[SeaBIOS] [PATCH] smbios: ensure comparison SMBIOS string can't be paragraph aligned

[Bruce Rogers]

>>> On 3/30/2015 at 10:02 PM, Kevin O'Connor <kevin at koconnor.net> wrote: 
> On Mon, Mar 30, 2015 at 05:06:30PM -0600, Bruce Rogers wrote:
>> The SMBIOS anchor string _SM_ is stored within SeaBIOS to validate
>> an SMBIOS entry point structure. There is the possibility (observed)
>> that this comparison string ends up paragraph aligned and mistakenly
>> found during a search for the real SMBIOS entry point. Ensure it will
>> never end up on a paragraph boundary by storing it at odd alignment.
> 
> Thanks.
> 
> What OS was this on?  It's really an OS bug as the OS needs to check
> both the signature and the checksum.
> 
> My preferred approach to addressing this would be to turn
> p->anchor_string into a u32 and do an integer compare instead of a
> string compare.  Although technically this can lead to the same
> potential issue, in practice it should not happen because SeaBIOS'
> init code is relocated out of the f-segment during startup (while
> static strings are generally not).
> 
> -Kevin

This was actually flagged by the QEMU make check test in
tests/bios-tables-test.c, where the code has asserts based on the
first find of the _SM_ string found on a paragraph boundary in the
f segment. It assumes that the string found is sufficient to identify
the smbios entry point structure.

I read in http://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.0.0.pdf
where there are number of steps needed to verify the entry-point, beyond
finding the anchor string.  So I assume that the make check test needs to
be fixed.

But I wonder if bios creators are also supposed to ensure that there is no
such string findable on a paragraph boundary so as to avoid any potential
confusion? I don't have expereince writing bios's so I am only guessing
at that.

Bruce