Date:   Fri, 20 Jan 2023 13:54:12 +0100
From:   Babis Chalios <bchalios@...zon.es>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Olivia Mackall <olivia@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jason Wang <jasowang@...hat.com>,
        <linux-crypto@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>, <amit@...nel.org>,
        <graf@...zon.de>, <Jason@...c4.com>, <xmarcalx@...zon.co.uk>
Subject: Re: [PATCH 1/2] virtio-rng: implement entropy leak feature



On 20/1/23 13:32, Michael S. Tsirkin wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>
>
>
> On Thu, Jan 19, 2023 at 07:43:47PM +0100, Babis Chalios wrote:
>> Implement the virtio-rng feature that allows a guest driver to request
>> from the device to perform certain operations in the event of an
>> "entropy leak", such as when taking a VM snapshot or restoring a VM from
>> a snapshot. The guest can request one of two operations: (i) fill a
>> buffer with random bytes, or (ii) perform a memory copy between two
>> bytes.
>>
>> The feature is similar to Microsoft's Virtual Machine Generation ID and
>> it can be used to (1) avoid the race-condition that exists in our
>> current VMGENID implementation, between the time vcpus are resumed and
>> the ACPI notification is being handled and (2) provide mechanisms for
>> notifying user-space about snapshot-related events.
>>
>> This commit implements the protocol between guest and device.
>> Additionally, it makes sure there is always a request for random bytes
>> in the event of entropy leak in-flight. Once such an event is observed,
>> the driver feeds these bytes to as entropy using
>> `add_device_randomness`.
>>
>> Keep in mind that this commit does not solve the race-condition issue,
>> it adds fresh entropy whenever the driver handles the used buffer from
>> the fill-on-leak request. In order to close the race window, we need to
>> expose some API so that other kernel subsystems can request directly
>> notifications from the device.
>>
>> Signed-off-by: Babis Chalios <bchalios@...zon.es>
>> ---
>>   drivers/char/hw_random/virtio-rng.c | 200 +++++++++++++++++++++++++++-
>>   include/uapi/linux/virtio_rng.h     |   3 +
>>   2 files changed, 197 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
>> index a6f3a8a2aca6..389a091a8801 100644
>> --- a/drivers/char/hw_random/virtio-rng.c
>> +++ b/drivers/char/hw_random/virtio-rng.c
>> @@ -4,12 +4,14 @@
>>    *  Copyright (C) 2007, 2008 Rusty Russell IBM Corporation
>>    */
>>
>> +#include "asm-generic/errno.h"
> Why are you pulling this in?
> If you really need errno the thing to include
> is linux/errno.h
>
> checkpatch has a rule to catch such cases:
>
> # warn if <asm/foo.h> is #included and <linux/foo.h> is available and includes
> # itself <asm/foo.h> (uses RAW line)
>
> did you run checkpatch?
>
You are right, this is wrong. I think my LSP client pulled this in and I 
missed it because, strangely enough, I did ran
checkpatch but it didn't complain about it.

>>   #include <linux/err.h>
>>   #include <linux/hw_random.h>
>>   #include <linux/scatterlist.h>
>>   #include <linux/spinlock.h>
>>   #include <linux/virtio.h>
>>   #include <linux/virtio_rng.h>
>> +#include <linux/random.h>
>>   #include <linux/module.h>
>>   #include <linux/slab.h>
>>
>> @@ -18,6 +20,12 @@ static DEFINE_IDA(rng_index_ida);
>>   struct virtrng_info {
>>        struct hwrng hwrng;
>>        struct virtqueue *vq;
>> +     /* Leak queues */
>> +     bool has_leakqs;
>> +     struct virtqueue *leakq[2];
>> +     spinlock_t lock;
>> +     int active_leakq;
>> +
>>        char name[25];
>>        int index;
>>        bool hwrng_register_done;
