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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Date:   Thu, 22 Oct 2020 12:04:52 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     David Laight <David.Laight@...lab.com>,
        Christoph Hellwig <hch@....de>,
        David Hildenbrand <david@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "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 11:13 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Thu, Oct 22, 2020 at 7:54 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> > On Thu, Oct 22, 2020 at 9:35 AM David Laight <David.Laight@...lab.com> wrote:
> > >
> > > Which makes it a bug in the kernel C syscall wrappers.
> > > They need to explicitly mask the high bits of 32bit
> > > arguments on arm64 but not x86-64.
> >
> > Why not x86-64? Wouldn't it be *any* LP64 ISA?
>
> x86-64 is slightly special because most instructions on a 32-bit
> argument clear the upper 32 bits, while on most architectures
> the same instruction would leave the upper bits unchanged.

Oh interesting, depends on the operations too on x86_64 IIUC?

>
> > Attaching a patch that uses the proper width, but I'm pretty sure
> > there's still a signedness issue .  Greg, would you mind running this
> > through the wringer?
>
> I would not expect this to change anything for the bug that Greg
> is chasing, unless there is also a bug in clang.
>
> In the version before the patch, we get a 64-bit argument from
> user space, which may consist of the intended value in the lower
> bits plus garbage in the upper bits. However, vlen only gets
> passed down  into import_iovec() without any other operations
> on it, and since import_iovec takes a 32-bit argument, this is
> where it finally gets narrowed.

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.

>
> After your patch, the SYSCALL_DEFINE3() does the narrowing
> conversion with the same clearing of the upper bits.
>
> If there is a problem somewhere leading up to import_iovec(),
> it would have to in some code that expects to get a 32-bit
> register argument but gets called with a register that has
> garbage in the upper bits /without/ going through a correct
> sanitizing function like SYSCALL_DEFINE3().
>
>       Arnd



-- 
Thanks,
~Nick Desaulniers