Date: Tue, 29 Mar 2016 17:11:04 -0600 From: Scotty Bauer <sbauer@....utah.edu> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, the arch/x86 maintainers <x86@...nel.org>, Andi Kleen <ak@...ux.intel.com>, Andy Lutomirski <luto@...capital.net>, Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, wmealing@...hat.com Subject: Re: [PATCH v4 0/4] SROP Mitigation: Sigreturn Cookies On 03/29/2016 04:54 PM, Linus Torvalds wrote: > On Tue, Mar 29, 2016 at 2:53 PM, Scott Bauer <sbauer@....utah.edu> wrote: >> >> These patches implement the necessary changes to generate a cookie >> which will be placed above signal frame upon signal delivery to userland. >> The cookie is generated using a per-process random value xor'd with >> the address where the cookie will be stored on the stack. > > Side note: wouldn't it be better to make the cookie something that > doesn't make it trivial to figure out the random value in case you > already have access to a signal stack? > > Maybe there could be a stronger variation of this that makes the > cookie be something like a single md5 round (not a full md5). > Something fast, and not necessarily secure, but something that needs > more than one single CPU instruction to figure out. > > So you could do 4 32 > > - the random value > - the low 32 bits of the address of the cookie > - the low 32 bits of the return point stack and instruction pointer > > Yes, yes, md5 is not cryptographically secure, and making it a single > iteration rather than the full four makes it even less so, but if the > attacker can generate long arbitrary code, then the whole SROP is > pointless to begin with, no? > Yeah I had toyed with using hashes, I used hash_64 not md5 which is like 14 extra instructions or something. Anyway Daniel Micay pointed out we could use SipHash https://131002.net/siphash/, but there's no siphash for me to use in the kernel and I'm the *last* person on earth to start porting/implementing 'crypto' algos. Anyway, we all sort of agreed that if you have enough arbitrary execution already to cause a signal, leak the cookie, do some xor magic to get the per-process secret then you probably don't really need to SROP in your exploit. Although you did mention an interesting attack which is force a signal then muck with an existing legitimate frame, which I would like to protect against now. > In contrast, with the plain xor, the SROP would be a trivial operation > if you can just force it to happen within the context of a signal, so > that you can just re-use the signal return stack as-is. But mixing in > the returning IP and SP would make it *much* harder to use the > sigreturn as an attack vector. > > I realize that this would likely need to be a separate and non-default > extra hardening mode, because there are *definitely* applications that > take signals and then update the return address (maybe single-stepping > over instructions etc). But for a *lot* of applications, signal return > implies changing no signal state at all, and mixing in the returning > IP and SP would seem to be a fundamentally stronger cookie. > > No? It's not hard to implement So I can try it. When you say an extra hardening mode do you mean hide it behind a sysctl or some sort of compile time CONFIG?
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