Re: [UBSAN] iwlmvm's iwl_mvm_enable_txq accesses IEEE80211_INVAL_HW_QUEUE

[Johannes Berg <johannes@sipsolutions.net>]

On Fri, 2017-06-23 at 09:48 +0200, Jiri Slaby wrote:
> 
> mac80211_queue is 255 which is IEEE80211_INVAL_HW_QUEUE, so it should
> not be worked with at all.

Funny you should find this today :-)

> The invalid queue is hopefully handled in ieee80211_check_queues
> after
> drv_add_interface in ieee80211_do_open:
> 
> res = drv_add_interface(local, sdata);
> if (res)
>         goto err_stop;
> res = ieee80211_check_queues(sdata,
>         ieee80211_vif_type_p2p(&sdata->vif));
> 
> 
> But the mvm driver still should not blindly shift 1 by 255 in
> iwl_mvm_enable_txq. Should the check for the invalid queue be before
> adding the interface in mac80211? Or should drivers check it in their
> add_interface?

Everything is actually handled well afaict, because the bug won't
matter - this is a queue we'll never really stop, so we won't be
looking at the (invalid) result of the calculation. I had actually been
under the impression that it wasn't undefined but would just result in
0; that's clearly not true but also doesn't matter. AFAICT it even
results in 0x80000000 (which makes some sense, since 255 % 32 == 31),
and that's a queue number (31) that's too big for mac80211 anyway, so
it would warn if we were to ever try to stop it.

In later versions of the code, however, there *was* indeed a bug - we
were using a u8 instead of a u32 in the code we have internally. While
fixing *that* bug, I also made this catch the case of
IEEE80211_INVAL_HW_QUEUE so we'll not modify the bitmap at all, which
is the correct thing to do.

IOW - yes, it's not nice, but it also shouldn't matter, and even if for
some strange reason we tried to stop the queue we'd just get a warning
from mac80211.

johannes