Date: Fri, 5 Apr 2019 07:14:11 -0600 From: Andy Lutomirski <luto@...capital.net> To: "Reshetova, Elena" <elena.reshetova@...el.com> Cc: Kees Cook <keescook@...omium.org>, Andy Lutomirski <luto@...nel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Jann Horn <jannh@...gle.com>, "Perla, Enrico" <enrico.perla@...el.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, Greg KH <gregkh@...uxfoundation.org> Subject: Re: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon syscall On Apr 5, 2019, at 4:14 AM, Reshetova, Elena <elena.reshetova@...el.com> wrote: >> On Thu, Apr 4, 2019 at 4:41 AM Reshetova, Elena >> <elena.reshetova@...el.com> wrote: >>> What I still don't fully understand here (due to my little knowledge of >>> compilers) and afraid of is that the asm code that alloca generates (see my version) >>> and the alignment might differ on the different targets, etc. >> >> I guess it's possible, but for x86_64, since appears to be consistent. > > So, yes, I double checked this now with just printing all possible offsets I get for rsp > from do_syscall_64, it is indeed 33 different offsets, so it is indeed more like 5 bits of entropy. > We can increase it, if we want and people are ok with losing a bit more stack space. > >> >>> If you tried it on yours, can you send me the asm code that it produced for you? >>> Is it different from mine? >> >> You can compare compiler outputs here. Here's gcc vs clang for this code: >> https://godbolt.org/z/WJSbN8 >> You can adjust compiler versions, etc. > > Oh, this is handy! Thank you for the link! > > > So, should I resend to lkml (with some cosmetic fixes) or how to proceed with this? > I will also update the randomness bit info. > > Go ahead and send a new version, please.
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