Re: [PATCH v2] module: fix validate_section_offset() overflow bug on 64-bit

[Shuah Khan]

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@xxxxxxxxxxxxxxxxxxx>
> > ---
> > 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.

thanks,
-- Shuah