linux-kernel - Re: [PATCH] vmcore: prevent PT_NOTE p_memsz overflow during header update

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Message-ID: <20140203154704.GA10795@redhat.com>
Date:	Mon, 3 Feb 2014 10:47:04 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Pearson, Greg" <greg.pearson@...com>,
	"d.hatayama@...fujitsu.com" <d.hatayama@...fujitsu.com>,
	"holzheu@...ux.vnet.ibm.com" <holzheu@...ux.vnet.ibm.com>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"paul.gortmaker@...driver.com" <paul.gortmaker@...driver.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vmcore: prevent PT_NOTE p_memsz overflow during header
 update

On Sun, Feb 02, 2014 at 02:25:25PM -0800, Eric W. Biederman wrote:
> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
> > On Sat, 1 Feb 2014 01:07:29 +0000 "Pearson, Greg" <greg.pearson@...com> wrote:
> >
> >> As far as I know the only consequence of dropping a PT_NOTE entry is 
> >> that it would not be available in the crash dump for use in debugging. 
> >> I'm not sure how important this data might be for triage. I'm guessing 
> >> that in cases where one of these strange PT_NOTE entries shows up with a 
> >> size that causes an overflow it probably isn't even a real PT_NOTE entry 
> >> so dropping it won't matter, but that's a guess at this point since I'm 
> >> still trying to figure out how the bogus entries were created.
> >
> > Can we detect the crazy-huge notes, skip them and then proceed with
> > the following sanely-sized ones?
> 
> The only way we can have following sanely-sized notes is if they are in
> a separate note segment (one of our extensions for kdump and
> /proc/vmcore merges them together).

This processing is happening before we have merged ELF notes. Previous
kernel/kexec-tools prepared per cpu PT_NOTE type ELF note. One for
each cpu. And by default it prepares only one ELF note per PT_NOTE. So
there should not be more notes in the same PT_NOTE.

Also even if there are, n_namesz and n_descsz values seem so high that
after skipping these nothing valid should be after that.

So I will not be too worried about skipping seemingly corrupted ELf
notes. I think giving a warning makes sense though. Is somebody
overwriting the memory area in kenrel reserved for per cpu PT_NOTE.

Thanks
Vivek
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