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Date: Wed, 11 May 2022 16:36:47 +0800 From: xiujianfeng <xiujianfeng@...wei.com> To: Kees Cook <keescook@...omium.org>, Nicholas Piggin <npiggin@...il.com> CC: <benh@...nel.crashing.org>, <christophe.leroy@...roup.eu>, <mark.rutland@....com>, <mpe@...erman.id.au>, <paulus@...ba.org>, <tglx@...utronix.de>, <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 2022/5/11 0:19, Kees Cook 写道: > On Tue, May 10, 2022 at 07:23:46PM +1000, Nicholas Piggin wrote: >> 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. > Agreed! :) > >>> [...] >>> @@ -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. > I welcome it being used for any userspace controllable entry to the > kernel! :) > > Also, related, have you validated the result using the LKDTM test? > See tools/testing/selftests/lkdtm/stack-entropy.sh not yet, I tested it by printing the address of local variable directly, will do before I send v2, thanks. >>> @@ -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. >> >> 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). > How does this read? I can send a "real" patch if it looks good: > > > diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h > index 1468caf001c0..ad3e80275c74 100644 > --- a/include/linux/randomize_kstack.h > +++ b/include/linux/randomize_kstack.h > @@ -40,8 +40,11 @@ DECLARE_PER_CPU(u32, kstack_offset); > */ > #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF) > > -/* > - * These macros must be used during syscall entry when interrupts and > +/** > + * add_random_kstack_offset - Increase stack utilization by previously > + * chosen random offset > + * > + * This should be used in the syscall entry path when interrupts and > * preempt are disabled, and after user registers have been stored to > * the stack. > */ > @@ -55,6 +58,24 @@ DECLARE_PER_CPU(u32, kstack_offset); > } \ > } while (0) > > +/** > + * choose_random_kstack_offset - Choose the random offsset for the next > + * add_random_kstack_offset() > + * > + * This should only be used during syscall exit when interrupts and > + * preempt are disabled, and before user registers have been restored > + * from the stack. This is done to frustrate attack attempts from > + * userspace to learn the offset: > + * - Maximize the timing uncertainty visible from userspace: if the > + * the offset is chosen at syscall entry, userspace has much more > + * control over the timing between chosen offsets. "How long will we > + * be in kernel mode?" tends to be more difficult to know than "how > + * long will be be in user mode?" > + * - Reduce the lifetime of the new offset sitting in memory during > + * kernel mode execution. Exposures of "thread-local" (e.g. current, > + * percpu, etc) memory contents tends to be easier than arbitrary > + * location memory exposures. > + */ > #define choose_random_kstack_offset(rand) do { \ > if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ > &randomize_kstack_offset)) { \ > >
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