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Date:	Fri, 05 Jun 2009 16:04:40 +0200
From:	Felix Fietkau <>
To:	Bob Copeland <>
Subject: Re: [PATCH] mac80211: fix minstrel single-rate memory corruption

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 on the
> regression list (also
> Cc:
> Reported-by: Sergey S. Kostyliov <>
> Reported-by: Ognjen Maric <>
> Signed-off-by: Bob Copeland <>
> ---
> 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 <>

- Felix
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