Re: [PATCH] ALSA: asihpi: fix a potential double-fetch bug when copying puhm

[Takashi Iwai <tiwai@suse.de>]

On Tue, 19 Sep 2017 15:54:22 +0200,
Meng Xu wrote:
> 
> Hi Takashi,
> 
> Thanks for the reply. In my opinion, many security issues
> are in fact unhandled corner cases and this could be one.
> 
> In the first fetch, get_user(hm->h.size, (u16 __user *)puhm),
> only 2 bytes from puhm are copied in and later it is ensured
> that hm->h.size (which is also hm->m0.size given hm is a union)
> is no larger than sizeof(*hm). However, this relation is broken
> after the second fetch, copy_from_user(hm, puhm, hm->h.size).
> 
> As a concrete example, a user could put 0x000A when the first
> fetch happens which make hm->h.size <= sizeof(*hm) and later
> races to change it to 0xFFFF in the second fetch. What makes it
> even worse is this call: hpi_send_recv_f(&hm->m0, &hr->r0, file),
> which sends &hm->m0 to a lot of destinations. If any of the
> downstream functions assumes that hm->m0.size <= sizeof(*hm)
> which is actually not, an exploit can be constructed.
> 
> In fact, similar issues have caused vulnerabilities before as in
> https://bugzilla.kernel.org/show_bug.cgi?id=116651
> https://bugzilla.kernel.org/show_bug.cgi?id=120131,
> and more recently the fix in sched/perf
> https://marc.info/?l=linux-kernel&m=150401225812533&w=2
> 
> Feel free to let us know your opinion.

OK, now it's clearer, it's about the overwrite by the second
copy_from_user() call.  I took the patch as is now.


thanks,

Takashi