linux-kernel - Re: [PATCH] arm: mm: Poison freed init memory

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110106090741.GQ8638@n2100.arm.linux.org.uk>
Date:	Thu, 6 Jan 2011 09:07:41 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm: mm: Poison freed init memory

On Wed, Jan 05, 2011 at 09:25:55PM -0800, Stephen Boyd wrote:
> On 01/05/2011 12:26 PM, Russell King - ARM Linux wrote:
> > On Wed, Jan 05, 2011 at 11:47:25AM -0800, Stephen Boyd wrote:
> >> Poisoning __init marked memory can be useful when tracking down
> >> obscure memory corruption bugs. When a pointer is 0xCCCCCCCC in an
> >
> > That's a bad idea for a value.  With a 3GB page offset and 256MB or
> > more memory, accesses to such an address will always succeed.
> >
> > There's two things to be considered when selecting a possible poison
> > value:
> >
> > 1. what value is guaranteed to provoke an undefined instruction exception?
> > 2. what value when used as an address and dereferenced is mostly always
> >    going to abort?
> >
> > 1 for ARM mode implies an 0xe7fXXXfX value.  For Thumb mode 0xdeXX.  We
> > use this space for breakpoints.
> >
> > 2 unfortunately depends on the platform. 
> 
> A coworker proposed we use a SWI instruction. We could do that if the
> poison is 0xEF and then do something in the SWI handler where that
> number causes us to blow up?

Doesn't work with EABI - the comment field in the SWI instruction is
ignored on EABI.

> If I'm following correctly, point 1 is about __init functions and point
> 2 is about __initdata. I'm more concerned about __initdata because
> __init functions called from non __init marked functions are usually
> caught with section mismatch checks. Also, if we're jumping to
> 0xCCCCCCCC we're probably not in the text section of the kernel with a

But, as I pointed out, you don't know that 0xCCCCCCCC isn't a valid
address _and_ on modern platforms it won't fault.  So it's pointless
to use it as a poison value.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/