>> @@ -29,27 +37,159 @@ struct virtrng_info {
>>        /* minimal size returned by rng_buffer_size() */
>>   #if SMP_CACHE_BYTES < 32
>>        u8 data[32];
>> +     u8 leak_data[32];
>>   #else
>>        u8 data[SMP_CACHE_BYTES];
>> +     u8 leak_data[SMP_CACHE_BYTES];
>>   #endif
>>   };
>>
>> +/* Swaps the queues and returns the new active leak queue. */
>> +static struct virtqueue *swap_leakqs(struct virtrng_info *vi)
>> +{
>> +     vi->active_leakq = 1 - vi->active_leakq;
>> +     return vi->leakq[vi->active_leakq];
>> +}
>> +
>> +static struct virtqueue *get_active_leakq(struct virtrng_info *vi)
>> +{
>> +     return vi->leakq[vi->active_leakq];
>> +}
>> +
>> +int add_fill_on_leak_request(struct virtrng_info *vi, struct virtqueue *vq, void *data, size_t len)
>> +{
>> +     struct scatterlist sg;
>> +     int ret;
>> +
>> +     sg_init_one(&sg, data, len);
>> +     ret = virtqueue_add_inbuf(vq, &sg, 1, data, GFP_KERNEL);
>> +     if (ret)
>> +             goto err;
>> +
>> +err:
>> +     return ret;
>> +}
>> +
>> +int virtrng_fill_on_leak(struct virtrng_info *vi, void *data, size_t len)
>> +{
>> +     struct virtqueue *vq;
>> +     unsigned long flags;
>> +     int ret;
>> +
>> +     if (!vi->has_leakqs)
>> +             return -EOPNOTSUPP;
>> +
>> +     spin_lock_irqsave(&vi->lock, flags);
>> +
>> +     vq = get_active_leakq(vi);
>> +     ret = add_fill_on_leak_request(vi, vq, data, len);
>> +     if (ret)
>> +             virtqueue_kick(vq);
>> +
>> +     spin_unlock_irqrestore(&vi->lock, flags);
>> +
>> +     return ret;
>> +}
>> +
>> +int add_copy_on_leak_request(struct virtrng_info *vi, struct virtqueue *vq,
>> +             void *to, void *from, size_t len)
>> +{
>> +     int ret;
>> +     struct scatterlist out, in, *sgs[2];
>> +
>> +     sg_init_one(&out, from, len);
>> +     sgs[0] = &out;
>> +     sg_init_one(&in, to, len);
>> +     sgs[1] = &in;
>> +
>> +     ret = virtqueue_add_sgs(vq, sgs, 1, 1, to, GFP_KERNEL);
>> +     if (ret)
>> +             goto err;
>> +
>> +err:
>> +     return ret;
>> +}
>> +
>> +int virtrng_copy_on_leak(struct virtrng_info *vi, void *to, void *from, size_t len)
>> +{
>> +     struct virtqueue *vq;
>> +     unsigned long flags;
>> +     int ret;
>> +
>> +     if (!vi->has_leakqs)
>> +             return -EOPNOTSUPP;
>> +
>> +     spin_lock_irqsave(&vi->lock, flags);
>> +
>> +     vq = get_active_leakq(vi);
>> +     ret = add_copy_on_leak_request(vi, vq, to, from, len);
>> +     if (ret)
>> +             virtqueue_kick(vq);
>> +
>> +     spin_unlock_irqrestore(&vi->lock, flags);
>> +
>> +     return ret;
>> +}
>> +
>> +static void entropy_leak_detected(struct virtqueue *vq)
>> +{
>> +     struct virtrng_info *vi = vq->vdev->priv;
>> +     struct virtqueue *activeq;
>> +     unsigned int len;
>> +     unsigned long flags;
>> +     void *buffer;
>> +     bool kick_activeq = false;
>> +
>> +     spin_lock_irqsave(&vi->lock, flags);
>> +
>> +     activeq = get_active_leakq(vi);
>> +     /* Drain all the used buffers from the queue */
>> +     while ((buffer = virtqueue_get_buf(vq, &len)) != NULL) {
>> +             if (vq == activeq) {
>> +                     pr_debug("%s: entropy leak detected!", vi->name);
>> +                     activeq = swap_leakqs(vi);
>> +             }
>> +
>> +             if (buffer == vi->leak_data) {
>> +                     add_device_randomness(vi->leak_data, sizeof(vi->leak_data));
>> +
>> +                     /* Ensure we always have a pending request for random bytes on entropy
>> +                      * leak. Do it here, after we have swapped leak queues, so it gets handled
>> +                      * with the next entropy leak event.
