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[PATCH] virtio-console: avoid DMA from stack

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  [PATCH] virtio-console: avoid DMA from stack Omar Sandoval <osandov@osandov.com> - 2017-02-01 09:10 +0100
    Re: [PATCH] virtio-console: avoid DMA from stack Amit Shah <amit.shah@redhat.com> - 2017-02-01 14:50 +0100

#1571317 — [PATCH] virtio-console: avoid DMA from stack

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

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#1571570

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

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