Re: Use of uninitialized memory with CONFIG_HW_RANDOM_VIRTIO

[Laurent Vivier <lvivier@redhat.com>]

Hi Alexander,

On 12/09/2021 19:05, Alexander Potapenko wrote:
> Hi Laurent,
> 
> Do you by any chance have an update on this?

I'm sorry I didn't have the time until now.

I try today.

Could you give more details how to reproduce this?
(kernel version, .config, tools to run?)

Thanks,
Laurent
> Thanks,
> Alex
> 
> On Fri, Aug 20, 2021 at 6:15 PM Laurent Vivier <lvivier@redhat.com> wrote:
>>
>> On 19/08/2021 22:58, Michael S. Tsirkin wrote:
>>> On Fri, Nov 13, 2020 at 06:26:16PM +0100, Alexander Potapenko wrote:
>>>> Hi Amos, Rusty, Amit, Michael,
>>>>
>>>> I am hitting something that I believe to be a minor problem in the
>>>> virtio RNG driver.
>>>> When running the kernel under KMSAN with "-device virtio-rng-pci"
>>>> passed to QEMU, I am seeing reports about rng_fillbuf in
>>>> drivers/char/hw_random/core.c being used before initialization (see
>>>> the report below).
>>>>
>>>> This can be verified by initializing rng_fillbuf with 'A' as follows:
>>>> ==========================================
>>>> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
>>>> index 8c1c47dd9f46..44d609a5796a 100644
>>>> --- a/drivers/char/hw_random/core.c
>>>> +++ b/drivers/char/hw_random/core.c
>>>> @@ -439,8 +439,11 @@ static int hwrng_fillfn(void *unused)
>>>>                 if (IS_ERR(rng) || !rng)
>>>>                         break;
>>>>                 mutex_lock(&reading_mutex);
>>>> +               memset(rng_fillbuf, 'A', rng_buffer_size());
>>>> +               rng_fillbuf[rng_buffer_size()-1] = 0;
>>>>                 rc = rng_get_data(rng, rng_fillbuf,
>>>>                                   rng_buffer_size(), 1);
>>>> +               pr_err("rng_fillbuf: %s\n", rng_fillbuf);
>>>>                 mutex_unlock(&reading_mutex);
>>>>                 put_rng(rng);
>>>>                 if (rc <= 0) {
>>>> ==========================================
>>>>
>>>> and booting the kernel: the first call of hwrng_fillfn() will print
>>>> "AAAAAAA.." instead of random data.
>>>>
>>>> For some reason on that first iteration vi->busy is true here:
>>>> https://elixir.bootlin.com/linux/latest/source/drivers/char/hw_random/virtio-rng.c#L62,
>>>> therefore the buffer is not being sent to virtio ring.
>>>>
>>>> While probably being benign, this bug is preventing syzkaller from
>>>> finding more bugs, so it would be nice to fix it.
>>>> Perhaps the easiest solution is to kzalloc rng_fillbuf, but if it's
>>>> critical for this driver to not skip even the first read, then maybe
>>>> you have better ideas?
>>>>
>>>> KMSAN report follows:
>>>>
>>>> =====================================================
>>>> BUG: KMSAN: uninit-value in _mix_pool_bytes+0x7d2/0x950
>>>> drivers/char/random.c:570
>>>> CPU: 0 PID: 2711 Comm: hwrng Not tainted 5.9.0-rc8-syzkaller #0
>>>> Hardware name: Google Google Compute Engine/Google Compute Engine,
>>>> BIOS Google 01/01/2011
>>>> Call Trace:
>>>>  __dump_stack lib/dump_stack.c:77 [inline]
>>>>  dump_stack+0x21c/0x280 lib/dump_stack.c:118
>>>>  kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:122
>>>>  __msan_warning+0x5f/0xa0 mm/kmsan/kmsan_instr.c:201
>>>>  _mix_pool_bytes+0x7d2/0x950 drivers/char/random.c:570
>>>>  mix_pool_bytes+0xca/0x2a0 drivers/char/random.c:599
>>>>  add_hwgenerator_randomness+0x4ac/0x500 drivers/char/random.c:2319
>>>>  hwrng_fillfn+0x6ae/0x940 drivers/char/hw_random/core.c:452
>>>>  kthread+0x51c/0x560 kernel/kthread.c:293
>>>>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
>>>>
>>>> Uninit was created at:
>>>>  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:129 [inline]
>>>>  kmsan_internal_poison_shadow+0x5c/0xf0 mm/kmsan/kmsan.c:112
>>>>  kmsan_slab_alloc+0x8d/0xe0 mm/kmsan/kmsan_hooks.c:80
>>>>  slab_alloc_node mm/slub.c:2903 [inline]
>>>>  slab_alloc mm/slub.c:2912 [inline]
>>>>  kmem_cache_alloc_trace+0x61e/0xc90 mm/slub.c:2929
>>>>  kmalloc include/linux/slab.h:554 [inline]
>>>>  hwrng_modinit+0x103/0x2ef drivers/char/hw_random/core.c:621
>>>>  do_one_initcall+0x371/0x9c0 init/main.c:1208
>>>>  do_initcall_level+0x1e5/0x3c6 init/main.c:1281
>>>>  do_initcalls+0x127/0x1cb init/main.c:1297
>>>>  do_basic_setup+0x33/0x36 init/main.c:1317
>>>>  kernel_init_freeable+0x238/0x38b init/main.c:1517
>>>>  kernel_init+0x1f/0x840 init/main.c:1406
>>>>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
>>>> =====================================================
>>>>
>>>> Thanks,
>>>> Alex
>>>
>>>
>>> Cc Laurent - I think he said he was going to look at virtio rng.
>>
>> I will have look next week.
>>
>> Thanks,
>> Laurent
>>
> 
> 

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