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

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Date:   Tue, 9 May 2017 23:53:01 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 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.

> 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.

Dropped, but we'll need to fix that eventually.

> 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.

> 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.