Re: [qemu-s390x] [PATCH] s390-bios: Skip bootmap signature entries

[Cornelia Huck]

On Mon, 6 May 2019 12:18:42 +0200
Christian Borntraeger <address@hidden> wrote:

> On 06.05.19 12:16, Thomas Huth wrote:
> > On 06/05/2019 12.10, David Hildenbrand wrote:  
> >> On 06.05.19 12:01, David Hildenbrand wrote:  
> >>> On 29.04.19 15:09, Jason J. Herne wrote:  
> >>>> Newer versions of zipl have the ability to write signature entries to 
> >>>> the boot
> >>>> script for secure boot. We don't yet support secure boot, but we need to 
> >>>> skip
> >>>> over signature entries while reading the boot script in order to 
> >>>> maintain our
> >>>> ability to boot guest operating systems that have a secure bootloader.
> >>>>
> >>>> Signed-off-by: Jason J. Herne <address@hidden>
> >>>> Reviewed-by: Farhan Ali <address@hidden>
> >>>> ---
> >>>>  pc-bios/s390-ccw/bootmap.c | 19 +++++++++++++++++--
> >>>>  pc-bios/s390-ccw/bootmap.h | 10 ++++++----
> >>>>  2 files changed, 23 insertions(+), 6 deletions(-)
> >>>>
> >>>> diff --git a/pc-bios/s390-ccw/bootmap.c b/pc-bios/s390-ccw/bootmap.c
> >>>> index 7aef65a..d13b7cb 100644
> >>>> --- a/pc-bios/s390-ccw/bootmap.c
> >>>> +++ b/pc-bios/s390-ccw/bootmap.c
> >>>> @@ -254,7 +254,14 @@ static void run_eckd_boot_script(block_number_t 
> >>>> bmt_block_nr,
> >>>>      memset(sec, FREE_SPACE_FILLER, sizeof(sec));
> >>>>      read_block(block_nr, sec, "Cannot read Boot Map Script");
> >>>>  
> >>>> -    for (i = 0; bms->entry[i].type == BOOT_SCRIPT_LOAD; i++) {
> >>>> +    for (i = 0; bms->entry[i].type == BOOT_SCRIPT_LOAD ||
> >>>> +                bms->entry[i].type == BOOT_SCRIPT_SIGNATURE; i++) {
> >>>> +
> >>>> +        /* We don't support secure boot yet, so we skip signature 
> >>>> entries */
> >>>> +        if (bms->entry[i].type == BOOT_SCRIPT_SIGNATURE) {
> >>>> +            continue;
> >>>> +        }
> >>>> +
> >>>>          address = bms->entry[i].address.load_address;
> >>>>          block_nr = eckd_block_num(&bms->entry[i].blkptr.xeckd.bptr.chs);
> >>>>  
> >>>> @@ -489,7 +496,15 @@ static void zipl_run(ScsiBlockPtr *pte)
> >>>>  
> >>>>      /* Load image(s) into RAM */
> >>>>      entry = (ComponentEntry *)(&header[1]);
> >>>> -    while (entry->component_type == ZIPL_COMP_ENTRY_LOAD) {
> >>>> +    while (entry->component_type == ZIPL_COMP_ENTRY_LOAD ||
> >>>> +           entry->component_type == ZIPL_COMP_ENTRY_SIGNATURE) {
> >>>> +
> >>>> +        /* We don't support secure boot yet, so we skip signature 
> >>>> entries */
> >>>> +        if (entry->component_type == ZIPL_COMP_ENTRY_SIGNATURE) {
> >>>> +            entry++;
> >>>> +            continue;
> >>>> +        }
> >>>> +
> >>>>          zipl_load_segment(entry);
> >>>>  
> >>>>          entry++;
> >>>> diff --git a/pc-bios/s390-ccw/bootmap.h b/pc-bios/s390-ccw/bootmap.h
> >>>> index a085212..94f53a5 100644
> >>>> --- a/pc-bios/s390-ccw/bootmap.h
> >>>> +++ b/pc-bios/s390-ccw/bootmap.h
> >>>> @@ -98,8 +98,9 @@ typedef struct ScsiMbr {
> >>>>  #define ZIPL_COMP_HEADER_IPL    0x00
> >>>>  #define ZIPL_COMP_HEADER_DUMP   0x01
> >>>>  
> >>>> -#define ZIPL_COMP_ENTRY_LOAD    0x02
> >>>> -#define ZIPL_COMP_ENTRY_EXEC    0x01
> >>>> +#define ZIPL_COMP_ENTRY_EXEC      0x01
> >>>> +#define ZIPL_COMP_ENTRY_LOAD      0x02
> >>>> +#define ZIPL_COMP_ENTRY_SIGNATURE 0x03
> >>>>  
> >>>>  typedef struct XEckdMbr {
> >>>>      uint8_t magic[4];   /* == "xIPL"        */
> >>>> @@ -117,8 +118,9 @@ typedef struct BootMapScriptEntry {
> >>>>      BootMapPointer blkptr;
> >>>>      uint8_t pad[7];
> >>>>      uint8_t type;   /* == BOOT_SCRIPT_* */
> >>>> -#define BOOT_SCRIPT_EXEC 0x01
> >>>> -#define BOOT_SCRIPT_LOAD 0x02
> >>>> +#define BOOT_SCRIPT_EXEC      0x01
> >>>> +#define BOOT_SCRIPT_LOAD      0x02
> >>>> +#define BOOT_SCRIPT_SIGNATURE 0x03
> >>>>      union {
> >>>>          uint64_t load_address;
> >>>>          uint64_t load_psw;
> >>>>  
> >>>
> >>> Naive question from me:
> >>>
> >>> Can't we place the signatures somewhere else, and instead associate them
> >>> with entries? This avoids breaking backwards compatibility for the sake
> >>> of signatures we want unmodified zipl loaders to ignore.
> >>>  
> >>
> >>
> >> ... but I guess this is already documented somewhere internally and
> >> other components have been adjusted. IOW, cannot be changed anymore.
> >>
> >> Guess our implementation should have tolerated other entries than
> >> "BOOT_SCRIPT_LOAD" right from the beginning.  
> > 
> > Hmm, now we only tolerate the _LOAD and _SIGNATURE entries, but still
> > nothing else... would it make sense to rewrite the code a little bit to
> > tolerate all other kind of entries, but just act on the well-known _LOAD
> > entries, so that we do not step into this trap in the future anymore?  
> 
> I think we should not. Those entries might have sematic elements that the 
> guest
> wants to enforce. I do not think that this will come, but imagine a boot entry
> that mandates some security wishes (e.g. do only run on non-shared cores).

Can we split the namespace for BOOT_SCRIPT into 'ignore if you don't
know what that is' and 'fail if you don't know what that is'? I'm
completely confused how 'optional' those entries are supposed to be...