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

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Date:   Mon, 18 Oct 2021 14:57:45 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     jeyu@...nel.org, linux-kernel@...r.kernel.org, mbenes@...e.com,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH v2] module: fix validate_section_offset() overflow bug on
 64-bit

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.

thanks,
-- Shuah