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

[Ard Biesheuvel]

On Fri, 31 May 2024 at 03:32, Ross Philipson <ross.philipson@xxxxxxxxxx> 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@xxxxxxxxxx>

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?

> +       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.


> +               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.

> +               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.

> +       /*
> +        * 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.

> +       /* 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
>