Re: [PATCH v5 4/4] x86: re-enable rng seeding via SetupData

[Eric Biggers]

Hi Jason,

On Wed, Sep 21, 2022 at 11:31:34AM +0200, Jason A. Donenfeld wrote:
> This reverts 3824e25db1 ("x86: disable rng seeding via setup_data"), but
> for 7.2 rather than 7.1, now that modifying setup_data is safe to do.
> 
> Cc: Laurent Vivier <laurent@vivier.eu>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Peter Maydell <peter.maydell@linaro.org>
> Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
> Cc: Richard Henderson <richard.henderson@linaro.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Acked-by: Gerd Hoffmann <kraxel@redhat.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  hw/i386/microvm.c | 2 +-
>  hw/i386/pc_piix.c | 3 ++-
>  hw/i386/pc_q35.c  | 3 ++-
>  3 files changed, 5 insertions(+), 3 deletions(-)
> 

After upgrading to QEMU 7.2, Linux 6.1 no longer boots with some configs.  There
is no output at all.  I bisected it to this commit, and I verified that the
following change to QEMU's master branch makes the problem go away:

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index b48047f50c..42f5b07d2f 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -441,6 +441,7 @@ static void pc_i440fx_8_0_machine_options(MachineClass *m)
     pc_i440fx_machine_options(m);
     m->alias = "pc";
     m->is_default = true;
+    PC_MACHINE_CLASS(m)->legacy_no_rng_seed = true;
 }

I've attached the kernel config I am seeing the problem on.

For some reason, the problem also goes away if I disable CONFIG_KASAN.

Any idea what is causing this?

- Eric

.config
Description: Text document