On 13/09/2021 10:25, Alexander Potapenko wrote:
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.
Thank you for the detailed steps.
The problem happens because we mix two different buffers:
- in add_early_randomness() we provide rng_buffer but don't wait it is full (see [1])
- in hwrng_fillfn() we provide rng_fillbuf, and we wait data here, but we received the
signal from QEMU that there are data, but these data are in rng_buffer while we expect
them in rng_fillbuf.
There are several ways to fix/workaround that:
1- ignore the read when wait=0 :
diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index a90001e02bf7..8466d76566fd 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -59,15 +59,15 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool
wait)
if (vi->hwrng_removed)
return -ENODEV;
+ if (!wait)
+ return 0;
+
if (!vi->busy) {
vi->busy = true;
reinit_completion(&vi->have_data);
register_buffer(vi, buf, size);
}
- if (!wait)
- return 0;
-
ret = wait_for_completion_killable(&vi->have_data);
if (ret < 0)
return ret;
2- Use an internal intermediate buffer in virtio-rng, at a cost of a copy,
I have some patches (somewhere) I can refresh to do that.
3- modify hw_random/core.c to use only one buffer
Thanks,
Laurent
[1] 78887832e765 ("hwrng: core - don't wait on add_early_randomness()")
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
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