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Re: [PATCH v2 1/2] x86/head: check base address alignment
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Wed, 3 May 2023 11:33:09 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 03 May 2023 09:33:33 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Wed, May 03, 2023 at 10:08:20AM +0200, Jan Beulich wrote:
> On 02.05.2023 16:59, Roger Pau Monne wrote:
> > Ensure that the base address is 2M aligned, or else the page table
> > entries created would be corrupt as reserved bits on the PDE end up
> > set.
> >
> > We have encountered a broken firmware where grub2 would end up loading
> > Xen at a non 2M aligned region when using the multiboot2 protocol, and
> > that caused a very difficult to debug triple fault.
> >
> > If the alignment is not as required by the page tables print an error
> > message and stop the boot. Also add a build time check that the
> > calculation of symbol offsets don't break alignment of passed
> > addresses.
> >
> > The check could be performed earlier, but so far the alignment is
> > required by the page tables, and hence feels more natural that the
> > check lives near to the piece of code that requires it.
> >
> > Note that when booted as an EFI application from the PE entry point
> > the alignment check is already performed by
> > efi_arch_load_addr_check(), and hence there's no need to add another
> > check at the point where page tables get built in
> > efi_arch_memory_setup().
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> Would you mind if, while committing, ...
>
> > @@ -146,6 +148,9 @@ bad_cpu:
> > not_multiboot:
> > add $sym_offs(.Lbad_ldr_msg),%esi # Error message
> > jmp .Lget_vtb
> > +not_aligned:
>
> ... a .L prefix was added to this label, bringing it out of sync with the
> earlier one, but in line with e.g. ...
>
> > + add $sym_offs(.Lbag_alg_msg),%esi # Error message
> > + jmp .Lget_vtb
> > .Lmb2_no_st:
>
> ... this one? I don't think the label is particularly useful to have in
> the symbol table (nor are not_multiboot and likely a few others).
Hm, right, yes, I don't think having those on the symbol table is
helpful, please adjust.
Thanks, Roger.
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