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

[H. Peter Anvin]

On February 2, 2023 6:38:12 AM PST, James Bottomley <jejb@linux.ibm.com> wrote:
>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,
>

NAK. We need to fix the actual problem of the kernel stomping on memory it 
shouldn't, not paper around it.