linux-kernel - Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

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Date:   Wed, 10 May 2017 04:12:54 +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: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

On Wed, May 10, 2017 at 03:45:24AM +0100, Al Viro wrote:

> FWIW, some parts of that queue are obviously sane; it's the conversions of
> kernel_write() and friends to ->read_iter/->write_iter() that are non-starters.

Egads...  OK, I *have* misread what you are doing there.  Your vfs_iter_read()
works for files sans ->read_iter().  For strange values of "works" - you
hardwire "it's either iovec or kvec iterator" into its calling conventions,
which is a trouble waiting to happen.

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.

Broken commit: "net: don't play with address limits in kernel_recvmsg".
It would be OK if it was only about data.  Unfortunately, that's not
true in one case: svc_udp_recvfrom() wants ->msg_control.

Another delicate place: you can't assume that write() always advances
file position by its (positive) return value.  btrfs stuff is sensitive
to that.

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.