Date: Tue, 23 Jun 2020 13:42:30 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Kees Cook' <keescook@...omium.org>, Arvind Sankar <nivedita@...m.mit.edu> CC: Thomas Gleixner <tglx@...utronix.de>, Elena Reshetova <elena.reshetova@...el.com>, "x86@...nel.org" <x86@...nel.org>, "Andy Lutomirski" <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, "Catalin Marinas" <catalin.marinas@....com>, Will Deacon <will@...nel.org>, "Mark Rutland" <mark.rutland@....com>, Alexander Potapenko <glider@...gle.com>, Alexander Popov <alex.popov@...ux.com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Jann Horn <jannh@...gle.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: RE: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall From: Kees Cook > Sent: 23 June 2020 01:56 > On Mon, Jun 22, 2020 at 08:05:10PM -0400, Arvind Sankar wrote: > > But I still don't see anything _stopping_ the compiler from optimizing > > this better in the future. The "=m" is not a barrier: it just informs > > the compiler that the asm produces an output value in *ptr (and no other > > outputs). If nothing can consume that output, it doesn't stop the > > compiler from freeing the allocation immediately after the asm instead > > of at the end of the function. > > Ah, yeah, I get what you mean. > > > I'm talking about something like > > asm volatile("" : : "r" (ptr) : "memory"); > > which tells the compiler that the asm may change memory arbitrarily. > > Yeah, I will adjust it. > > > Here, we don't use it really as a barrier, but to tell the compiler that > > the asm may have stashed the value of ptr somewhere in memory, so it's > > not free to reuse the space that it pointed to until the function > > returns (unless it can prove that nothing accesses memory, not just that > > nothing accesses ptr). Do you need another asm volatile("" : : "r" (ptr) : "memory"); (or similar) at the bottom of the function - that the compiler thinks might access the memory whose address it thought got saved earlier? I wonder if it would be easier to allocate the stack space in the asm wrapper? At least as an architecture option. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.