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Subject: Re: [PATCH v6] efi: Do not import certificates from UEFI Secure
 Boot for T2 Macs
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Aditya Garg <gargaditya08@live.com>,
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Cc:     "linux-integrity@vger.kernel.org" <linux-integrity@vger.kernel.org>,
        "keyrings@vger.kernel.org" <keyrings@vger.kernel.org>,
        "linux-security-module@vger.kernel.org" 
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        Orlando Chamberlain <redecorating@protonmail.com>,
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Date:   Fri, 15 Apr 2022 12:26:29 -0400
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On Fri, 2022-04-15 at 06:19 +0000, Aditya Garg wrote:
> From: Aditya Garg <gargaditya08@live.com>
> 
> On Apple T2 Macs, when Linux attempts to read the db and dbx efi variables
> at early boot to load UEFI Secure Boot certificates, a page fault occurs
> in Apple firmware code and EFI runtime services are disabled with the
> following logs:
> 
> [Firmware Bug]: Page fault caused by firmware at PA: 0xffffb1edc0068000
> WARNING: CPU: 3 PID: 104 at arch/x86/platform/efi/quirks.c:735 efi_crash_gracefully_on_page_fault+0x50/0xf0
> (Removed some logs from here)
> 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 skips reading these UEFI variables and thus prevents the crash.

Instead of "This patch skips reading" say "Avoid reading".

> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Aditya Garg <gargaditya08@live.com>

After making these minor changes, both above and below, 
	Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>

> ---
> v2 :- Reduce code size of the table.
> v3 :- Close the brackets which were left open by mistake.
> v4 :- Fix comment style issues, remove blank spaces and limit use of dmi_first_match()
> v4 RESEND :- Add stable to cc
> v5 :- Rewrite the description
> v6 :- Make description more clear
>  .../platform_certs/keyring_handler.h          |  8 +++++
>  security/integrity/platform_certs/load_uefi.c | 33 +++++++++++++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/security/integrity/platform_certs/keyring_handler.h b/security/integrity/platform_certs/keyring_handler.h
> index 284558f30..212d894a8 100644
> --- a/security/integrity/platform_certs/keyring_handler.h
> +++ b/security/integrity/platform_certs/keyring_handler.h
> @@ -35,3 +35,11 @@ efi_element_handler_t get_handler_for_mok(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 5f45c3c07..1a7e7d597 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 +13,31 @@
>  #include "../integrity.h"
>  #include "keyring_handler.h"
>  
> +/*
> + * On T2 Macs reading the reading the db and dbx efi variables to load UEFI
> + * Secure Boot certificates causes occurrence of a page fault in Apple's
> + * firmware and a crash disabling EFI runtime services. The following quirk
> + * skips reading these variables.
> + */
> +static const struct dmi_system_id uefi_skip_cert[] = {
> +	{ 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.
> @@ -138,6 +164,13 @@ 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) {
> +		pr_err("Getting UEFI Secure Boot Certs is not supported on T2 Macs.\n");

Replace "Getting" with "Reading".

thanks,

Mimi

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