netdev - RE: [PATCH 14/17] MIPS: bpf: Prevent kernel fall over for >=32bit shifts

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Date:	Mon, 23 Jun 2014 11:08:26 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Markos Chandras' <Markos.Chandras@...tec.com>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>
CC:	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH 14/17] MIPS: bpf: Prevent kernel fall over for >=32bit
 shifts

From: Markos Chandras
> On 06/23/2014 10:44 AM, David Laight wrote:
> > From: Markos Chandras
> >> Remove BUG_ON() if the shift immediate is >=32 to avoid
> >> kernel crashes due to malicious user input. Since the micro-assembler
> >> will not allow an immediate greater or equal to 32, we will use the
> >> maximum value which is 31. This will do the correct thing on either 32-
> >> or 64-bit cores since no 64-bit instructions are being used in JIT.
> >
> > I'm not sure that bounding the shift to 31 bits 'is the correct thing'.
> > I'd have thought that emulating the large shift or masking the shift
> > to 5 bits are equally 'correct'.
> >
> > ...
> Hi David,
> 
> Since we use 32-bit registers (or rather, we ignore the top 32bits on
> MIPS64), shifting >= 32 will always result to 0.
> Alexei suggested [1] to allow large shifts and emulate them, so this
> patch aims to do that by treating >=32 shift values as 31. Please tell
> me if I got this wrong.

Shifting by 31 converts 0xffffffff to 1, not 0.

	David



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