[Qemu-devel] [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs

[Qemu-devel] [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs

[marcandre.lureau]

From: Marc-André Lureau <marcandre.lureau@redhat.com>

Since qemu 2.9, DMA write operations are allowed. However, usage of this
interface from kernel or user-space is strongly discouraged by the
maintainers. This patch is meant for experimentations for now.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 drivers/firmware/qemu_fw_cfg.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index 54b569da3257..e2f2ad1c9c0c 100644
--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c
@@ -524,9 +524,28 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
 	return fw_cfg_read_blob(entry->f.select, buf, pos, count, true);
 }
 
+static ssize_t fw_cfg_sysfs_write_raw(struct file *filp, struct kobject *kobj,
+				      struct bin_attribute *bin_attr,
+				      char *buf, loff_t pos, size_t count)
+{
+	struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
+
+	if (!fw_cfg_dma_enabled())
+		return -ENOTSUPP;
+
+	if (pos > entry->f.size)
+		return -EINVAL;
+
+	if (count > entry->f.size - pos)
+		count = entry->f.size - pos;
+
+	return fw_cfg_write_blob(entry->f.select, buf, pos, count);
+}
+
 static struct bin_attribute fw_cfg_sysfs_attr_raw = {
-	.attr = { .name = "raw", .mode = S_IRUSR },
+	.attr = { .name = "raw", .mode = S_IRUSR | S_IWUSR },
 	.read = fw_cfg_sysfs_read_raw,
+	.write = fw_cfg_sysfs_write_raw,
 };
 
 /*
-- 
2.14.1.146.gd35faa819

Re: [Qemu-devel] [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs

[Hatayama, Daisuke]

> From: Marc-André Lureau <marcandre.lureau@redhat.com>
>
> Since qemu 2.9, DMA write operations are allowed. However, usage of this
> interface from kernel or user-space is strongly discouraged by the
> maintainers. This patch is meant for experimentations for now.
>

Could you (or maintainers?) tell me how experimental the DMA write
operations from kernel are?

From some technical reason?
Or simply there has not been enough test yet so far?

> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  drivers/firmware/qemu_fw_cfg.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> index 54b569da3257..e2f2ad1c9c0c 100644
> --- a/drivers/firmware/qemu_fw_cfg.c
> +++ b/drivers/firmware/qemu_fw_cfg.c
> @@ -524,9 +524,28 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
>       return fw_cfg_read_blob(entry->f.select, buf, pos, count, true);
>  }
>
> +static ssize_t fw_cfg_sysfs_write_raw(struct file *filp, struct kobject *kobj,
> +                                   struct bin_attribute *bin_attr,
> +                                   char *buf, loff_t pos, size_t count)
> +{
> +     struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> +
> +     if (!fw_cfg_dma_enabled())
> +             return -ENOTSUPP;
> +
> +     if (pos > entry->f.size)
> +             return -EINVAL;
> +
> +     if (count > entry->f.size - pos)
> +             count = entry->f.size - pos;
> +
> +     return fw_cfg_write_blob(entry->f.select, buf, pos, count);
> +}
> +
>  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> -     .attr = { .name = "raw", .mode = S_IRUSR },
> +     .attr = { .name = "raw", .mode = S_IRUSR | S_IWUSR },
>       .read = fw_cfg_sysfs_read_raw,
> +     .write = fw_cfg_sysfs_write_raw,
>  };
>
>  /*
> --
> 2.14.1.146.gd35faa819

Thanks.
HATAYAMA, Daisuke

Re: [Qemu-devel] [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs

[Michael S. Tsirkin]

On Mon, Oct 30, 2017 at 11:07:20AM +0000, Hatayama, Daisuke wrote:
> > From: Marc-André Lureau <marcandre.lureau@redhat.com>
> >
> > Since qemu 2.9, DMA write operations are allowed. However, usage of this
> > interface from kernel or user-space is strongly discouraged by the
> > maintainers. This patch is meant for experimentations for now.
> >
> 
> Could you (or maintainers?) tell me how experimental the DMA write
> operations from kernel are?
> 
> From some technical reason?
> Or simply there has not been enough test yet so far?

The concern is security, talking from userspace to hypervisor
might become a trivial DOS vector.

If there's need for a specific entry to be accessible from userspace,
I'd rather white-list it.

> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > ---
> >  drivers/firmware/qemu_fw_cfg.c | 21 ++++++++++++++++++++-
> >  1 file changed, 20 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> > index 54b569da3257..e2f2ad1c9c0c 100644
> > --- a/drivers/firmware/qemu_fw_cfg.c
> > +++ b/drivers/firmware/qemu_fw_cfg.c
> > @@ -524,9 +524,28 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
> >       return fw_cfg_read_blob(entry->f.select, buf, pos, count, true);
> >  }
> >
> > +static ssize_t fw_cfg_sysfs_write_raw(struct file *filp, struct kobject *kobj,
> > +                                   struct bin_attribute *bin_attr,
> > +                                   char *buf, loff_t pos, size_t count)
> > +{
> > +     struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> > +
> > +     if (!fw_cfg_dma_enabled())
> > +             return -ENOTSUPP;
> > +
> > +     if (pos > entry->f.size)
> > +             return -EINVAL;
> > +
> > +     if (count > entry->f.size - pos)
> > +             count = entry->f.size - pos;
> > +
> > +     return fw_cfg_write_blob(entry->f.select, buf, pos, count);
> > +}
> > +
> >  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> > -     .attr = { .name = "raw", .mode = S_IRUSR },
> > +     .attr = { .name = "raw", .mode = S_IRUSR | S_IWUSR },
> >       .read = fw_cfg_sysfs_read_raw,
> > +     .write = fw_cfg_sysfs_write_raw,
> >  };
> >
> >  /*
> > --
> > 2.14.1.146.gd35faa819
> 
> Thanks.
> HATAYAMA, Daisuke
> 

Re: [Qemu-devel] [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs

[Hatayama, Daisuke]

> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org
> [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Michael S. Tsirkin
> Sent: Tuesday, October 31, 2017 1:19 AM
> To: Hatayama, Daisuke <d.hatayama@jp.fujitsu.com>
> Cc: 'marcandre.lureau@redhat.com' <marcandre.lureau@redhat.com>;
> 'somlo@cmu.edu' <somlo@cmu.edu>; linux-kernel@vger.kernel.org;
> 'qemu-devel@nongnu.org' <qemu-devel@nongnu.org>
> Subject: Re: [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs
> 
> On Mon, Oct 30, 2017 at 11:07:20AM +0000, Hatayama, Daisuke wrote:
> > > From: Marc-André Lureau <marcandre.lureau@redhat.com>
> > >
> > > Since qemu 2.9, DMA write operations are allowed. However, usage of this
> > > interface from kernel or user-space is strongly discouraged by the
> > > maintainers. This patch is meant for experimentations for now.
> > >
> >
> > Could you (or maintainers?) tell me how experimental the DMA write
> > operations from kernel are?
> >
> > From some technical reason?
> > Or simply there has not been enough test yet so far?
> 
> The concern is security, talking from userspace to hypervisor
> might become a trivial DOS vector.
> 
> If there's need for a specific entry to be accessible from userspace,
> I'd rather white-list it.
> 

Thanks for the explanation.

If so, I guess the white-list is done in userspace qemu side,
and no additional change would be needed for this kernel-side patch set for the white-list feature.
Is this understanding correct?

> > > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > > ---
> > >  drivers/firmware/qemu_fw_cfg.c | 21 ++++++++++++++++++++-
> > >  1 file changed, 20 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/firmware/qemu_fw_cfg.c
> b/drivers/firmware/qemu_fw_cfg.c
> > > index 54b569da3257..e2f2ad1c9c0c 100644
> > > --- a/drivers/firmware/qemu_fw_cfg.c
> > > +++ b/drivers/firmware/qemu_fw_cfg.c
> > > @@ -524,9 +524,28 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp,
> struct kobject *kobj,
> > >       return fw_cfg_read_blob(entry->f.select, buf, pos, count, true);
> > >  }
> > >
> > > +static ssize_t fw_cfg_sysfs_write_raw(struct file *filp, struct kobject
> *kobj,
> > > +                                   struct bin_attribute *bin_attr,
> > > +                                   char *buf, loff_t pos, size_t count)
> > > +{
> > > +     struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> > > +
> > > +     if (!fw_cfg_dma_enabled())
> > > +             return -ENOTSUPP;
> > > +
> > > +     if (pos > entry->f.size)
> > > +             return -EINVAL;
> > > +
> > > +     if (count > entry->f.size - pos)
> > > +             count = entry->f.size - pos;
> > > +
> > > +     return fw_cfg_write_blob(entry->f.select, buf, pos, count);
> > > +}
> > > +
> > >  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> > > -     .attr = { .name = "raw", .mode = S_IRUSR },
> > > +     .attr = { .name = "raw", .mode = S_IRUSR | S_IWUSR },
> > >       .read = fw_cfg_sysfs_read_raw,
> > > +     .write = fw_cfg_sysfs_write_raw,
> > >  };
> > >
> > >  /*
> > > --
> > > 2.14.1.146.gd35faa819
> >
> > Thanks.
> > HATAYAMA, Daisuke
> >
> 

