Hi Laurent,
I took the latest kernel (5.15-rc1,
6880fa6c56601bb8ed59df6c30fd390cc5f6dd8f) and a slightly modified
config from syzbot (see attached)
The latter has a lot of unnecessary debug checks, but those should not
affect the RNG.
You then need to apply the following patch to the kernel:
====================================================
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index a3db27916256d..a4cba9f0ff8cb 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -433,8 +433,11 @@ 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 run the kernel under QEMU.
On my machine I'm seeing the following output:
$ cat log | strings | grep rng_fillbuf
[ 4.901931][ T897] rng_fillbuf:
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
[ 4.903104][ T897] rng_fillbuf: >
[ 4.903641][ T897] rng_fillbuf:
[ 4.904846][ T897] rng_fillbuf: ?
[ 4.913442][ T897] rng_fillbuf: [
, which denotes that the first call to rng_get_data() leaves
rng_fillbuf uninitialized.
HTH,
Alex
On Mon, Sep 13, 2021 at 8:52 AM Laurent Vivier <lvivier@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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
> >>
> >
> >
>
--
Alexander Potapenko
Software Engineer
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
Attachment:
config
Description: Binary data
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