linux-kernel - Re: [PATCH v2] module: fix validate_section

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Date:   Tue, 19 Oct 2021 09:22:47 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     jeyu@...nel.org, linux-kernel@...r.kernel.org, mbenes@...e.com
Subject: Re: [PATCH v2] module: fix validate_section_offset() overflow bug on
 64-bit

On Mon, Oct 18, 2021 at 02:57:45PM -0600, Shuah Khan wrote:
> On 10/18/21 2:20 PM, Luis Chamberlain wrote:
> > On Mon, Oct 18, 2021 at 11:35:11AM -0600, Shuah Khan wrote:
> > > validate_section_offset() uses unsigned long local variable to
> > > add/store shdr->sh_offset and shdr->sh_size on all platforms.
> > > unsigned long is too short when sh_offset is Elf64_Off which
> > > would be the case on 64bit ELF headers.
> > > 
> > > This problem was found while adding an error message to print
> > > sh_offset and sh_size. If sh_offset + sh_size exceed the size
> > > of the local variable, the checks for overflow and offset/size
> > > being too large will not find the problem and call the section
> > > offset valid. This failure might cause problems later on.
> > > 
> > > Fix the overflow problem using the right size local variable when
> > > CONFIG_64BIT is defined.
> > > 
> > > Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
> > > ---
> > > Changes since v1:
> > > - Updated commit log to describe the fix clearly. No code
> > >    changes.
> > 
> > Thanks! But the implications of your fix is beyond what is described.
> > Although not a real issue today in practice.
> > 
> > I think we should extend it with something like this, let me know
> > what you think (I can just ammend the commit log, no resend would
> > be needed):
> > 
> > Without this fix applied we were shorting the design of modules to
> > have section headers placed within the 32-bit boundary (4 GiB) instead of
> > 64-bits when on 64-bit architectures (which allows for up to 16,777,216
> > TiB). In practice this just meant we were limiting modules to below
> > 4 GiB even on 64-bit systems. This then should not really affect any
> > real-world use case as modules these days obviously should likely never
> > exceed 1 GiB in size. A specially crafted invalid module might succeed to
> > skip validation in validate_section_offset() due to this mistake, but in such
> > case no impact is observed through code inspection given the correct data
> > types are used for the copy of the module when needed on move_module() when
> > the section type is not SHT_NOBITS (which indicates no the section
> > occupies no space on the file).
> > 
> 
> Sounds good to me.

OK pushed with the change above added. Thanks!

  Luis