Date: Mon, 11 Feb 2019 07:54:59 -0800 From: Andy Lutomirski <luto@...capital.net> To: "Reshetova, Elena" <elena.reshetova@...el.com> Cc: Andy Lutomirski <luto@...nel.org>, Jann Horn <jannh@...gle.com>, "Perla, Enrico" <enrico.perla@...el.com>, Peter Zijlstra <peterz@...radead.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, "tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>, "keescook@...omium.org" <keescook@...omium.org>, "tytso@....edu" <tytso@....edu> Subject: Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon system call On Feb 10, 2019, at 10:39 PM, Reshetova, Elena <elena.reshetova@...el.com> wrote: >> On Sat, Feb 9, 2019 at 3:13 AM Reshetova, Elena >> <elena.reshetova@...el.com> wrote: >>> >>>> On Fri, Feb 08, 2019 at 01:20:09PM +0000, Reshetova, Elena wrote: >>>>>> On Fri, Feb 08, 2019 at 02:15:49PM +0200, Elena Reshetova wrote: >>>> >>>>>> >>>>>> Why can't we change the stack offset periodically from an interrupt or >>>>>> so, and then have every later entry use that. >>>>> >>>>> Hm... This sounds more complex conceptually - we cannot touch >>>>> stack when it is in use, so we have to periodically probe for a >>>>> good time (when process is in userspace I guess) to change it from an >> interrupt? >>>>> IMO trampoline stack provides such a good clean place for doing it and we >>>>> have stackleak there doing stack cleanup, so would make sense to keep >>>>> these features operating together. >>>> >>>> The idea was to just change a per-cpu (possible per-task if you ctxsw >>>> it) offset that is used on entry to offset the stack. >>>> So only entries after the change will have the updated offset, any >>>> in-progress syscalls will continue with their current offset and will be >>>> unaffected. >>> >>> Let me try to write this into simple steps to make sure I understand your >>> approach: >>> >>> - create a new per-stack value (and potentially its per-cpu "shadow") called >> stack_offset = 0 >>> - periodically issue an interrupt, and inside it walk the process tree and >>> update stack_offset randomly for each process >>> - when a process makes a new syscall, it subtracts stack_offset value from >> top_of_stack() >>> and that becomes its new top_of_stack() for that system call. >>> >>> Smth like this? >> >> I'm proposing somthing that is conceptually different. > > OK, looks like I fully misunderstand what you meant indeed. > The reason I didn’t reply to your earlier answer is that I started to look > into unwinder code & logic to get at least a slight clue on how things > can be done since I haven't looked in it almost at all before (I wasn't changing > anything with regards to it, so I didn't have to). So, I meant to come back > with a more rigid answer that just "let me study this first"... Fair enough. > > You are, >> conceptually, changing the location of the stack. I'm suggesting that >> you leave the stack alone and, instead, randomize how you use the >> stack. > > > So, yes, instead of having: > > allocated_stack_top > random_offset > actual_stack_top > pt_regs > ... > and so on > > We will have smth like: > > allocated_stack_top = actual_stack_top > pt_regs > random_offset > ... > > So, conceptually we have the same amount of randomization with > both approaches, but it is applied very differently. Exactly. > > Security-wise I will have to think more if second approach has any negative > consequences, in addition to positive ones. As a paranoid security person, > you might want to merge both approaches and randomize both places (before and > after pt_regs) with different offsets, but I guess this would be out of question, right? It’s not out of the question, but it needs some amount of cost vs benefit analysis. The costs are complexity, speed, and a reduction in available randomness for any given amount of memory consumed. > > I am not that experienced with exploits , but we have been > talking now with Jann and Enrico on this, so I think it is the best they comment > directly here. I am just wondering if having pt_regs in a fixed place can > be an advantage for an attacker under any scenario... If an attacker has write-what-where (i.e. can write controlled values to controlled absolute virtual addresses), then I expect that pt_regs is a pretty low ranking target. But it may be a fairly juicy target if you have a stack buffer overflow that lets an attacker write to a controlled *offset* from the stack. We used to keep thread_info at the bottom of the stack, and that was a great attack target. But there’s an easier mitigation: just do regs->cs |= 3 or something like that in the exit code. Then any such attack can only corrupt *user* state. The performance impact would be *very* low, since this could go in the asm path that’s only used for IRET to user mode.
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