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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