Date: Fri, 5 Apr 2019 10:14:45 +0000 From: "Reshetova, Elena" <elena.reshetova@...el.com> To: Kees Cook <keescook@...omium.org> CC: Andy Lutomirski <luto@...nel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Andy Lutomirski <luto@...capital.net>, 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 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. Best Regards, Elena.
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