Re: [PATCH] x86: fix q35 kernel measurements broken due to rng seeding

[James Bottomley <jejb@linux.ibm.com>]

On Wed, 2023-02-01 at 15:48 -0500, Jason A. Donenfeld wrote:
[...]
> But it sounds like you might now have a concrete suggestion on
> something even better. I'm CCing hpa, as this is his wheelhouse, and
> maybe you two can divise the next step while I'm away. Maybe the pad9
> thing you mentioned is the super nice solution we've been searching
> for this whole time. When I'm home in 10 days and have internet
> again, I'll take a look at where thing's are out and try to figure
> out how I can be productive again with it.

OK, so just FYI HPA, this is the patch I'm thinking of sending to
linux-kernel to reserve space in struct boot_params for this.  If you
could take a look and advise on the location before I send the final
patch, I'd be grateful.  I took space in _pad9 because that's the
standard method (add on to end), but it does strike me we could also
use all of _pad8 for the (the addition is only 48 bytes) or even _pad3
+ hd0_info + hd1_info.

James

---

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 9338c68e7413..0120ab77dac9 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -308,7 +308,7 @@ _start:
 	# Part 2 of the header, from the old setup.S
 
 		.ascii	"HdrS"		# header signature
-		.word	0x020f		# header version number (>= 0x0105)
+		.word	0x0210		# header version number (>= 0x0105)
 					# or else old loadlin-1.5 will fail)
 		.globl realmode_swtch
 realmode_swtch:	.word	0, 0		# default_switch, SETUPSEG
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 01d19fc22346..c614ff0755f2 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -181,6 +181,19 @@ struct ima_setup_data {
 	__u64 size;
 } __attribute__((packed));
 
+/*
+ * Define a boot_param area for the RNG seed which can be used via the
+ * setup_data mechanism (so must have a setup_data header) but which
+ * is embedded in boot_params because qemu has been unable to find
+ * a safe data space for it.  The value RNG_SEED_LENGTH must not
+ * change (pad length dependent on it) and must match the value in QEMU
+ */
+#define RNG_SEED_LENGTH	32
+struct random_seed_data {
+	struct	setup_data s;
+	__u8	data[RNG_SEED_LENGTH];
+} __attribute__((packed));
+
 /* The so-called "zeropage" */
 struct boot_params {
 	struct screen_info screen_info;			/* 0x000 */
@@ -228,7 +241,8 @@ struct boot_params {
 	struct boot_e820_entry e820_table[E820_MAX_ENTRIES_ZEROPAGE]; /* 0x2d0 */
 	__u8  _pad8[48];				/* 0xcd0 */
 	struct edd_info eddbuf[EDDMAXNR];		/* 0xd00 */
-	__u8  _pad9[276];				/* 0xeec */
+	struct random_seed_data random_seed;		/* 0xeec */
+	__u8  _pad9[228];				/* 0xf1c */
 } __attribute__((packed));
 
 /**
diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
index 6b58610a1552..fb719682579d 100644
--- a/arch/x86/kernel/kexec-bzimage64.c
+++ b/arch/x86/kernel/kexec-bzimage64.c
@@ -110,13 +110,10 @@ static int setup_e820_entries(struct boot_params *params)
 	return 0;
 }
 
-enum { RNG_SEED_LENGTH = 32 };
-
 static void
-setup_rng_seed(struct boot_params *params, unsigned long params_load_addr,
-	       unsigned int rng_seed_setup_data_offset)
+setup_rng_seed(struct boot_params *params, unsigned long params_load_addr)
 {
-	struct setup_data *sd = (void *)params + rng_seed_setup_data_offset;
+	struct setup_data *sd = &params->random_seed.s;
 	unsigned long setup_data_phys;
 
 	if (!rng_is_initialized())
@@ -125,7 +122,8 @@ setup_rng_seed(struct boot_params *params, unsigned long params_load_addr,
 	sd->type = SETUP_RNG_SEED;
 	sd->len = RNG_SEED_LENGTH;
 	get_random_bytes(sd->data, RNG_SEED_LENGTH);
-	setup_data_phys = params_load_addr + rng_seed_setup_data_offset;
+	setup_data_phys = params_load_addr + offsetof(struct boot_params,
+						      random_seed);
 	sd->next = params->hdr.setup_data;
 	params->hdr.setup_data = setup_data_phys;
 }
@@ -306,7 +304,7 @@ setup_boot_parameters(struct kimage *image, struct boot_params *params,
 	}
 
 	/* Setup RNG seed */
-	setup_rng_seed(params, params_load_addr, setup_data_offset);
+	setup_rng_seed(params, params_load_addr);
 
 	/* Setup EDD info */
 	memcpy(params->eddbuf, boot_params.eddbuf,