netdev - Re: [PATCH net v2] atm: Fix the cleanup on alloc_mpc failure in atm_mpoa_mpoad

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Message-ID: <CAC_ur0rRtuVS7GTxUuEU=yOfrsTRZH0dcGwgoQCy9wB_b6pE2A@mail.gmail.com>
Date: Tue, 30 Sep 2025 20:43:11 +0530
From: Cortex Auth <deepak.takumi.120@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Deepak Sharma <deepak.sharma.472935@...il.com>, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, horms@...nel.org, pwn9uin@...il.com, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kernel-mentees@...ts.linux.dev, david.hunter.linux@...il.com, 
	skhan@...uxfoundation.org, 
	syzbot+740e04c2a93467a0f8c8@...kaller.appspotmail.com, 
	syzbot+07b635b9c111c566af8b@...kaller.appspotmail.com
Subject: Re: [PATCH net v2] atm: Fix the cleanup on alloc_mpc failure in atm_mpoa_mpoad_attach

On Tue, Sep 30, 2025 at 8:01 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 9/30/25 3:33 PM, Deepak Sharma wrote:
> > On Tue, Sep 30, 2025 at 2:15 PM Paolo Abeni <pabeni@...hat.com> wrote:
> >> AFAICS the mpc_timer can rearm itself, so this the above is not enough
> >> and you should use timer_shutdown_sync() instead.
> >
> > Hi,
> >
> > As I understand it, `timer_shutdown_sync` will prevent any further
> > re-arming of the timer. I think this is not what we want here; since even if
> > we somehow fail to allocate our first MPOA client object on our first
> > ioctl call,
> > and hence end up wanting to disarm the timer, maybe on next call we can
> > allocate it successfully, and we would want that caches are processed
> > (which are processed for every time out). So we still want it to be
> > possible that
> > we can re-arm it.
>
> Ah, I missed the goal here is just being able to rearm the timer (i.e.
> there is no related UaF).
>
> Given the above, I think you could instead simply replace add_timer()
> with mod_timer().
>
> /P
>

I think yeah we could do that.

I have just been going with what code seems to have wanted to do;
Arm the timer if `mpcs` was NULL (no MPOA client existed)
And if there's any error, delete it as (was done in case of error by
the `_notifier` call, where we have no MPOA client yet).

I just extended it to the `alloc_mpoa` failure too, because
in that case too `mpcs` remains NULL

`mod_timer` would still work, because the timer callback will not do much
if it finds the `mpcs` to be NULL

If it sounds good, I can go ahead with it

Thanks,

Deepak