Re: [PATCH v3 RESEND] efi: Do not import certificates from UEFI Secure Boot for T2 Macs

[Mimi Zohar <zohar@linux.ibm.com>]

On Sun, 2022-04-10 at 10:49 +0000, Aditya Garg wrote:
> From: Aditya Garg <gargaditya08@live.com>
> 
> On T2 Macs, the secure boot is handled by the T2 Chip. If enabled, only
> macOS and Windows are allowed to boot on these machines. Thus we need to
> disable secure boot for Linux.

The end result might be "disable secure boot for Linux", but that isn't
what the code is actually doing.  As a result of not being able to read
or load certificates, secure boot cannot be enabled.  Please be more
precise.

> If we boot into Linux after disabling
> secure boot, if CONFIG_LOAD_UEFI_KEYS is enabled, EFI Runtime services
> fail to start, with the following logs in dmesg
> 
> Call Trace:
>  <TASK>
>  page_fault_oops+0x4f/0x2c0
>  ? search_bpf_extables+0x6b/0x80
>  ? search_module_extables+0x50/0x80
>  ? search_exception_tables+0x5b/0x60
>  kernelmode_fixup_or_oops+0x9e/0x110
>  __bad_area_nosemaphore+0x155/0x190
>  bad_area_nosemaphore+0x16/0x20
>  do_kern_addr_fault+0x8c/0xa0
>  exc_page_fault+0xd8/0x180
>  asm_exc_page_fault+0x1e/0x30
> (Removed some logs from here)
>  ? __efi_call+0x28/0x30
>  ? switch_mm+0x20/0x30
>  ? efi_call_rts+0x19a/0x8e0
>  ? process_one_work+0x222/0x3f0
>  ? worker_thread+0x4a/0x3d0
>  ? kthread+0x17a/0x1a0
>  ? process_one_work+0x3f0/0x3f0
>  ? set_kthread_struct+0x40/0x40
>  ? ret_from_fork+0x22/0x30
>  </TASK>
> ---[ end trace 1f82023595a5927f ]---
> efi: Froze efi_rts_wq and disabled EFI Runtime Services
> integrity: Couldn't get size: 0x8000000000000015
> integrity: MODSIGN: Couldn't get UEFI db list
> efi: EFI Runtime Services are disabled!
> integrity: Couldn't get size: 0x8000000000000015
> integrity: Couldn't get UEFI dbx list
> integrity: Couldn't get size: 0x8000000000000015
> integrity: Couldn't get mokx list
> integrity: Couldn't get size: 0x80000000
> 
> This patch prevents querying of these UEFI variables, since these Macs
> seem to use a non-standard EFI hardware
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Aditya Garg <gargaditya08@live.com>
> ---
> v2 :- Reduce code size of the table.
> V3 :- Close the brackets which were left open by mistake.
>  .../platform_certs/keyring_handler.h          |  8 ++++
>  security/integrity/platform_certs/load_uefi.c | 48 +++++++++++++++++++
>  2 files changed, 56 insertions(+)
> 
> diff --git a/security/integrity/platform_certs/keyring_handler.h b/security/integrity/platform_certs/keyring_handler.h
> index 2462bfa08..cd06bd607 100644
> --- a/security/integrity/platform_certs/keyring_handler.h
> +++ b/security/integrity/platform_certs/keyring_handler.h
> @@ -30,3 +30,11 @@ efi_element_handler_t get_handler_for_db(const efi_guid_t *sig_type);
>  efi_element_handler_t get_handler_for_dbx(const efi_guid_t *sig_type);
>  
>  #endif
> +
> +#ifndef UEFI_QUIRK_SKIP_CERT
> +#define UEFI_QUIRK_SKIP_CERT(vendor, product) \
> +		 .matches = { \
> +			DMI_MATCH(DMI_BOARD_VENDOR, vendor), \
> +			DMI_MATCH(DMI_PRODUCT_NAME, product), \
> +		},
> +#endif
> diff --git a/security/integrity/platform_certs/load_uefi.c b/security/integrity/platform_certs/load_uefi.c
> index 08b6d12f9..f246c8732 100644
> --- a/security/integrity/platform_certs/load_uefi.c
> +++ b/security/integrity/platform_certs/load_uefi.c
> @@ -3,6 +3,7 @@
>  #include <linux/kernel.h>
>  #include <linux/sched.h>
>  #include <linux/cred.h>
> +#include <linux/dmi.h>
>  #include <linux/err.h>
>  #include <linux/efi.h>
>  #include <linux/slab.h>
> @@ -12,6 +13,32 @@
>  #include "../integrity.h"
>  #include "keyring_handler.h"
>  
> +/* Apple Macs with T2 Security chip don't support these UEFI variables.

