linux-kernel - Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov

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Message-ID: <CAKwvOdkLHozVUs85Wx-_qo2OfbdkKwtmaJfQFJfvxi_vpEYxWQ@mail.gmail.com>
Date:   Thu, 22 Oct 2020 13:11:14 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Arnd Bergmann <arnd@...db.de>,
        David Laight <David.Laight@...lab.com>,
        Christoph Hellwig <hch@....de>,
        David Hildenbrand <david@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        David Howells <dhowells@...hat.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move
 rw_copy_check_uvector() into lib/iov_iter.c"

On Thu, Oct 22, 2020 at 12:25 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Thu, Oct 22, 2020 at 12:04:52PM -0700, Nick Desaulniers wrote:
>
> > Passing an `unsigned long` as an `unsigned int` does no such
> > narrowing: https://godbolt.org/z/TvfMxe (same vice-versa, just tail
> > calls, no masking instructions).
> > So if rw_copy_check_uvector() is inlined into import_iovec() (looking
> > at the mainline@...8ae406999), then children calls of
> > `rw_copy_check_uvector()` will be interpreting the `nr_segs` register
> > unmodified, ie. garbage in the upper 32b.
>
> FWIW,
>
> void f(unsinged long v)
> {
>         if (v != 1)
>                 printf("failed\n");
> }
>
> void g(unsigned int v)
> {
>         f(v);
> }
>
> void h(unsigned long v)
> {
>         g(v);
> }
>
> main()
> {
>         h(0x100000001);
> }

A good/analogous example, but things get weird when the leaf node in
the call chain is inline asm: https://godbolt.org/z/s19TY5

(I'm not sure that's precisely what's going on here; I'll need to dive
more into the calls rw_copy_check_uvector() makes to see if there's
inline asm somewhere, pretty sure calls to get_user with `nr_regs`
exist).

>
> must not produce any output on a host with 32bit int and 64bit long, regardless of
> the inlining, having functions live in different compilation units, etc.
>
> Depending upon the calling conventions, compiler might do truncation in caller or
> in a callee, but it must be done _somewhere_.



-- 
Thanks,
~Nick Desaulniers