Re: [PATCH v9 19/19] x86: EFI stub DRTM launch support for Secure Launch

ross.philipson
2024-06-04T17:23:16Z

On 5/31/24 4:09 AM, Ard Biesheuvel wrote:
> On Fri, 31 May 2024 at 03:32, Ross Philipson <ross.philipson@oracle.com> wrote:
>>
>> This support allows the DRTM launch to be initiated after an EFI stub
>> launch of the Linux kernel is done. This is accomplished by providing
>> a handler to jump to when a Secure Launch is in progress. This has to be
>> called after the EFI stub does Exit Boot Services.
>>
>> Signed-off-by: Ross Philipson <ross.philipson@oracle.com>
>
> Just some minor remarks below. The overall approach in this patch
> looks fine now.
>
>
>> ---
>> drivers/firmware/efi/libstub/x86-stub.c | 98 +++++++++++++++++++++++++
>> 1 file changed, 98 insertions(+)
>>
>> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
>> index d5a8182cf2e1..a1143d006202 100644
>> --- a/drivers/firmware/efi/libstub/x86-stub.c
>> +++ b/drivers/firmware/efi/libstub/x86-stub.c
>> @@ -9,6 +9,8 @@
>> #include <linux/efi.h>
>> #include <linux/pci.h>
>> #include <linux/stddef.h>
>> +#include <linux/slr_table.h>
>> +#include <linux/slaunch.h>
>>
>> #include <asm/efi.h>
>> #include <asm/e820/types.h>
>> @@ -830,6 +832,97 @@ static efi_status_t efi_decompress_kernel(unsigned long *kernel_entry)
>> return efi_adjust_memory_range_protection(addr, kernel_text_size);
>> }
>>
>> +#if (IS_ENABLED(CONFIG_SECURE_LAUNCH))
>
> IS_ENABLED() is mostly used for C conditionals not CPP ones.
>
> It would be nice if this #if could be dropped, and replaced with ... (see below)
>
>
>> +static bool efi_secure_launch_update_boot_params(struct slr_table *slrt,
>> + struct boot_params *boot_params)
>> +{
>> + struct slr_entry_intel_info *txt_info;
>> + struct slr_entry_policy *policy;
>> + struct txt_os_mle_data *os_mle;
>> + bool updated = false;
>> + int i;
>> +
>> + txt_info = slr_next_entry_by_tag(slrt, NULL, SLR_ENTRY_INTEL_INFO);
>> + if (!txt_info)
>> + return false;
>> +
>> + os_mle = txt_os_mle_data_start((void *)txt_info->txt_heap);
>> + if (!os_mle)
>> + return false;
>> +
>> + os_mle->boot_params_addr = (u32)(u64)boot_params;
>> +
>
> Why is this safe?

The size of the boot_params_addr is a holdover from the legacy boot
world when boot params were always loaded at a low address. We will
increase the size of the field.

>
>> + policy = slr_next_entry_by_tag(slrt, NULL, SLR_ENTRY_ENTRY_POLICY);
>> + if (!policy)
>> + return false;
>> +
>> + for (i = 0; i < policy->nr_entries; i++) {
>> + if (policy->policy_entries[i].entity_type == SLR_ET_BOOT_PARAMS) {
>> + policy->policy_entries[i].entity = (u64)boot_params;
>> + updated = true;
>> + break;
>> + }
>> + }
>> +
>> + /*
>> + * If this is a PE entry into EFI stub the mocked up boot params will
>> + * be missing some of the setup header data needed for the second stage
>> + * of the Secure Launch boot.
>> + */
>> + if (image) {
>> + struct setup_header *hdr = (struct setup_header *)((u8 *)image->image_base + 0x1f1);
>
> Could we use something other than a bare 0x1f1 constant here? struct
> boot_params has a struct setup_header at the correct offset, so with
> some casting of offsetof() use, we can make this look a lot more self
> explanatory.

Yes we can do this.

>
>
>> + u64 cmdline_ptr, hi_val;
>> +
>> + boot_params->hdr.setup_sects = hdr->setup_sects;
>> + boot_params->hdr.syssize = hdr->syssize;
>> + boot_params->hdr.version = hdr->version;
>> + boot_params->hdr.loadflags = hdr->loadflags;
>> + boot_params->hdr.kernel_alignment = hdr->kernel_alignment;
>> + boot_params->hdr.min_alignment = hdr->min_alignment;
>> + boot_params->hdr.xloadflags = hdr->xloadflags;
>> + boot_params->hdr.init_size = hdr->init_size;
>> + boot_params->hdr.kernel_info_offset = hdr->kernel_info_offset;
>> + hi_val = boot_params->ext_cmd_line_ptr;
>
> We have efi_set_u64_split() for this.

Ok I will use that then.

>
>> + cmdline_ptr = boot_params->hdr.cmd_line_ptr | hi_val << 32;
>> + boot_params->hdr.cmdline_size = strlen((const char *)cmdline_ptr);;
>> + }
>> +
>> + return updated;
>> +}
>> +
>> +static void efi_secure_launch(struct boot_params *boot_params)
>> +{
>> + struct slr_entry_dl_info *dlinfo;
>> + efi_guid_t guid = SLR_TABLE_GUID;
>> + dl_handler_func handler_callback;
>> + struct slr_table *slrt;
>> +
>
> ... a C conditional here, e.g.,
>
> if (!IS_ENABLED(CONFIG_SECURE_LAUNCH))
> return;
>
> The difference is that all the code will get compile test coverage
> every time, instead of only in configs that enable
> CONFIG_SECURE_LAUNCH.
>
> This significantly reduces the risk that your stuff will get broken
> inadvertently.

Understood, I will address these as you suggest.

>
>> + /*
>> + * The presence of this table indicated a Secure Launch
>> + * is being requested.
>> + */
>> + slrt = (struct slr_table *)get_efi_config_table(guid);
>> + if (!slrt || slrt->magic != SLR_TABLE_MAGIC)
>> + return;
>> +
>> + /*
>> + * Since the EFI stub library creates its own boot_params on entry, the
>> + * SLRT and TXT heap have to be updated with this version.
>> + */
>> + if (!efi_secure_launch_update_boot_params(slrt, boot_params))
>> + return;
>> +
>> + /* Jump through DL stub to initiate Secure Launch */
>> + dlinfo = slr_next_entry_by_tag(slrt, NULL, SLR_ENTRY_DL_INFO);
>> +
>> + handler_callback = (dl_handler_func)dlinfo->dl_handler;
>> +
>> + handler_callback(&dlinfo->bl_context);
>> +
>> + unreachable();
>> +}
>> +#endif
>> +
>> static void __noreturn enter_kernel(unsigned long kernel_addr,
>> struct boot_params *boot_params)
>> {
>> @@ -957,6 +1050,11 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
>> goto fail;
>> }
>>
>> +#if (IS_ENABLED(CONFIG_SECURE_LAUNCH))
>
> ... and drop this #if as well.

Yes.

Thanks
Ross

>
>> + /* If a Secure Launch is in progress, this never returns */
>> + efi_secure_launch(boot_params);
>> +#endif
>> +
>> /*
>> * Call the SEV init code while still running with the firmware's
>> * GDT/IDT, so #VC exceptions will be handled by EFI.
>> --
>> 2.39.3
>>