Re: [Xen-devel] [PATCH] xsplice: Use ld-embedded build-ids

[Martin Pohlack <mpohlack@xxxxxxxxxx>]

On 11.08.2015 16:12, Jan Beulich wrote:
>>>> On 05.08.15 at 16:09, <mpohlack@xxxxxxxxx> wrote:
>> Todo:
>>   * Should be moved to sysctl to only allow Dom0 access
> 
> Because of?

The discussion in this thread:

[Xen-devel] [RFC PATCH v3.1 2/2] xsplice: Add hook for build_id

was:
----------------------------------------------------------------------
>> Martin Pohlack:
>> We should not expose the build_id to normal guests, but only to Dom0.
>>
>> A build_id uniquely identifies a specific build and I don't see how that
>> information would be required from DomU.  It might actually help an
>> attacker to build his return-oriented programming exploit against a
>> specific build.
>>
>> The normal version numbers should be enough to know about capabilities
>> and API.
>
> Andrew Cooper:
> 
> It will need its own XSM hook, but need not be strictly limited to just
> dom0.
----------------------------------------------------------------------

>>   * Maybe convert to binary transport to userland instead of printable form
> 
> Indeed.
> 
>> @@ -360,11 +366,30 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
>> arg)
>>  
>>      case XENVER_build_id:
>>      {
>> -        xen_build_id_t build_id;
>> +        xen_build_id_t ascii_id;
>> +        Elf_Note * n = (Elf_Note *)&__note_gnu_build_id_start;
>> +        char * binary_id;
>> +        int i;
>> +
>> +        memset(ascii_id, 0, sizeof(ascii_id));
>> +
>> +        /* check if we really have a build-id */
>> +        if ( NT_GNU_BUILD_ID != n->type )
>> +            return 0;
> 
> This needs to signal an error.

Yes, ENOSYS, (or ENOENT, ENODATA)?

>> +
>> +        /* sanity check, name should be "GNU" for ld-generated build-id */
>> +        if ( 0 != strncmp(ELFNOTE_NAME(n), "GNU", n->namesz))
>> +            return 0;
> 
> Same here.
> 
>> +        binary_id = (char *)ELFNOTE_DESC(n);
>> +
>> +        /* convert to printable format */
>> +        for (i = 0; i < n->descsz && (i + 1) * 2 < sizeof(xen_build_id_t); 
>> i++)
>> +        {
>> +            snprintf(&ascii_id[i * 2], 3, "%02hhx", binary_id[i]);
>> +        }
> 
> No need for the braces, and no need for the"hh" modifier.
> 
> Jan
> 

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