Date: Wed, 10 May 2017 08:27:47 +0100 From: Al Viro <viro@...IV.linux.org.uk> To: Christoph Hellwig <hch@...radead.org> Cc: Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>, Greg KH <greg@...ah.com>, Thomas Garnier <thgarnie@...gle.com>, Martin Schwidefsky <schwidefsky@...ibm.com>, Heiko Carstens <heiko.carstens@...ibm.com>, Dave Hansen <dave.hansen@...el.com>, Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>, David Howells <dhowells@...hat.com>, René Nyffenegger <mail@...enyffenegger.ch>, Andrew Morton <akpm@...ux-foundation.org>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Oleg Nesterov <oleg@...hat.com>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Paolo Bonzini <pbonzini@...hat.com>, Rik van Riel <riel@...hat.com>, Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Christian Borntraeger <borntraeger@...ibm.com>, Russell King <linux@...linux.org.uk>, Will Deacon <will.deacon@....com>, Catalin Marinas <catalin.marinas@....com>, Mark Rutland <mark.rutland@....com>, James Morse <james.morse@....com>, linux-s390 <linux-s390@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, the arch/x86 maintainers <x86@...nel.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl> Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode On Tue, May 09, 2017 at 11:53:01PM -0700, Christoph Hellwig wrote: > On Wed, May 10, 2017 at 04:12:54AM +0100, Al Viro wrote: > > What's the point? What's wrong with having kernel_read()/kernel_readv()/etc.? > > You still have set_fs() in there; doing that one level up in call chain would > > be just fine... IDGI. > > The problem is that they modify the address limit, which the whole > subthread here wants to get rid of. And you *still* do the same. Christoph, this is ridiculous - the worst part of the area is not a couple of functions in fs/read_write.c, it's a fucking lot of ->read() and ->write() instances in shitty driver code, pardon the redundance. And _that_ is still done under set_fs(KERNEL_DS). Claiming that set_fs() done one function deeper in callchain (both in fs/read_write.c) is somehow better because it reduces the amount of code under that thing... Get real, please - helpers that encapsulate those set_fs() pairs (a-la kernel_read(), etc.) absolutely make sense and converting their open-coded instances to calls of those helpers is clearly a good thing. However, we are not * getting rid of low-quality code run under KERNEL_DS * gettind rid of set_fs() itself * getting a generic kernel_read() variant that would really take an iov_iter. That's what I'm objecting to. Centralized kernel_readv() et.al. - sure, and fs/read_write.c is the right place for those. No arguments here. Conversion to those - absolutely; drivers have no fucking business touching set_fs() at all. But your primitives are trouble waiting to happen. Let them take kvec arrays. And let them, in case when there's no ->read_iter()/->write_iter(), do set_fs(). Statically, without this if (iter->type & ITER_KVEC) ... stuff. > > Another delicate place: you can't assume that write() always advances > > file position by its (positive) return value. btrfs stuff is sensitive > > to that. > > If we don't want to assume that we need to pass pointer to pos to > kernel_read/write. Which might be a good idea in general. Yes. > > ashmem probably _is_ OK with demanding ->read_iter(), but I'm not sure > > about blind asma->file->f_pos += ret. That's begging for races. Actually, > > scratch that - it *is* racy. > > I think the proper fix is to not even bother to maintain f_pos of the > backing file, as we don't ever use it - all reads from it pass in > an explicit position anyway. vfs_llseek() used by ashmem_llseek()...
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