Please refer to Documentation/process/coding-style.rst for the format
of multi-line comments.

> + * The T2 chip manages the Secure Boot and does not allow Linux to boot
> + * if it is turned on. If turned off, an attempt to get certificates
> + * causes a crash, so we simply return 0 for them in each function.
> + */
> +

No need for a blank line here.

> +static const struct dmi_system_id uefi_skip_cert[] = {
> +
No need for a blank here either.

> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro15,1") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro15,2") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro15,3") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro15,4") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro16,1") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro16,2") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro16,3") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookPro16,4") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookAir8,1") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookAir8,2") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacBookAir9,1") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacMini8,1") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "MacPro7,1") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "iMac20,1") },
> +	{ UEFI_QUIRK_SKIP_CERT("Apple Inc.", "iMac20,2") },
> +	{ }
> +};
> +
>  /*
>   * Look to see if a UEFI variable called MokIgnoreDB exists and return true if
>   * it does.
> @@ -21,12 +48,18 @@
>   * is set, we should ignore the db variable also and the true return indicates
>   * this.
>   */
> +
Or here

>  static __init bool uefi_check_ignore_db(void)
>  {
>  	efi_status_t status;
>  	unsigned int db = 0;
>  	unsigned long size = sizeof(db);
>  	efi_guid_t guid = EFI_SHIM_LOCK_GUID;
> +	const struct dmi_system_id *dmi_id;
> +
> +	dmi_id = dmi_first_match(uefi_skip_cert);
> +	if (dmi_id)
> +		return 0;

The function returns a bool.  Return either "true" or "false".

>  
>  	status = efi.get_variable(L"MokIgnoreDB", &guid, NULL, &size, &db);
>  	return status == EFI_SUCCESS;
> @@ -41,6 +74,11 @@ static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
>  	unsigned long lsize = 4;
>  	unsigned long tmpdb[4];
>  	void *db;
> +	const struct dmi_system_id *dmi_id;
> +
> +	dmi_id = dmi_first_match(uefi_skip_cert);
> +	if (dmi_id)
> +		return 0;

The return value here should be NULL.

>  
>  	*status = efi.get_variable(name, guid, NULL, &lsize, &tmpdb);
>  	if (*status == EFI_NOT_FOUND)
> @@ -85,6 +123,11 @@ static int __init load_moklist_certs(void)
>  	unsigned long moksize;
>  	efi_status_t status;
>  	int rc;
> +	const struct dmi_system_id *dmi_id;
> +
> +	dmi_id = dmi_first_match(uefi_skip_cert);
> +	if (dmi_id)
> +		return 0;
>  
>  	/* First try to load certs from the EFI MOKvar config table.
>  	 * It's not an error if the MOKvar config table doesn't exist
> @@ -138,6 +181,11 @@ static int __init load_uefi_certs(void)
>  	unsigned long dbsize = 0, dbxsize = 0, mokxsize = 0;
>  	efi_status_t status;
>  	int rc = 0;
> +	const struct dmi_system_id *dmi_id;
> +
> +	dmi_id = dmi_first_match(uefi_skip_cert);
> +	if (dmi_id)
> +		return 0;

uefi_check_ignore_db(), get_cert_list(), uefi_check_ignore_db(), and
/load_moklist_certs() are all defined all static and are gated here by
this dmi_first_match().  There's probably no need for any of the other
calls to dmi_first_match().

Like in all the other cases, there should be some sort of message.  At
minimum, there should be a pr_info().

>  
>  	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE))
>  		return false;

thanks,

Mimi