[PATCH] virtio-console: avoid DMA from stack

[PATCH] virtio-console: avoid DMA from stack

[Omar Sandoval]

From: Omar Sandoval <osandov@fb.com>

put_chars() stuffs the buffer it gets into an sg, but that buffer may be
on the stack. This breaks with CONFIG_VMAP_STACK=y (for me, it
manifested as printks getting turned into NUL bytes).

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
Patch based on v4.10-rc6.

 drivers/char/virtio_console.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 17857beb4892..3cbf4c95e446 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1136,6 +1136,8 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 {
 	struct port *port;
 	struct scatterlist sg[1];
+	void *data;
+	int ret;
 
 	if (unlikely(early_put_chars))
 		return early_put_chars(vtermno, buf, count);
@@ -1144,8 +1146,14 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 	if (!port)
 		return -EPIPE;
 
-	sg_init_one(sg, buf, count);
-	return __send_to_port(port, sg, 1, count, (void *)buf, false);
+	data = kmemdup(buf, count, GFP_ATOMIC);
+	if (!data)
+		return -ENOMEM;
+
+	sg_init_one(sg, data, count);
+	ret = __send_to_port(port, sg, 1, count, data, false);
+	kfree(data);
+	return ret;
 }
 
 /*
-- 
2.11.0

Re: [PATCH] virtio-console: avoid DMA from stack

[Amit Shah]

On (Wed) 01 Feb 2017 [00:02:27], Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> put_chars() stuffs the buffer it gets into an sg, but that buffer may be
> on the stack. This breaks with CONFIG_VMAP_STACK=y (for me, it
> manifested as printks getting turned into NUL bytes).

Seems reasonable.  I wonder since all implementations of hvc do a
memcpy, if we can abstract it - but that'll need some work.

Reviewed-by: Amit Shah <amit.shah@redhat.com>

Michael, please add to the virtio queue.

		Amit

Re: [PATCH] virtio-console: avoid DMA from stack

[Omar Sandoval]

On Wed, Feb 01, 2017 at 07:17:12PM +0530, Amit Shah wrote:
> On (Wed) 01 Feb 2017 [00:02:27], Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@fb.com>
> > 
> > put_chars() stuffs the buffer it gets into an sg, but that buffer may be
> > on the stack. This breaks with CONFIG_VMAP_STACK=y (for me, it
> > manifested as printks getting turned into NUL bytes).
> 
> Seems reasonable.  I wonder since all implementations of hvc do a
> memcpy, if we can abstract it - but that'll need some work.
> 
> Reviewed-by: Amit Shah <amit.shah@redhat.com>
> 
> Michael, please add to the virtio queue.
> 
> 		Amit

Hi, Michael,

I don't see this in Linus' master or in your git tree, is it going to go
in for -rc1?

Thanks.

Re: [PATCH] virtio-console: avoid DMA from stack

[Michael S. Tsirkin]

On Wed, Mar 01, 2017 at 12:01:10PM -0800, Omar Sandoval wrote:
> On Wed, Feb 01, 2017 at 07:17:12PM +0530, Amit Shah wrote:
> > On (Wed) 01 Feb 2017 [00:02:27], Omar Sandoval wrote:
> > > From: Omar Sandoval <osandov@fb.com>
> > > 
> > > put_chars() stuffs the buffer it gets into an sg, but that buffer may be
> > > on the stack. This breaks with CONFIG_VMAP_STACK=y (for me, it
> > > manifested as printks getting turned into NUL bytes).
> > 
> > Seems reasonable.  I wonder since all implementations of hvc do a
> > memcpy, if we can abstract it - but that'll need some work.
> > 
> > Reviewed-by: Amit Shah <amit.shah@redhat.com>
> > 
> > Michael, please add to the virtio queue.
> > 
> > 		Amit
> 
> Hi, Michael,
> 
> I don't see this in Linus' master or in your git tree, is it going to go
> in for -rc1?
> 
> Thanks.

Yes.

Re: [PATCH] virtio-console: avoid DMA from stack

[Jan Dakinevich]

I also faced with the same issue. Could you clarify it for me whether it
is safe to allocate memory inside console driver handler? For example,
what would happen if put_chars was triggered by fail in another memory
allocation?

On 02/01/2017 11:02 AM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> put_chars() stuffs the buffer it gets into an sg, but that buffer may be
> on the stack. This breaks with CONFIG_VMAP_STACK=y (for me, it
> manifested as printks getting turned into NUL bytes).
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> Patch based on v4.10-rc6.
> 
>  drivers/char/virtio_console.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index 17857beb4892..3cbf4c95e446 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -1136,6 +1136,8 @@ static int put_chars(u32 vtermno, const char *buf, int count)
>  {
>  	struct port *port;
>  	struct scatterlist sg[1];
> +	void *data;
> +	int ret;
>  
>  	if (unlikely(early_put_chars))
>  		return early_put_chars(vtermno, buf, count);
> @@ -1144,8 +1146,14 @@ static int put_chars(u32 vtermno, const char *buf, int count)
>  	if (!port)
>  		return -EPIPE;
>  
> -	sg_init_one(sg, buf, count);
> -	return __send_to_port(port, sg, 1, count, (void *)buf, false);
> +	data = kmemdup(buf, count, GFP_ATOMIC);
> +	if (!data)
> +		return -ENOMEM;
> +
> +	sg_init_one(sg, data, count);
> +	ret = __send_to_port(port, sg, 1, count, data, false);
> +	kfree(data);
> +	return ret;
>  }
>  
>  /*
> 

-- 
Best regards,
Jan Dakinevich