Date: Fri, 12 May 2017 23:57:55 +0100 From: Al Viro <viro@...IV.linux.org.uk> To: Rik van Riel <riel@...hat.com> Cc: Kees Cook <keescook@...omium.org>, Russell King - ARM Linux <linux@...linux.org.uk>, Linus Torvalds <torvalds@...ux-foundation.org>, Mark Rutland <mark.rutland@....com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Greg KH <greg@...ah.com>, Heiko Carstens <heiko.carstens@...ibm.com>, LKML <linux-kernel@...r.kernel.org>, David Howells <dhowells@...hat.com>, Dave Hansen <dave.hansen@...el.com>, "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, linux-s390 <linux-s390@...r.kernel.org>, the arch/x86 maintainers <x86@...nel.org>, Will Deacon <will.deacon@....com>, Christian Borntraeger <borntraeger@...ibm.com>, René Nyffenegger <mail@...enyffenegger.ch>, Catalin Marinas <catalin.marinas@....com>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Arnd Bergmann <arnd@...db.de>, Brian Gerst <brgerst@...il.com>, Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Linux API <linux-api@...r.kernel.org>, Oleg Nesterov <oleg@...hat.com>, Daniel Micay <danielmicay@...il.com>, James Morse <james.morse@....com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Martin Schwidefsky <schwidefsky@...ibm.com>, Paolo Bonzini <pbonzini@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Thomas Garnier <thgarnie@...gle.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com> Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode On Fri, May 12, 2017 at 05:47:55PM -0400, Rik van Riel wrote: > > Seriously, look at these beasts. Overwriting ->addr_limit is nowhere > > near > > the top threat. If attacker can overwrite thread_info, you have > > lost. > > That is why THREAD_INFO_IN_TASK exists. It moves > the struct thread_info to a location away from the > stack, which means a stack overflow will not overwrite > the thread_info. ... in which case such attacks on ->addr_limit also become a non-issue. AFAICS, we are mixing several unrelated issues here: * amount of places where set_fs() is called. Sure, reducing it is a good idea and we want to move to primitives like kernel_write() et.al. Fewer users => lower odds of screwing it up. * making sure that remaining callers are properly paired. Ditto. * switching to ->read_iter()/->write_iter() where it makes sense. Again, no problem with that. * providing sane environment for places like perf/oprofile. Again, a good idea, and set_fs(USER_DS) is only a part of what's needed there. * switching _everything_ to ->read_iter()/->write_iter(). Flat-out insane and AFAICS nobody is signing up for that. * getting rid of set_fs() entirely. I'm afraid that it's not feasible without the previous one and frankly, I don't see much point. * sanity-checking on return to userland. Maybe useful, maybe not. * taking thread_info out of the way of stack overflows. Reasonable, but has very little to do with the rest of that. * protecting against Lovecraftian horrors slithering in from the outer space only to commit unspeakable acts against ->addr_limit and ignoring much tastier targets next to it, but then what do you expect from degenerate spawn of Great Old Ones - sanity?
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