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Message-ID: <87946d71-5c94-1045-2e96-347423820d5c@oracle.com>
Date: Sun, 4 Feb 2018 13:55:25 -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 02/04/2018 10:35 AM, Arnd Bergmann wrote:
> On Sat, Feb 3, 2018 at 6:08 PM, Boris Ostrovsky
> <boris.ostrovsky@...cle.com> wrote:
>>
>> 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.
> It was not an optimization but a correctness fix to avoid overflowing
> the caller stack on the copy-back operation. What I tried to explain
> in my commit message is that the same fix is also needed on
> the copy-out before it. It's only a read access beyond the end
> of a local variable, but not both the static checks and kasan-stack
> get alarmed about it.
>
Yes, I understand that. I was referring to:
Move the fallback code into out-of-line functions, and handle all of
the operations known by this old a hypervisor individually: *Some don't
require copying back anything at all*, and for the rest use the
individual argument structures' sizes rather than the container's
in the original commit. I.e. prior to that commit we *were* copying back
for all commands (although possibly with potentially incorrect size). So
not copying back for some commands was an optimization.
In any case,
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
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