Re: [PATCH] x86: Don't add RNG seed to Linux cmdline for SEV guests

[Jason A. Donenfeld]

On Tue, Feb 07, 2023 at 08:41:16AM +0000, Dov Murik wrote:
> Recent feature to supply RNG seed to the guest kernel modifies the
> kernel command-line by adding extra data at its end; this breaks
> measured boot with SEV and OVMF, and possibly signed boot.
> 
> Specifically SEV doesn't miss this feature because it uses UEFI/OVMF
> which has its own way of getting random seed (not to mention that
> getting the random seed from the untrusted host breaks the confidential
> computing trust model).
> 
> Disable the RNG seed feature in SEV guests.
> 
> Fixes: eac7a7791bb6 ("x86: don't let decompressed kernel image clobber 
> setup_data")
> Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> 
> ---
> 
> There might be a need for a wider change to the ways setup_data entries
> are handled in x86_load_linux(); here I just try to restore the
> situation for SEV guests prior to the addition of the SETUP_RNG_SEED
> entry.
> 
> Recent discussions on other (safer?) ways to pass this setup_data entry:
> [1] 
> da39abab9785aea2a2e7652ed6403b6268aeb31f.camel@linux.ibm.com/">https://lore.kernel.org/qemu-devel/da39abab9785aea2a2e7652ed6403b6268aeb31f.camel@linux.ibm.com/
> 
> Note that in qemu 7.2.0 this is broken as well -- there the
> SETUP_RNG_SEED entry is appended to the Linux kernel data (and therefore
> modifies and breaks the measurement of the kernel in SEV measured boot).
> A similar fix will be needed there (but I fear this patch cannot be
> applied as-is).
> ---
>  hw/i386/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> index eaff4227bd..e65a83f8df 100644
> --- a/hw/i386/x86.c
> +++ b/hw/i386/x86.c
> @@ -1103,7 +1103,7 @@ void x86_load_linux(X86MachineState *x86ms,
>          load_image_size(dtb_filename, setup_data->data, dtb_size);
>      }
>  
> -    if (!legacy_no_rng_seed && protocol >= 0x209) {
> +    if (!legacy_no_rng_seed && protocol >= 0x209 && !sev_enabled()) {
>          setup_data_offset = cmdline_size;
>          cmdline_size += sizeof(SetupData) + RNG_SEED_LENGTH;
>          kernel_cmdline = g_realloc(kernel_cmdline, cmdline_size);
> 
> base-commit: 6661b8c7fe3f8b5687d2d90f7b4f3f23d70e3e8b
> -- 
> 2.25.1
> 

Thanks for catching this. The same thing should also probably be set on
the dtb handling, and the commit message updated to reflect that this is
setup_data-specific, not just for the rng seed.

I'm concurrently working on a related bug fix in this same code. If you
want, I can roll a v2 of your patch (retaining your authorship) and
submit that together, if you want.

Jason