Re: [PATCH] media: dvb-core: add upper bound check in DMX_SET_BUFFER_SIZE ioctl

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

On Wed, 22 Jul 2026 15:37:19 -0400
"Cen Zhang (Microsoft)" <blbllhy@gmail.com> wrote:

> dvb_dvr_set_buffer_size() and dvb_dmxdev_set_buffer_size() pass the
> user-supplied size argument directly to vmalloc() without any upper
> bound check. This allows excessive kernel memory allocation via the
> DMX_SET_BUFFER_SIZE ioctl, which can lead to system-wide OOM conditions.
> 
>   Kernel panic - not syncing: System is deadlocked on memory
> 
>   Call Trace:
>    out_of_memory+0x12fd/0x1370
>    __alloc_frozen_pages_noprof+0x2620/0x2fa0
>    __vmalloc_node_range_noprof+0x7fa/0x1490
>    dvb_dvr_do_ioctl+0x11e/0x260 (drivers/media/dvb-core/dmxdev.c:296)
>    dvb_usercopy+0x15b/0x360
> 
> Fix by cap both functions at 64 MB and return -EINVAL for oversized
> requests.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Fixes: a095be4b030c ("V4L/DVB (7659): dvb-core: Implement DMX_SET_BUFFER_SIZE for dvr")
> Reported-by: AutonomousCodeSecurity@microsoft.com
> Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
> ---
>  drivers/media/dvb-core/dmxdev.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/media/dvb-core/dmxdev.c b/drivers/media/dvb-core/dmxdev.c
> index 3c8bc75e4d6c..b4dc87945be1 100644
> --- a/drivers/media/dvb-core/dmxdev.c
> +++ b/drivers/media/dvb-core/dmxdev.c
> @@ -20,6 +20,9 @@
>  #include <media/dmxdev.h>
>  #include <media/dvb_vb2.h>
>  
> +/* 64 MB upper bound for DVB ring buffer allocations */
> +#define DVB_BUFFER_SIZE_MAX (64 * 1024 * 1024)

Having a limit is good, but why 64MB?

Can you provide me more details about the test scenario: e.g. with
what TV standards had you test it, and such.

Regards,
Mauro

Thanks,
Mauro