linux-kernel - Re: [PATCH 2/2] efi: Validate UEFI boot variables

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Message-ID: <20120502145412.GA20058@srcf.ucam.org>
Date:	Wed, 2 May 2012 15:54:12 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/2] efi: Validate UEFI boot variables

On Wed, May 02, 2012 at 04:55:06AM +0100, Ben Hutchings wrote:

> This validation is crap; you're not accounting for the size of the node
> or invalid lengths.  Try:

Good catch.

> Isn't this slightly too restrictive?  The '& 0xff' results in many
> non-ASCII characters aliasing hex digits and potentially causes a
> variable to be validated as if it was special.  I would think the
> correct condition is:

Mm. Yeah, I guess that should probably be permitted.

> 		if (var->VariableName[i] > 127 ||
> 		    hex_to_bin(var->VariableName[i]) < 0)
> 
> Presumably the variable should also be ignored if there are any more
> characters after the 4 hex digits?

The spec doesn't permit any such names, so I don't think it makes any 
difference in practice. But in theory, we should probably either 
explicitly permit or reject them rather than treating them as if they're 
boot variables.

> > +	/* A valid entry must be at least 6 bytes */
> > +	if (len < 6)
> > +		return false;
> 
> Surely 8 bytes - otherwise you don't even have space for the
> description's null terminator.

Yes, I guess that could trip things up.

> > +	filepathlength = buffer[4] | buffer[5] << 8;
> > +
> > +	/*
> > +	 * There's no stored length for the description, so it has to be
> > +	 * found by hand
> > +	 */
> > +	desclength = utf16_strsize((efi_char16_t *)(buffer + 6), len) + 2;
> [...]
> 
> Second argument should be len - 6.

Sure.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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