>> +                      */
>> +                     add_fill_on_leak_request(vi, activeq, vi->leak_data, sizeof(vi->leak_data));
>> +                     kick_activeq = true;
>> +             }
>> +     }
>> +
>> +     if (kick_activeq)
>> +             virtqueue_kick(activeq);
>> +
>> +     spin_unlock_irqrestore(&vi->lock, flags);
>> +}
>> +
>>   static void random_recv_done(struct virtqueue *vq)
>>   {
>>        struct virtrng_info *vi = vq->vdev->priv;
>> +     unsigned long flags;
>>
>> +     spin_lock_irqsave(&vi->lock, flags);
>>        /* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */
>>        if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
>> -             return;
>> +             goto unlock;
>>
>>        vi->data_idx = 0;
>>
>>        complete(&vi->have_data);
>> +
>> +unlock:
>> +     spin_unlock_irqrestore(&vi->lock, flags);
>>   }
>>
>>   static void request_entropy(struct virtrng_info *vi)
>>   {
>>        struct scatterlist sg;
>> +     unsigned long flags;
>>
>>        reinit_completion(&vi->have_data);
>>        vi->data_avail = 0;
>> @@ -57,10 +197,12 @@ static void request_entropy(struct virtrng_info *vi)
>>
>>        sg_init_one(&sg, vi->data, sizeof(vi->data));
>>
>> +     spin_lock_irqsave(&vi->lock, flags);
>>        /* There should always be room for one buffer. */
>>        virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_KERNEL);
>>
>>        virtqueue_kick(vi->vq);
>> +     spin_unlock_irqrestore(&vi->lock, flags);
>>   }
>>
>>   static unsigned int copy_data(struct virtrng_info *vi, void *buf,
>> @@ -126,6 +268,40 @@ static void virtio_cleanup(struct hwrng *rng)
>>        complete(&vi->have_data);
>>   }
>>
>> +static int init_virtqueues(struct virtrng_info *vi, struct virtio_device *vdev)
>> +{
>> +     int ret = -ENOMEM, total_vqs = 1;
>> +     struct virtqueue *vqs[3];
>> +     const char *names[3];
>> +     vq_callback_t *callbacks[3];
>> +
>> +     if (vi->has_leakqs)
>> +             total_vqs = 3;
>> +
>> +     callbacks[0] = random_recv_done;
>> +     names[0] = "input";
>> +     if (vi->has_leakqs) {
>> +             callbacks[1] = entropy_leak_detected;
>> +             names[1] = "leakq.1";
>> +             callbacks[2] = entropy_leak_detected;
>> +             names[2] = "leakq.2";
>> +     }
>> +
>> +     ret = virtio_find_vqs(vdev, total_vqs, vqs, callbacks, names, NULL);
>> +     if (ret)
>> +             goto err;
>> +
>> +     vi->vq = vqs[0];
>> +
>> +     if (vi->has_leakqs) {
>> +             vi->leakq[0] = vqs[1];
>> +             vi->leakq[1] = vqs[2];
>> +     }
>> +
>> +err:
>> +     return ret;
>> +}
>> +
>>   static int probe_common(struct virtio_device *vdev)
>>   {
>>        int err, index;
>> @@ -152,18 +328,24 @@ static int probe_common(struct virtio_device *vdev)
>>        };
>>        vdev->priv = vi;
>>
>> -     /* We expect a single virtqueue. */
>> -     vi->vq = virtio_find_single_vq(vdev, random_recv_done, "input");
>> -     if (IS_ERR(vi->vq)) {
>> -             err = PTR_ERR(vi->vq);
>> -             goto err_find;
>> +     vi->has_leakqs = virtio_has_feature(vdev, VIRTIO_RNG_F_LEAK);
>> +     if (vi->has_leakqs) {
>> +             spin_lock_init(&vi->lock);
>> +             vi->active_leakq = 0;
>>        }
>>
>> +     err = init_virtqueues(vi, vdev);
>> +     if (err)
>> +             goto err_find;
>> +
>>        virtio_device_ready(vdev);
>>
>>        /* we always have a pending entropy request */
>>        request_entropy(vi);
>>
>> +     /* we always have a fill_on_leak request pending */
>> +     virtrng_fill_on_leak(vi, vi->leak_data, sizeof(vi->leak_data));
>> +
>>        return 0;
>>
>>   err_find:
>> @@ -246,7 +428,13 @@ static const struct virtio_device_id id_table[] = {
>>        { 0 },
>>   };
>>
>> +static unsigned int features[] = {
>> +     VIRTIO_RNG_F_LEAK,
>> +};
>> +
>>   static struct virtio_driver virtio_rng_driver = {
>> +     .feature_table = features,
>> +     .feature_table_size = ARRAY_SIZE(features),
>>        .driver.name =  KBUILD_MODNAME,
>>        .driver.owner = THIS_MODULE,
>>        .id_table =     id_table,
>> diff --git a/include/uapi/linux/virtio_rng.h b/include/uapi/linux/virtio_rng.h
>> index c4d5de896f0c..d9774951547e 100644
>> --- a/include/uapi/linux/virtio_rng.h
>> +++ b/include/uapi/linux/virtio_rng.h
>> @@ -5,4 +5,7 @@
>>   #include <linux/virtio_ids.h>
>>   #include <linux/virtio_config.h>
>>
>> +/* The feature bitmap for virtio entropy device */
>> +#define VIRTIO_RNG_F_LEAK 0
>> +
>>   #endif /* _LINUX_VIRTIO_RNG_H */
>> --
>> 2.38.1
>>
>> Amazon Spain Services sociedad limitada unipersonal, Calle Ramirez de Prado 5, 28045 Madrid. Registro Mercantil de Madrid . Tomo 22458 . Folio 102 . Hoja M-401234 . CIF B84570936

Amazon Spain Services sociedad limitada unipersonal, Calle Ramirez de Prado 5, 28045 Madrid. Registro Mercantil de Madrid . Tomo 22458 . Folio 102 . Hoja M-401234 . CIF B84570936

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