linux-kernel - Re: [PATCH] xen: hypercall: fix out-of-bounds memcpy

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Date:   Sat, 3 Feb 2018 12:08:14 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Juergen Gross <jgross@...e.com>, Nicolas Pitre <nico@...aro.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Jan Beulich <jbeulich@...e.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen: hypercall: fix out-of-bounds memcpy



On 02/03/2018 10:12 AM, Arnd Bergmann wrote:
> On Sat, Feb 3, 2018 at 12:33 AM, Boris Ostrovsky
> <boris.ostrovsky@...cle.com> wrote:
>> On 02/02/2018 10:32 AM, Arnd Bergmann wrote:
>>> The legacy hypercall handlers were originally added with
>>> a comment explaining that "copying the argument structures in
>>> HYPERVISOR_event_channel_op() and HYPERVISOR_physdev_op() into the local
>>> variable is sufficiently safe" and only made sure to not write
>>> past the end of the argument structure, the checks in linux/string.h
>>> disagree with that, when link-time optimizations are used:
>>>
>>> In function 'memcpy',
>>>      inlined from 'pirq_query_unmask' at drivers/xen/fallback.c:53:2,
>>>      inlined from '__startup_pirq' at drivers/xen/events/events_base.c:529:2,
>>>      inlined from 'restore_pirqs' at drivers/xen/events/events_base.c:1439:3,
>>>      inlined from 'xen_irq_resume' at drivers/xen/events/events_base.c:1581:2:
>>> include/linux/string.h:350:3: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
>>>     __read_overflow2();
>>>     ^
>>> make[3]: *** [ccLujFNx.ltrans15.ltrans.o] Error 1
>>> make[3]: Target 'all' not remade because of errors.
>>> lto-wrapper: fatal error: make returned 2 exit status
>>> compilation terminated.
>>> ld: error: lto-wrapper failed
>>>
>>> This changes the functions so that each argument is accessed with
>>> exactly the correct length based on the command code.
>>>
>>> Fixes: cf47a83fb06e ("xen/hypercall: fix hypercall fallback code for very old hypervisors")
>>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>>> ---
>>>   drivers/xen/fallback.c | 94 ++++++++++++++++++++++++++++----------------------
>>>   1 file changed, 53 insertions(+), 41 deletions(-)
>>>
> 
>>>        default:
>>> -             WARN_ON(rc != -ENOSYS);
>>> -             break;
>>> +             return -ENOSYS;
>>>        }
>>>
>>> +     memcpy(&op.u, arg, len);
>>> +     rc = _hypercall1(int, event_channel_op_compat, &op);
>>> +     memcpy(arg, &op.u, len);
>>
>>
>> We don't copy back for all commands, only those that are COPY_BACK.
> 
> Not sure what you mean. Is it harmful to copy back the data for the others
> in any way? Otherwise I wouldn't micro-optimize this.


I should have checked the original commit for fallback.c --- the code 
that it replaced was doing copybacks for all hypercalls and selective 
copybacks is an optimization introduced in that commit.

-boris