Re: [PATCH v3] ALSA: control: Validate element info in compat ioctl

[Sreeraj S Kurup <sreekuttan2156239@gmail.com>]

Hi Takashi,

The validation in snd_ctl_elem_write() happens too late. Before calling 
snd_ctl_elem_write(), the compat handler calls copy_ctl_value_from_user(), which 
uses the type and count returned by get_ctl_type() to determine how many values to copy.

If the control's info callback returns an invalid count, the integer and boolean paths 
can access beyond the fixed-size integer value array. Other paths can also calculate a 
copy size larger than the available kernel buffer.

The read path has a similar issue. It uses the same type and count when copying the values 
back to userspace, which can result in an out-of-bounds read from the kernel buffer.

So, validation in snd_ctl_elem_write() cannot prevent these problems because the unsafe 
accesses can happen before it is called.

Using __snd_ctl_elem_info() in get_ctl_type() ensures that the type and count are validated 
before they are used by the compat handlers. It also reuses the existing element ID handling 
and access checks.

Therefore, I think this validation is necessary.

Best regards,

Sreeraj