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

[James Bottomley]

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,