Re: [PATCH qemu] x86: don't let decompressed kernel image clobber setup_data

[H. Peter Anvin]

On 12/31/22 10:22, Jason A. Donenfeld wrote:
> On Sat, Dec 31, 2022 at 03:24:32PM +0100, Borislav Petkov wrote:
> > On Sat, Dec 31, 2022 at 02:51:28PM +0100, Jason A. Donenfeld wrote:
> > > That failure is unrelated to the ident mapping issue Peter and
> > > I discussed. The original failure is described in the commit message:
> > > decompression clobbers the data, so sd->next points to garbage.
> >
> > Right
>
> So with that understanding confirmed, I'm confused at your surprise that
> hpa's unrelated fix to the different issue didn't fix this issue.

If decompression does clobber the data, then we *also* need to figure 
out why that is. There are basically three possibilities:

1. If physical KASLR is NOT used:

        a. The boot loader doesn't honor the kernel safe area properly;
        b. Somewhere in the process a bug in the calculation of the
           kernel safe area has crept in.

2. If physical KASLR IS used:

        The decompressor doesn't correctly keep track of nor relocate
        all the keep-out zones before picking a target address.

One is a bootloader bug, two is a kernel bug. My guess is (2) is the 
culprit, but (1b) should be checked, too.

        -hpa