Re: [PATCH] arm64/mm: Sanity check PTE address before runtime P4D/PUD folding

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Nov 04, 2024 at 02:57:21PM +0100, Ard Biesheuvel wrote:
> On Mon, 4 Nov 2024 at 14:50, Catalin Marinas <catalin.marinas@arm.com> wrote:
> > On Fri, Nov 01, 2024 at 04:58:01PM +0100, Ard Biesheuvel wrote:
> > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > > index dd5dcf7ae056..0d729adf894c 100644
> > > --- a/arch/arm64/include/asm/pgtable.h
> > > +++ b/arch/arm64/include/asm/pgtable.h
> > > @@ -740,6 +740,11 @@ static inline bool pud_table(pud_t pud) { return true; }
> > >                                PUD_TYPE_TABLE)
> > >  #endif
> > >
> > > +static inline long sign_of(unsigned long addr)
> > > +{
> > > +     return (int)(addr >> 24) >> 31L; // bit 55 is the sign bit
> > > +}
> >
> > That's a pretty generic name that trickles into the core code. It should
> > be renamed to something that suggests arm64 addresses (and maybe some
> > underscores to imply private).
> 
> Fair enough.
> 
> > Also, this assumes untagged addresses but
> > I haven't checked whether that's the case for all call sites.
> 
> Isn't bit #55 always guaranteed to be the sign bit?

Ah, true, we have the cast to (int) which removes any tag bits.

Why's the 31L? Does it affect the type? (it saves me from having to dig
out the C standard).

> > >  extern pgd_t init_pg_dir[];
> > >  extern pgd_t init_pg_end[];
> > >  extern pgd_t swapper_pg_dir[];
> > > @@ -932,6 +937,8 @@ static inline phys_addr_t p4d_page_paddr(p4d_t p4d)
> > >
> > >  static inline pud_t *p4d_to_folded_pud(p4d_t *p4dp, unsigned long addr)
> > >  {
> > > +     VM_BUG_ON(((u64)p4dp / sizeof(p4d_t) - sign_of(addr)) % PTRS_PER_P4D);
> > > +
> > >       return (pud_t *)PTR_ALIGN_DOWN(p4dp, PAGE_SIZE) + pud_index(addr);
> > >  }
> >
> > I think I get it but please add a comment in the code, otherwise in a
> > week time I'll wonder what this is.
> >
> > Even better if it could be written in a less cryptic way ;).
> >
> 
> Yeah, I tried to keep it concise so the code for the VM_BUG_ON()
> doesn't bloat the function.

Fine but add a comment along the lines of the commit log.

-- 
Catalin