Re: [PATCH v9 08/19] x86: Secure Launch kernel early boot stub

From: Ard Biesheuvel
Date: Tue Jun 04 2024 - 16:55:01 EST

On Tue, 4 Jun 2024 at 19:34, <ross.philipson@xxxxxxxxxx> wrote:
>
> On 6/4/24 10:27 AM, Ard Biesheuvel wrote:
> > On Tue, 4 Jun 2024 at 19:24, <ross.philipson@xxxxxxxxxx> wrote:
> >>
> >> On 5/31/24 6:33 AM, Ard Biesheuvel wrote:
> >>> On Fri, 31 May 2024 at 13:00, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >>>>
> >>>> Hello Ross,
> >>>>
> >>>> On Fri, 31 May 2024 at 03:32, Ross Philipson <ross.philipson@xxxxxxxxxx> wrote:
> >>>>>
> >>>>> The Secure Launch (SL) stub provides the entry point for Intel TXT (and
> >>>>> later AMD SKINIT) to vector to during the late launch. The symbol
> >>>>> sl_stub_entry is that entry point and its offset into the kernel is
> >>>>> conveyed to the launching code using the MLE (Measured Launch
> >>>>> Environment) header in the structure named mle_header. The offset of the
> >>>>> MLE header is set in the kernel_info. The routine sl_stub contains the
> >>>>> very early late launch setup code responsible for setting up the basic
> >>>>> environment to allow the normal kernel startup_32 code to proceed. It is
> >>>>> also responsible for properly waking and handling the APs on Intel
> >>>>> platforms. The routine sl_main which runs after entering 64b mode is
> >>>>> responsible for measuring configuration and module information before
> >>>>> it is used like the boot params, the kernel command line, the TXT heap,
> >>>>> an external initramfs, etc.
> >>>>>
> >>>>> Signed-off-by: Ross Philipson <ross.philipson@xxxxxxxxxx>
> >>>>> ---
> >>>>> Documentation/arch/x86/boot.rst | 21 +
> >>>>> arch/x86/boot/compressed/Makefile | 3 +-
> >>>>> arch/x86/boot/compressed/head_64.S | 30 +
> >>>>> arch/x86/boot/compressed/kernel_info.S | 34 ++
> >>>>> arch/x86/boot/compressed/sl_main.c | 577 ++++++++++++++++++++
> >>>>> arch/x86/boot/compressed/sl_stub.S | 725 +++++++++++++++++++++++++
> >>>>> arch/x86/include/asm/msr-index.h | 5 +
> >>>>> arch/x86/include/uapi/asm/bootparam.h | 1 +
> >>>>> arch/x86/kernel/asm-offsets.c | 20 +
> >>>>> 9 files changed, 1415 insertions(+), 1 deletion(-)
> >>>>> create mode 100644 arch/x86/boot/compressed/sl_main.c
> >>>>> create mode 100644 arch/x86/boot/compressed/sl_stub.S
> >>>>>
> >>>>> diff --git a/Documentation/arch/x86/boot.rst b/Documentation/arch/x86/boot.rst
> >>>>> index 4fd492cb4970..295cdf9bcbdb 100644
> >>>>> --- a/Documentation/arch/x86/boot.rst
> >>>>> +++ b/Documentation/arch/x86/boot.rst
> >>>>> @@ -482,6 +482,14 @@ Protocol: 2.00+
> >>>>> - If 1, KASLR enabled.
> >>>>> - If 0, KASLR disabled.
> >>>>>
> >>>>> + Bit 2 (kernel internal): SLAUNCH_FLAG
> >>>>> +
> >>>>> + - Used internally by the setup kernel to communicate
> >>>>> + Secure Launch status to kernel proper.
> >>>>> +
> >>>>> + - If 1, Secure Launch enabled.
> >>>>> + - If 0, Secure Launch disabled.
> >>>>> +
> >>>>> Bit 5 (write): QUIET_FLAG
> >>>>>
> >>>>> - If 0, print early messages.
> >>>>> @@ -1028,6 +1036,19 @@ Offset/size: 0x000c/4
> >>>>>
> >>>>> This field contains maximal allowed type for setup_data and setup_indirect structs.
> >>>>>
> >>>>> +============ =================
> >>>>> +Field name: mle_header_offset
> >>>>> +Offset/size: 0x0010/4
> >>>>> +============ =================
> >>>>> +
> >>>>> + This field contains the offset to the Secure Launch Measured Launch Environment
> >>>>> + (MLE) header. This offset is used to locate information needed during a secure
> >>>>> + late launch using Intel TXT. If the offset is zero, the kernel does not have
> >>>>> + Secure Launch capabilities. The MLE entry point is called from TXT on the BSP
> >>>>> + following a success measured launch. The specific state of the processors is
> >>>>> + outlined in the TXT Software Development Guide, the latest can be found here:
> >>>>> + https://urldefense.com/v3/__https://www.intel.com/content/dam/www/public/us/en/documents/guides/intel-txt-software-development-guide.pdf__;!!ACWV5N9M2RV99hQ!Mng0gnPhOYZ8D02t1rYwQfY6U3uWaypJyd1T2rsWz3QNHr9GhIZ9ANB_-cgPExxX0e0KmCpda-3VX8Fj$
> >>>>> +
> >>>>>
> >>>>
> >>>> Could we just repaint this field as the offset relative to the start
> >>>> of kernel_info rather than relative to the start of the image? That
> >>>> way, there is no need for patch #1, and given that the consumer of
> >>>> this field accesses it via kernel_info, I wouldn't expect any issues
> >>>> in applying this offset to obtain the actual address.
> >>>>
> >>>>
> >>>>> The Image Checksum
> >>>>> ==================
> >>>>> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> >>>>> index 9189a0e28686..9076a248d4b4 100644
> >>>>> --- a/arch/x86/boot/compressed/Makefile
> >>>>> +++ b/arch/x86/boot/compressed/Makefile
> >>>>> @@ -118,7 +118,8 @@ vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
> >>>>> vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_mixed.o
> >>>>> vmlinux-objs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
> >>>>>
