Re: [PATCH 2/2] x86: fix setup of brk area

[Juergen Gross]

On 23.06.22 10:09, Jan Beulich wrote:
> On 22.06.2022 18:10, Juergen Gross wrote:
> > Commit e32683c6f7d2 ("x86/mm: Fix RESERVE_BRK() for older binutils")
> > put the brk area into the .bss segment, causing it not to be cleared
> > initially.
>
> This reads contradictively: If the area was put in .bss, it would be
> cleared. Thing is it is put in .bss..brk in the object files, while
> the linker script puts it in .brk (i.e. outside of .bss).

Hmm, yes, this should be reworded.

> > As the brk area is used to allocate early page tables, these
> > might contain garbage in not explicitly written entries.
>
> I'm surprised this lack of zero-initialization didn't cause any issue
> outside of PV Xen. Unless of course there never was the intention for
> users of the facility to assume blank pages coming from there, in
> which case Xen's use for early page tables would have been wrong (in
> not explicitly zeroing the space first).

Fun fact: Its not Xen's use for early page tables, but the kernel's
init code. It is used for bare metal, too.

The use case for initial page tables is the problematic one. Only the
needed page table entries are written by the kernel, so the other ones
keep their initial garbage values. As normally no uninitialized entries
are ever referenced, this will have no real impact.

With Xen, however, ALL entries are being validated, which led to the
early crash of dom0.


Juergen

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