Re: [PATCH v3 1/3] ArmPkg/ArmMmuLib AARCH64: limit recursion when freeing page tables

["Ard Biesheuvel" <ard.biesheuvel@linaro.org>]

On Thu, 26 Mar 2020 at 11:22, Leif Lindholm <leif@nuviainc.com> wrote:
>
> On Wed, Mar 25, 2020 at 16:29:38 +0100, Ard Biesheuvel wrote:
> > FreePageTablesRecursive () traverses the page table tree depth first
> > to free all pages that it finds, without taking into account the
> > level at which it is operating.
> >
> > Since TT_TYPE_TABLE_ENTRY aliases TT_TYPE_BLOCK_ENTRY_LEVEL3, we cannot
> > distinguish table entries from block entries unless we take the level
> > into account, and so we may be dereferencing garbage if we happen to
> > try and free a hierarchy of page tables that has level 3 pages in it.
> >
> > Let's fix this by passing the level into FreePageTablesRecursive (),
> > and limit the recursion to levels < 3.
> >
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
> LGTM
> Reviewed-by: Leif Lindholm <leif@nuviainc.com>
>
> One question, which does not require any action on this patch:
> Could that 3 be a #define (or Pcd) for future-proofing, or do the
> bindings already restrict us in ways that can't reasonably be tweaked?
>

We also have TT_TYPE_BLOCK_ENTRY_LEVEL3 (as opposed to
TT_TYPE_BLOCK_ENTRY for other levels) so the '3' is rather set in
stone, I'd say.

> > ---
> >  ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 18 ++++++++++++------
> >  1 file changed, 12 insertions(+), 6 deletions(-)
> >
> > diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> > index a43d468b73ca..d78918cf7ba8 100644
> > --- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> > +++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> > @@ -142,15 +142,21 @@ ReplaceTableEntry (
> >  STATIC
> >  VOID
> >  FreePageTablesRecursive (
> > -  IN  UINT64  *TranslationTable
> > +  IN  UINT64  *TranslationTable,
> > +  IN  UINTN   Level
> >    )
> >  {
> >    UINTN   Index;
> >
> > -  for (Index = 0; Index < TT_ENTRY_COUNT; Index++) {
> > -    if ((TranslationTable[Index] & TT_TYPE_MASK) == TT_TYPE_TABLE_ENTRY) {
> > -      FreePageTablesRecursive ((VOID *)(UINTN)(TranslationTable[Index] &
> > -                                               TT_ADDRESS_MASK_BLOCK_ENTRY));
> > +  ASSERT (Level <= 3);
> > +
> > +  if (Level < 3) {
> > +    for (Index = 0; Index < TT_ENTRY_COUNT; Index++) {
> > +      if ((TranslationTable[Index] & TT_TYPE_MASK) == TT_TYPE_TABLE_ENTRY) {
> > +        FreePageTablesRecursive ((VOID *)(UINTN)(TranslationTable[Index] &
> > +                                                 TT_ADDRESS_MASK_BLOCK_ENTRY),
> > +                                 Level + 1);
> > +      }
> >      }
> >    }
> >    FreePages (TranslationTable, 1);
> > @@ -254,7 +260,7 @@ UpdateRegionMappingRecursive (
> >            // possible for existing table entries, since we cannot revert the
> >            // modifications we made to the subhierarchy it represents.)
> >            //
> > -          FreePageTablesRecursive (TranslationTable);
> > +          FreePageTablesRecursive (TranslationTable, Level + 1);
> >          }
> >          return Status;
> >        }
> > --
> > 2.17.1
> >