linux-kernel - Re: [PATCH] capability: WARN when invalid capability is requested rather than BUG/panic

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Date:	Tue, 30 Sep 2008 10:36:28 -0400
From:	Eric Paris <eparis@...hat.com>
To:	James Morris <jmorris@...ei.org>
Cc:	linux-kernel@...r.kernel.org, sds@...ho.nsa.gov, morgan@...nel.org,
	serue@...ibm.com, selinux@...ho.nsa.gov
Subject: Re: [PATCH] capability: WARN when invalid capability is requested
	rather than BUG/panic

On Wed, 2008-10-01 at 00:23 +1000, James Morris wrote:
> On Tue, 30 Sep 2008, Eric Paris wrote:
> 
> > This patch adds a WARN_ONCE() to cap_capable() so we will stop
> > dereferencing random spots of memory and will cleanly tell the obviously
> > broken driver that it doesn't have that ridiculous permissions.  No idea
> > if the driver is going to handle EPERM but anything that calls capable
> > and doesn't expect a denial has got to be the worst piece of code ever
> > written.....  I could return EINVAL, but I think its clear that noone
> > has capabilities over 64 so clearly they don't have that permission.
> > 
> > This 'could' be considered a regression since 2.6.24.  Neither SELinux
> > nor the capabilities system had a problem with ginormous request values
> > until we got 64 bit support, although this is OBVIOUSLY a bug with the
> > out of tree closed source driver....
> 
> An issue here is whether we should be adding workarounds in the mainline 
> kernel for buggy closed drivers.  Papering over problems rather than 
> getting them fixed does not seem like a winning approach.  Especially 
> problems which are unexpectedly messing with kernel security APIs.

I don't know, looking at the feelings on "Can userspace bugs be kernel
regressions" leads me to believe that when we break something that once
worked we are supposed to fix it.

http://lwn.net/Articles/292143/

I don't think the proprietary closed source nature of the driver makes
it any less our problem to not make changes which cause the kernel to
esplode.

> Also, won't this encourage vendors of such drivers to continue with this 
> behavior, while discouraging those vendors who are doing the right thing?

Discouraging people who open source their drivers and put them in the
kernel?  obviously not.  encouraging crap?  well, I hope we fix
regressions no matter how they are found...

> Do we know if this even really helps the user?  For all we know, the 
> driver may simply crash differently with an -EPERM.

Well, before the 64 bit capabilities change we did:

(cap_t(c) & CAP_TO_MASK(flag))

so a huge value for "flag" got masked off.

After 64 bit capabilities we do:

((c).cap[CAP_TO_INDEX(flag)] & CAP_TO_MASK(flag))

so a huge flag causes an array index out of bounds and either explodes
here or continues onto SELinux where it BUG().

So this is regression.  It would have gotten an EPERM, now it gets a
BUG/panic.

Yes ATI needs to fix their driver, but we broke it and I don't remember
the driver not working on 2.6.24 and earlier....

-Eric

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