linux-kernel - Re: [PATCH] [RFC] mwifiex: Fix NULL pointer deref

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Message-ID: <ZnSHcZttq79cJS3l@google.com>
Date: Thu, 20 Jun 2024 12:48:01 -0700
From: Brian Norris <briannorris@...omium.org>
To: Kalle Valo <kvalo@...nel.org>
Cc: Sascha Hauer <s.hauer@...gutronix.de>, linux-wireless@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Lin <yu-hao.lin@....com>,
	Francesco Dolcini <francesco@...cini.it>
Subject: Re: [PATCH] [RFC] mwifiex: Fix NULL pointer deref

Hi Sascha,

On Wed, Jun 19, 2024 at 11:05:28AM +0300, Kalle Valo wrote:
> Sascha Hauer <s.hauer@...gutronix.de> writes:
> 
> > When an Access Point is repeatedly started it happens that the
> > interrupts handler is called with priv->wdev.wiphy being NULL, but
> > dereferenced in mwifiex_parse_single_response_buf() resulting in:
> >
> > | Unable to handle kernel NULL pointer dereference at virtual address 0000000000000140
...
> > | pc : mwifiex_get_cfp+0xd8/0x15c [mwifiex]
> > | lr : mwifiex_get_cfp+0x34/0x15c [mwifiex]
> > | sp : ffff8000818b3a70
> > | x29: ffff8000818b3a70 x28: ffff000006bfd8a5 x27: 0000000000000004
> > | x26: 000000000000002c x25: 0000000000001511 x24: 0000000002e86bc9
> > | x23: ffff000006bfd996 x22: 0000000000000004 x21: ffff000007bec000
> > | x20: 000000000000002c x19: 0000000000000000 x18: 0000000000000000
> > | x17: 000000040044ffff x16: 00500072b5503510 x15: ccc283740681e517
> > | x14: 0201000101006d15 x13: 0000000002e8ff43 x12: 002c01000000ffb1
> > | x11: 0100000000000000 x10: 02e8ff43002c0100 x9 : 0000ffb100100157
> > | x8 : ffff000003d20000 x7 : 00000000000002f1 x6 : 00000000ffffe124
> > | x5 : 0000000000000001 x4 : 0000000000000003 x3 : 0000000000000000
> > | x2 : 0000000000000000 x1 : 0001000000011001 x0 : 0000000000000000
> > | Call trace:
> > |  mwifiex_get_cfp+0xd8/0x15c [mwifiex]
> > |  mwifiex_parse_single_response_buf+0x1d0/0x504 [mwifiex]
> > |  mwifiex_handle_event_ext_scan_report+0x19c/0x2f8 [mwifiex]
> > |  mwifiex_process_sta_event+0x298/0xf0c [mwifiex]
> > |  mwifiex_process_event+0x110/0x238 [mwifiex]
> > |  mwifiex_main_process+0x428/0xa44 [mwifiex]
> > |  mwifiex_sdio_interrupt+0x64/0x12c [mwifiex_sdio]
> > |  process_sdio_pending_irqs+0x64/0x1b8
> > |  sdio_irq_work+0x4c/0x7c
> > |  process_one_work+0x148/0x2a0
> > |  worker_thread+0x2fc/0x40c
> > |  kthread+0x110/0x114
> > |  ret_from_fork+0x10/0x20
> > | Code: a94153f3 a8c37bfd d50323bf d65f03c0 (f940a000)
> > | ---[ end trace 0000000000000000 ]---
> >
> > Fix this by adding a NULL check before dereferencing this pointer.
> >
> > Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>
> >
> > ---
> >
> > This is the most obvious fix for this problem, but I am not sure if we
> > might want to catch priv->wdev.wiphy being NULL earlier in the call
> > chain.
> 
> I haven't looked at the call but the symptoms sound like that either we
> are enabling the interrupts too early or there's some kind of locking
> problem so that an other cpu doesn't see the change.

I agree with Kalle that there's a different underlying bug involved, and
(my conclusion:) we shouldn't whack-a-mole the NULL pointer without
addressing the underlying problem.

Looking a bit closer (and without much other context to go on): I believe 
that one potential underlying problem is the complete lack of locking
between cfg80211 entry points (such as mwifiex_add_virtual_intf() or
mwifiex_cfg80211_change_virtual_intf()) and most stuff in the main loop
(mwifiex_main_process()). The former call sites only hold the wiphy
lock, and the latter tends to ... mostly not hold any locks, but rely on
sequentialization with itself, and using its |main_proc_lock| for setup
and teardown. It's all really bad and ready to fall down like a house of
cards at any moment. Unfortunately, no one has spent time on
rearchitecting this driver.

So it's possible that mwifiex_process_event() (mwifiex_get_priv_by_id()
/ mwifiex_get_priv()) is getting a hold of a not-fully-initialized
'priv' structure.

BTW, in case I can reproduce and poke at your scenario, what exactly
is your test case? Are you just starting / killing / restarting hostapd
in a loop? Are you running a full network manager stack that's doing
something more complex (e.g., initiating scans)? Can you reproduce with
some more targeted set of `iw` commands? (`iw phy ... interface add ...;
iw dev ... del`) Is there anything else interesting in the dmesg logs?
(Some of the worst behaviors in this driver come when we see command
timeouts and mwifiex_reinit_sw(), for example.)

Or barring that, can you get some kind of trace of the nl80211 command
sequence, so it's clearer which command(s) are involved leading up to
the problem?

Brian