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

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