linux-kernel - Re: [PATCH] efi: Do not import certificates from UEFI Secure Boot for T2 Macs

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Message-ID: <20220209193705.GA15463@srcf.ucam.org>
Date:   Wed, 9 Feb 2022 19:37:05 +0000
From:   Matthew Garrett <mjg59@...f.ucam.org>
To:     Aditya Garg <gargaditya08@...e.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Jeremy Kerr <jk@...abs.org>,
        "joeyli.kernel@...il.com" <joeyli.kernel@...il.com>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "eric.snowberg@...cle.com" <eric.snowberg@...cle.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "jlee@...e.com" <jlee@...e.com>,
        "James.Bottomley@...senpartnership.com" 
        <James.Bottomley@...senPartnership.com>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "mic@...ikod.net" <mic@...ikod.net>,
        "dmitry.kasatkin@...il.com" <dmitry.kasatkin@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        Orlando Chamberlain <redecorating@...tonmail.com>,
        Aun-Ali Zaidi <admin@...eit.net>
Subject: Re: [PATCH] efi: Do not import certificates from UEFI Secure Boot
 for T2 Macs

On Wed, Feb 09, 2022 at 06:35:45PM +0000, Matthew Garrett wrote:

> The LOAD_UEFI_KEYS code isn't doing anything special here - it's just 
> trying to read some variables. If we simply disable that then the 
> expectation would be that reading the same variables from userland would 
> trigger the same failure. So the question is which of the variables that 
> LOAD_UEFI_KEYS accesses is triggering the failure, and what's special 
> about that? If it's a specific variable GUID or name that's failing, we 
> should block that on Apple hardware in order to avoid issues caused by 
> userland performing equivalent accesses.

ie, can you try something like this?

diff --git a/drivers/firmware/efi/runtime-wrappers.c b/drivers/firmware/efi/runtime-wrappers.c
index f3e54f6616f0..01cbd4811d1e 100644
--- a/drivers/firmware/efi/runtime-wrappers.c
+++ b/drivers/firmware/efi/runtime-wrappers.c
@@ -32,6 +32,8 @@
 #include <linux/stringify.h>
 #include <linux/workqueue.h>
 #include <linux/completion.h>
+#include <linux/ucs2_string.h>
+#include <linux/slab.h>
 
 #include <asm/efi.h>
 
@@ -203,6 +205,21 @@ static void efi_call_rts(struct work_struct *work)
 				       (efi_time_t *)arg2);
 		break;
 	case EFI_GET_VARIABLE:
+		unsigned long utf8_name_size;
+		char *utf8_name;
+		char guid_str[sizeof(efi_guid_t)+1];
+
+		utf8_name_size = ucs2_utf8size((efi_char16_t *)arg1);
+		utf8_name = kmalloc(utf8_name_size+1, GFP_KERNEL);
+		if (!utf8_name) {
+			printk(KERN_INFO "failed to allocate UTF8 buffer\n");
+			break;
+		}
+
+		ucs2_as_utf8(utf8_name, (efi_char16_t *)arg1, utf8_name_size + 1);
+		efi_guid_to_str((efi_guid_t *)arg2, guid_str);
+
+		printk(KERN_INFO "Reading EFI variable %s-%s\n", utf8_name, guid_str);
 		status = efi_call_virt(get_variable, (efi_char16_t *)arg1,
 				       (efi_guid_t *)arg2, (u32 *)arg3,
 				       (unsigned long *)arg4, (void *)arg5);