Re: [Qemu-devel] [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs

[Michael S. Tsirkin]

On Tue, Oct 31, 2017 at 09:55:39AM +0000, Hatayama, Daisuke wrote:
> 
> 
> > -----Original Message-----
> > From: linux-kernel-owner@vger.kernel.org
> > [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Michael S. Tsirkin
> > Sent: Tuesday, October 31, 2017 1:19 AM
> > To: Hatayama, Daisuke <d.hatayama@jp.fujitsu.com>
> > Cc: 'marcandre.lureau@redhat.com' <marcandre.lureau@redhat.com>;
> > 'somlo@cmu.edu' <somlo@cmu.edu>; linux-kernel@vger.kernel.org;
> > 'qemu-devel@nongnu.org' <qemu-devel@nongnu.org>
> > Subject: Re: [PATCH v3 5/5] RFC: fw_cfg: add DMA write operation in sysfs
> > 
> > On Mon, Oct 30, 2017 at 11:07:20AM +0000, Hatayama, Daisuke wrote:
> > > > From: Marc-André Lureau <marcandre.lureau@redhat.com>
> > > >
> > > > Since qemu 2.9, DMA write operations are allowed. However, usage of this
> > > > interface from kernel or user-space is strongly discouraged by the
> > > > maintainers. This patch is meant for experimentations for now.
> > > >
> > >
> > > Could you (or maintainers?) tell me how experimental the DMA write
> > > operations from kernel are?
> > >
> > > From some technical reason?
> > > Or simply there has not been enough test yet so far?
> > 
> > The concern is security, talking from userspace to hypervisor
> > might become a trivial DOS vector.
> > 
> > If there's need for a specific entry to be accessible from userspace,
> > I'd rather white-list it.
> > 
> 
> Thanks for the explanation.
> 
> If so, I guess the white-list is done in userspace qemu side,
> and no additional change would be needed for this kernel-side patch set for the white-list feature.
> Is this understanding correct?

No - host side has no idea whether it's guest userspace or
kernel initiating the access.

> > > > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > > > ---
> > > >  drivers/firmware/qemu_fw_cfg.c | 21 ++++++++++++++++++++-
> > > >  1 file changed, 20 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/firmware/qemu_fw_cfg.c
> > b/drivers/firmware/qemu_fw_cfg.c
> > > > index 54b569da3257..e2f2ad1c9c0c 100644
> > > > --- a/drivers/firmware/qemu_fw_cfg.c
> > > > +++ b/drivers/firmware/qemu_fw_cfg.c
> > > > @@ -524,9 +524,28 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp,
> > struct kobject *kobj,
> > > >       return fw_cfg_read_blob(entry->f.select, buf, pos, count, true);
> > > >  }
> > > >
> > > > +static ssize_t fw_cfg_sysfs_write_raw(struct file *filp, struct kobject
> > *kobj,
> > > > +                                   struct bin_attribute *bin_attr,
> > > > +                                   char *buf, loff_t pos, size_t count)
> > > > +{
> > > > +     struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> > > > +
> > > > +     if (!fw_cfg_dma_enabled())
> > > > +             return -ENOTSUPP;
> > > > +
> > > > +     if (pos > entry->f.size)
> > > > +             return -EINVAL;
> > > > +
> > > > +     if (count > entry->f.size - pos)
> > > > +             count = entry->f.size - pos;
> > > > +
> > > > +     return fw_cfg_write_blob(entry->f.select, buf, pos, count);
> > > > +}
> > > > +
> > > >  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> > > > -     .attr = { .name = "raw", .mode = S_IRUSR },
> > > > +     .attr = { .name = "raw", .mode = S_IRUSR | S_IWUSR },
> > > >       .read = fw_cfg_sysfs_read_raw,
> > > > +     .write = fw_cfg_sysfs_write_raw,
> > > >  };
> > > >
> > > >  /*
> > > > --
> > > > 2.14.1.146.gd35faa819
> > >
> > > Thanks.
> > > HATAYAMA, Daisuke
> > >
> > 
>