[PATCH] mac80211: fix minstrel single-rate memory corruption

[PATCH] mac80211: fix minstrel single-rate memory corruption

[Bob Copeland]

The minstrel rate controller periodically looks up rate indexes in
a sampling table.  When accessing a specific row and column, minstrel
correctly does a bounds check which, on the surface, appears to handle
the case where mi->n_rates < 2.  However, mi->sample_idx is actually
defined as an unsigned, so the right hand side is taken to be a huge
positive number when negative, and the check will always fail.

Consequently, the RC will overrun the array and cause random memory
corruption when communicating with a peer that has only a single rate.
The max value of mi->sample_idx is around 25 so casting to int should
have no ill effects.

Without the change, uptime is a few minutes under load with an AP
that has a single hard-coded rate, and both the AP and STA could
potentially crash.  With the change, both lasted 12 hours with a
steady load.

Thanks to Ognjen Maric for providing the single-rate clue so I could
reproduce this.

This fixes http://bugzilla.kernel.org/show_bug.cgi?id=12490 on the
regression list (also http://bugzilla.kernel.org/show_bug.cgi?id=13000).

Cc: stable@kernel.org
Reported-by: Sergey S. Kostyliov <rathamahata@gmail.com>
Reported-by: Ognjen Maric <ognjen.maric@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
---

John & Felix, the patch itself may be too subtle so feel free to do it a
different way.  However this is as minimal as it gets so I hope it can
be applied quickly to stable, and mainline if not too late.

 net/mac80211/rc80211_minstrel.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/net/mac80211/rc80211_minstrel.c b/net/mac80211/rc80211_minstrel.c
index d9233ec..2681dfa 100644
--- a/net/mac80211/rc80211_minstrel.c
+++ b/net/mac80211/rc80211_minstrel.c
@@ -216,7 +216,7 @@ minstrel_get_next_sample(struct minstrel_sta_info *mi)
 	unsigned int sample_ndx;
 	sample_ndx = SAMPLE_TBL(mi, mi->sample_idx, mi->sample_column);
 	mi->sample_idx++;
-	if (mi->sample_idx > (mi->n_rates - 2)) {
+	if ((int) mi->sample_idx > (mi->n_rates - 2)) {
 		mi->sample_idx = 0;
 		mi->sample_column++;
 		if (mi->sample_column >= SAMPLE_COLUMNS)
-- 
1.5.4.1

Re: [PATCH] mac80211: fix minstrel single-rate memory corruption

[Felix Fietkau]

Bob Copeland wrote:
> The minstrel rate controller periodically looks up rate indexes in
> a sampling table.  When accessing a specific row and column, minstrel
> correctly does a bounds check which, on the surface, appears to handle
> the case where mi->n_rates < 2.  However, mi->sample_idx is actually
> defined as an unsigned, so the right hand side is taken to be a huge
> positive number when negative, and the check will always fail.
> 
> Consequently, the RC will overrun the array and cause random memory
> corruption when communicating with a peer that has only a single rate.
> The max value of mi->sample_idx is around 25 so casting to int should
> have no ill effects.
> 
> Without the change, uptime is a few minutes under load with an AP
> that has a single hard-coded rate, and both the AP and STA could
> potentially crash.  With the change, both lasted 12 hours with a
> steady load.
> 
> Thanks to Ognjen Maric for providing the single-rate clue so I could
> reproduce this.
> 
> This fixes http://bugzilla.kernel.org/show_bug.cgi?id=12490 on the
> regression list (also http://bugzilla.kernel.org/show_bug.cgi?id=13000).
> 
> Cc: stable@kernel.org
> Reported-by: Sergey S. Kostyliov <rathamahata@gmail.com>
> Reported-by: Ognjen Maric <ognjen.maric@gmail.com>
> Signed-off-by: Bob Copeland <me@bobcopeland.com>
> ---
> 
> John & Felix, the patch itself may be too subtle so feel free to do it a
> different way.  However this is as minimal as it gets so I hope it can
> be applied quickly to stable, and mainline if not too late.
How about changing the type of sample_idx to signed instead of casting
it? It's just a cosmetic thing, so even if you leave at this you get my
Acked-by: Felix Fietkau <nbd@openwrt.org>

- Felix

Re: [PATCH] mac80211: fix minstrel single-rate memory corruption

[Bob Copeland]

On Fri, Jun 5, 2009 at 10:04 AM, Felix Fietkau<nbd@openwrt.org> wrote:
>> John & Felix, the patch itself may be too subtle so feel free to do it a
>> different way.  However this is as minimal as it gets so I hope it can
>> be applied quickly to stable, and mainline if not too late.
>
> How about changing the type of sample_idx to signed instead of casting
> it? It's just a cosmetic thing, so even if you leave at this you get my

Yeah, that is a better option for 2.6.31.  I would be a bit more
comfortable with this version for now though since it's exactly
the code I tested.

> Acked-by: Felix Fietkau <nbd@openwrt.org>

Thanks!

-- 
Bob Copeland %% www.bobcopeland.com

Re: [PATCH] mac80211: fix minstrel single-rate memory corruption

[Thiemo Nagel]

> This fixes http://bugzilla.kernel.org/show_bug.cgi?id=12490 on the
> regression list (also http://bugzilla.kernel.org/show_bug.cgi?id=13000).

The patch works well.  No kernel panics anymore.  You made my day!

Thank you,

Thiemo