err.h> >> #include <linux/compat.h> >> #include <linux/sched/debug.h> /* for show_regs */ >> +#include <linux/randomize_kstack.h> >> >> #include <asm/kup.h> >> #include <asm/cputime.h> >> @@ -82,6 +83,7 @@ notrace long system_call_exception(long r3, long r4, long r5, >> >> kuap_lock(); >> >> + add_random_kstack_offset(); >> regs->orig_gpr3 = r3; >> >> if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG)) > This looks like the right place. I wonder why other interrupts don't > get the same treatment. Userspace can induce the kernel to take a > synchronous interrupt, or wait for async ones. Smaller surface area > maybe but certain instruction emulation for example could result in > significant logic that depends on user state. Anyway that's for > hardening gurus to ponder. > >> @@ -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, do you mean the first one? It looks like cheaper too, or am I missing something? > to measure. 64-bit *may* be able to use a bit more than 256 > bytes of stack too -- we have 16 byte alignment minimum so this > gives only 4 bits of randomness AFAIKS. KSTACK_OFFSET_MAX limits entropy to 10 bits, and THREAD_SHIFT is 14 for ppc64 and 13 for ppc32, so can we just use 0x1FF for both or 0x1FF for 64bit and 0xFF for 32bit? what is your suggestion? thanks. > > Thanks, > Nick > .
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