Re: [PATCH v3 1/2] x86/sev: enforce RIP-relative accesses in early SEV/SME code

[Ard Biesheuvel]

On Fri, 2 Feb 2024 at 23:00, Kevin Loughlin <kevinloughlin@xxxxxxxxxx> wrote:
>
> On Wed, Jan 31, 2024 at 12:20 AM Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Jan 30, 2024 at 10:08:44PM +0000, Kevin Loughlin wrote:
> > > diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
> > > index 359ada486fa9..b65e66ee79c4 100644
> > > --- a/arch/x86/include/asm/mem_encrypt.h
> > > +++ b/arch/x86/include/asm/mem_encrypt.h
> > > @@ -17,6 +17,20 @@
> > >
> > >  #include <asm/bootparam.h>
> > >
> > > +/*
> > > + * Like the address operator "&", evaluates to the address of a LHS variable
> > > + * "var", but also enforces the use of RIP-relative logic. This macro can be
> > > + * used to safely access global data variables prior to kernel relocation.
> > > + */
> > > +#define RIP_RELATIVE_ADDR(var)               \
> > > +({                                   \
> > > +     void *rip_rel_ptr;              \
> > > +     asm ("lea "#var"(%%rip), %0"    \
> > > +             : "=r" (rip_rel_ptr)    \
> > > +             : "p" (&var));          \
> > > +     rip_rel_ptr;                    \
> > > +})
> > > +
> >
> > I don't think it is the right place for the macro. The next patch uses for
> > things unrelated to memory encryption.
>
> You're right; with the cleanup, I agree it becomes more general. We
> can move it to arch/x86/include/asm/asm.h.
>
> >
> > > @@ -239,14 +244,14 @@ unsigned long __head __startup_64(unsigned long physaddr,
> > >        */
> > >
> > >       next_pgt_ptr = fixup_pointer(&next_early_pgt, physaddr);
> > > -     pud = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
> > > -     pmd = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
> > > +     early_dynamic_pgts_ptr = fixup_pointer(early_dynamic_pgts, physaddr);
> > > +     pud = (pudval_t *) early_dynamic_pgts_ptr[(*next_pgt_ptr)++];
> > > +     pmd = (pmdval_t *) early_dynamic_pgts_ptr[(*next_pgt_ptr)++];
> > >
> >
> > This change doesn't belong to this patch. Maybe move it into the next
> > patch and combine with removing fixup_pointer().
>
> I'll put it in a separate commit even preceding this one, as it's
> actually a bug in the existing fixup pointer logic that I noticed when
> transitioning to the use of the RIP-relative macro. Specifically,
> early_dynamic_pgts is a global variable just like next_early_pgt and
> thus also needs to be fixed up to guarantee the correct address is
> used across toolchains.

No, this is not a bug.

early_dynamic_pgts is a two dimensional array

extern pmd_t early_dynamic_pgts[EARLY_DYNAMIC_PAGE_TABLES][PTRS_PER_PMD];

and so this expression

early_dynamic_pgts[(*next_pgt_ptr)++]

produces the kernel virtual address of one of the top level elements,
which itself is an array, and so this can be fixed up as usual.
IOW, the [] operation does not result in pointer dereference here,
only in pointer arithmetic.