> >>>>> -vmlinux-objs-$(CONFIG_SECURE_LAUNCH) += $(obj)/early_sha1.o $(obj)/early_sha256.o
> >>>>> +vmlinux-objs-$(CONFIG_SECURE_LAUNCH) += $(obj)/early_sha1.o $(obj)/early_sha256.o \
> >>>>> + $(obj)/sl_main.o $(obj)/sl_stub.o
> >>>>>
> >>>>> $(obj)/vmlinux: $(vmlinux-objs-y) FORCE
> >>>>> $(call if_changed,ld)
> >>>>> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> >>>>> index 1dcb794c5479..803c9e2e6d85 100644
> >>>>> --- a/arch/x86/boot/compressed/head_64.S
> >>>>> +++ b/arch/x86/boot/compressed/head_64.S
> >>>>> @@ -420,6 +420,13 @@ SYM_CODE_START(startup_64)
> >>>>> pushq $0
> >>>>> popfq
> >>>>>
> >>>>> +#ifdef CONFIG_SECURE_LAUNCH
> >>>>> + /* Ensure the relocation region is coverd by a PMR */
> >>>>
> >>>> covered
> >>>>
> >>>>> + movq %rbx, %rdi
> >>>>> + movl $(_bss - startup_32), %esi
> >>>>> + callq sl_check_region
> >>>>> +#endif
> >>>>> +
> >>>>> /*
> >>>>> * Copy the compressed kernel to the end of our buffer
> >>>>> * where decompression in place becomes safe.
> >>>>> @@ -462,6 +469,29 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
> >>>>> shrq $3, %rcx
> >>>>> rep stosq
> >>>>>
> >>>>> +#ifdef CONFIG_SECURE_LAUNCH
> >>>>> + /*
> >>>>> + * Have to do the final early sl stub work in 64b area.
> >>>>> + *
> >>>>> + * *********** NOTE ***********
> >>>>> + *
> >>>>> + * Several boot params get used before we get a chance to measure
> >>>>> + * them in this call. This is a known issue and we currently don't
> >>>>> + * have a solution. The scratch field doesn't matter. There is no
> >>>>> + * obvious way to do anything about the use of kernel_alignment or
> >>>>> + * init_size though these seem low risk with all the PMR and overlap
> >>>>> + * checks in place.
> >>>>> + */
> >>>>> + movq %r15, %rdi
> >>>>> + callq sl_main
> >>>>> +
> >>>>> + /* Ensure the decompression location is covered by a PMR */
> >>>>> + movq %rbp, %rdi
> >>>>> + movq output_len(%rip), %rsi
> >>>>> + callq sl_check_region
> >>>>> +#endif
> >>>>> +
> >>>>> + pushq %rsi
> >>>>
> >>>> This looks like a rebase error.
> >>>>
> >>>>> call load_stage2_idt
> >>>>>
> >>>>> /* Pass boot_params to initialize_identity_maps() */
> >>>>> diff --git a/arch/x86/boot/compressed/kernel_info.S b/arch/x86/boot/compressed/kernel_info.S
> >>>>> index c18f07181dd5..e199b87764e9 100644
> >>>>> --- a/arch/x86/boot/compressed/kernel_info.S
> >>>>> +++ b/arch/x86/boot/compressed/kernel_info.S
> >>>>> @@ -28,6 +28,40 @@ SYM_DATA_START(kernel_info)
> >>>>> /* Maximal allowed type for setup_data and setup_indirect structs. */
> >>>>> .long SETUP_TYPE_MAX
> >>>>>
> >>>>> + /* Offset to the MLE header structure */
> >>>>> +#if IS_ENABLED(CONFIG_SECURE_LAUNCH)
> >>>>> + .long rva(mle_header)
> >>>>
> >>>> ... so this could just be mle_header - kernel_info, and the consumer
> >>>> can do the math instead.
> >>>>
> >>>>> +#else
> >>>>> + .long 0
> >>>>> +#endif
> >>>>> +
> >>>>> kernel_info_var_len_data:
> >>>>> /* Empty for time being... */
> >>>>> SYM_DATA_END_LABEL(kernel_info, SYM_L_LOCAL, kernel_info_end)
> >>>>> +
> >>>>> +#if IS_ENABLED(CONFIG_SECURE_LAUNCH)
> >>>>> + /*
> >>>>> + * The MLE Header per the TXT Specification, section 2.1
> >>>>> + * MLE capabilities, see table 4. Capabilities set:
> >>>>> + * bit 0: Support for GETSEC[WAKEUP] for RLP wakeup
> >>>>> + * bit 1: Support for RLP wakeup using MONITOR address
> >>>>> + * bit 2: The ECX register will contain the pointer to the MLE page table
> >>>>> + * bit 5: TPM 1.2 family: Details/authorities PCR usage support
> >>>>> + * bit 9: Supported format of TPM 2.0 event log - TCG compliant
> >>>>> + */
> >>>>> +SYM_DATA_START(mle_header)
> >>>>> + .long 0x9082ac5a /* UUID0 */
> >>>>> + .long 0x74a7476f /* UUID1 */
> >>>>> + .long 0xa2555c0f /* UUID2 */
> >>>>> + .long 0x42b651cb /* UUID3 */
> >>>>> + .long 0x00000034 /* MLE header size */
> >>>>> + .long 0x00020002 /* MLE version 2.2 */
> >>>>> + .long rva(sl_stub_entry) /* Linear entry point of MLE (virt. address) */
> >>>>
> >>>> and these should perhaps be relative to mle_header?
> >>>>
> >>>>> + .long 0x00000000 /* First valid page of MLE */
> >>>>> + .long 0x00000000 /* Offset within binary of first byte of MLE */
> >>>>> + .long rva(_edata) /* Offset within binary of last byte + 1 of MLE */
> >>>>
> >>>> and here
> >>>>
> >>>
> >>> Ugh never mind - these are specified externally.
> >>
> >> Can you clarify your follow on comment here?
> >>
> >
> > I noticed that -as you pointed out in your previous reply- these
> > fields cannot be repainted at will, as they are defined by an external
> > specification.
> >
> > I'll play a bit more with this code tomorrow - I would *really* like
> > to avoid the need for patch #1, as it adds another constraint on how
> > we construct the boot image, and this is already riddled with legacy
> > and other complications.
>
> Yea I should have read forward through all your replies before
> responding to the first one but I think it clarified things as you point
> out here. We appreciate you help and suggestions.
>

