Re: [PATCH] x86/boot: Fix PVH boot during boot_info transition period

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/10/2024 3:12 pm, Roger Pau Monné wrote:
> On Tue, Oct 22, 2024 at 01:41:14PM +0100, Andrew Cooper wrote:
>> multiboot_fill_boot_info() taking the physical address of the multiboot_info
>> structure leads to a subtle use-after-free on the PVH path, with rather less
>> subtle fallout.
>>
>> The pointers used by __start_xen(), mbi and mod, are either:
>>
>>  - MB:  Directmap pointers into the trampoline, or
>>  - PVH: Xen pointers into .initdata, or
>>  - EFI: Directmap pointers into Xen.
>>
>> Critically, these either remain valid across move_xen() (MB, PVH), or rely on
>> move_xen() being inhibited (EFI).
>>
>> The conversion to multiboot_fill_boot_info(), taking only mbi_p, makes the 
>> PVH
>> path use directmap pointers into Xen, as well as move_xen() which invalidates
>> said pointers.
>>
>> Switch multiboot_fill_boot_info() to consume the same virtual addresses that
>> __start_xen() currently uses.  This keeps all the pointers valid for the
>> duration of __start_xen(), for all boot protocols.
>>
>> It can be safely untangled once multiboot_fill_boot_info() takes a full copy
>> the multiboot info data, and __start_xen() has been moved over to using the
>> new boot_info consistently.
>>
>> Right now, bi->{loader,cmdline,mods} are problematic.  Nothing uses
>> bi->mods[], and nothing uses bi->cmdline after move_xen().
>>
>> bi->loader is used after move_xen(), although only for cmdline_cook() of
>> dom0's cmdline, where it happens to be benign because PVH boot skips the
>> inspection of the bootloader name.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

>
> One nit below about dropping a const keyword.
>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
>>
>> This is more proof that Xen only boots by accident.  It certainly isn't by 
>> any
>> kind of design.
>>
>> https://gitlab.com/xen-project/people/andyhhp/xen/-/pipelines/1506947472
>> including the pending work to add a PVShim test
>>
>> This is the least-invasive fix given the rest of the Hyperlaunch series.
>>
>> A different option would to introduce EFI and PVH forms of
>> multiboot_fill_boot_info(), but that would involve juggling even more moving
>> parts during the transition period.
>> ---
>>  xen/arch/x86/setup.c | 25 ++++++++++++++++++++-----
>>  1 file changed, 20 insertions(+), 5 deletions(-)
>>
>> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
>> index db670258d650..e43b56d4e80f 100644
>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -283,11 +283,10 @@ struct boot_info __initdata xen_boot_info = {
>>      .cmdline = "",
>>  };
>>  
>> -static struct boot_info *__init multiboot_fill_boot_info(unsigned long 
>> mbi_p)
>> +static struct boot_info *__init multiboot_fill_boot_info(
>> +    multiboot_info_t *mbi, module_t *mods)
> Shouldn't mbi keep the const keyword?  Given there are no changes on
> how it's used in multiboot_fill_boot_info().

Fine.

FWIW,
https://gitlab.com/xen-project/people/andyhhp/xen/-/pipelines/1507153249
is previously-crashing consider_modules() change, with this fix in
place, shown now to be functioning.

~Andrew