linux-kernel - Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

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Date:   Mon, 16 Jul 2018 13:25:59 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-doc@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Lee Smith <Lee.Smith@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Evgeniy Stepanov <eugenis@...gle.com>
Subject: Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

On Thu, Jun 28, 2018 at 9:30 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> On Wed, Jun 27, 2018 at 5:05 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>> On Tue, Jun 26, 2018 at 7:29 PM, Catalin Marinas
>> <catalin.marinas@....com> wrote:
>>> While I support this work, as a maintainer I'd like to understand
>>> whether we'd be in a continuous chase of ABI breaks with every kernel
>>> release or we have a better way to identify potential issues. Is there
>>> any way to statically analyse conversions from __user ptr to long for
>>> example? Or, could we get the compiler to do this for us?
>>
>>
>> OK, got it, I'll try to figure out a way to find these conversions.
>
> I've prototyped a checker on top of clang static analyzer (initially
> looked at sparse, but couldn't find any documentation or examples).
> The results are here [1], search for "warning: user pointer cast".
> Sharing in case anybody wants to take a look, will look at them myself
> tomorrow.
>
> [1] https://gist.github.com/xairy/433edd5c86456a64026247cb2fef2115

So the checker reports ~100 different places where a __user pointer
being casted. I've looked through them and found 3 places where we
need to add untagging. Source code lines below come from 4.18-rc2+
(6f0d349d).

Place 1:

arch/arm64/mm/fault.c:302:34: warning: user pointer cast
current->thread.fault_address = (unsigned long)info->si_addr;

Compare a pointer with TASK_SIZE (1 << 48) to check whether it lies in
the kernel or in user space. Need to untag the address before
performing a comparison.

Place 2:

fs/namespace.c:2736:21: warning: user pointer cast
size = TASK_SIZE - (unsigned long)data;

A similar check performed by subtracting a pointer from TASK_SIZE.
Need to untag before subtracting.

Place 3:

drivers/usb/core/devio.c:1407:29: warning: user pointer cast
unsigned long uurb_start = (unsigned long)uurb->buffer;
drivers/usb/core/devio.c:1636:31: warning: user pointer cast
unsigned long uurb_start = (unsigned long)uurb->buffer;
drivers/usb/core/devio.c:1715:30: warning: user pointer cast
unsigned long uurb_start = (unsigned long)uurb->buffer;

The device keeps list of mmapped areas and searches them for provided
__user pointer. Need to untag before searching.

There are also a few cases of memory syscalls operating on __user
pointers instead of unsigned longs like mmap:

ipc/shm.c:1355:23: warning: user pointer cast
unsigned long addr = (unsigned long)shmaddr;
ipc/shm.c:1566:23: warning: user pointer cast
unsigned long addr = (unsigned long)shmaddr;
mm/migrate.c:1586:10: warning: user pointer cast
addr = (unsigned long)p;
mm/migrate.c:1660:24: warning: user pointer cast
unsigned long addr = (unsigned long)(*pages);

If we don't add untagging to mmap, we probably don't need it here.

The rest of reported places look fine as is. Full annotated results of
running the checker are here [2].

I'll add the 3 patches with fixes to v5 of this patchset.

Catalin, WDYT?

[2] https://gist.github.com/xairy/aabda57741919df67d79895356ba9b58