OK, so I have a solution that does not require kernel_info at a fixed offset:

- put this at the end of arch/x86/boot/compressed/vmlinux.lds.S

#ifdef CONFIG_SECURE_LAUNCH
PROVIDE(kernel_info_offset = ABSOLUTE(kernel_info - startup_32));
PROVIDE(mle_header_offset = kernel_info_offset +
ABSOLUTE(mle_header - startup_32));
PROVIDE(sl_stub_entry_offset = kernel_info_offset +
ABSOLUTE(sl_stub_entry - startup_32));
PROVIDE(_edata_offset = kernel_info_offset + ABSOLUTE(_edata
- startup_32));
#endif


and use this for the header fields:

/* Offset to the MLE header structure */
#if IS_ENABLED(CONFIG_SECURE_LAUNCH)
.long mle_header_offset - kernel_info
#else
.long 0
#endif



SYM_DATA_START(mle_header)
.long 0x9082ac5a /* UUID0 */
.long 0x74a7476f /* UUID1 */
.long 0xa2555c0f /* UUID2 */
.long 0x42b651cb /* UUID3 */
.long 0x00000034 /* MLE header size */
.long 0x00020002 /* MLE version 2.2 */
.long sl_stub_entry_offset - kernel_info /* Linear entry
point of MLE (virt. address) */
.long 0x00000000 /* First valid page of MLE */
.long 0x00000000 /* Offset within binary of first byte of MLE */
.long _edata_offset - kernel_info /* Offset within binary of
last byte + 1 of MLE */
.long 0x00000227 /* Bit vector of MLE-supported capabilities */
.long 0x00000000 /* Starting linear address of command line
(unused) */
.long 0x00000000 /* Ending linear address of command line (unused) */