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Date: Tue, 14 Nov 2017 20:12:23 -0800 From: Andy Lutomirski <luto@...capital.net> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Boqun Feng <boqun.feng@...il.com>, Dave Watson <davejwatson@...com>, linux-kernel <linux-kernel@...r.kernel.org>, linux-api <linux-api@...r.kernel.org>, Paul Turner <pjt@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Russell King <linux@....linux.org.uk>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>, Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>, Josh Triplett <josh@...htriplett.org>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Michael Kerrisk <mtk.manpages@...il.com> Subject: Re: [RFC PATCH for 4.15 00/24] Restartable sequences and CPU op vector v11 > On Nov 14, 2017, at 1:32 PM, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote: > > ----- On Nov 14, 2017, at 4:15 PM, Andy Lutomirski luto@...capital.net wrote: > > > One thing I kept however that diverge from your recommendation is the > "sign" parameter to the rseq syscall. I prefer this flexible > approach to a hardcoded signature value. We never know when we may > need to randomize or change this in the future. > > Regarding abort target signature the vs x86 disassemblers, I used a > 5-byte no-op on x86 32/64: > > x86-32: nopl <sig> > x86-64: nopl <sig>(%rip) I still don't see how this can possibly work well with libraries. If glibc or whatever issues the syscall and registers some signature, that signature *must* match the expectation of all libraries used in that thread or it's not going to work. I can see two reasonable ways to handle it: 1. The signature is just a well-known constant. If you have an rseq abort landing site, you end up with something like: nopl $11223344(%rip) landing_site: or whatever the constant is. 2. The signature varies depending on the rseq_cs in use. So you get: static struct rseq_cs this_cs = { .signature = 0x55667788; ... }; and then the abort landing site has: nopl $11223344(%rip) nopl $55667788(%rax) landing_site: The former is a bit easier to deal with. The latter has the nice property that you can't subvert one rseq_cs to land somewhere else, but it's not clear to me how what actual attack this prevents, so I think I prefer #1. I just think that your variant is asking for trouble down the road with incompatible userspace. --Andy
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