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

From: Ard Biesheuvel
Date: Tue Jun 04 2024 - 13:28:36 EST

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.