linux-hardening - Re: [PATCH] USB: ene_usb6250: Allocate enough memory for full object

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Date:   Mon, 6 Feb 2023 10:33:44 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, usb-storage@...ts.one-eyed-alien.net,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] USB: ene_usb6250: Allocate enough memory for full object

On Sat, Feb 04, 2023 at 02:43:47PM -0500, Alan Stern wrote:
> On Sat, Feb 04, 2023 at 10:35:46AM -0800, Kees Cook wrote:
> > The allocation of PageBuffer is 512 bytes in size, but the dereferencing
> > of struct ms_bootblock_idi (also size 512) happens at a calculated offset
> > within the allocation, which means the object could potentially extend
> > beyond the end of the allocation. Avoid this case by just allocating
> > enough space to catch any accesses beyond the end. Seen with GCC 13:
> 
> In principle, it would be better to add a runtime check for overflow.  
> Doing it this way means that the code could read an invalid value.
> 
> In fact, I get the impression that this code tries to load a data 
> structure which might straddle a page boundary by reading in just the 
> first page.  Either that, or else EntryOffset is always a multiple of 
> 512 so the error cannot arise.

Yeah, I couldn't figure it out. It seems like it might move in
non-512-byte steps too sometimes? Doubling the allocation (and zero-fill
it) seemed the safest way to cover it.

-Kees

-- 
Kees Cook