Re: [PATCH v7 3/7] mseal, system mappings: enable x86-64

["Thomas Weißschuh" <thomas.weissschuh@linutronix.de>]

On Tue, Feb 25, 2025 at 04:48:47PM -0800, Jeff Xu wrote:
> On Tue, Feb 25, 2025 at 12:08 AM Thomas Weißschuh
> <thomas.weissschuh@linutronix.de> wrote:
> > On Mon, Feb 24, 2025 at 10:52:42PM +0000, jeffxu@chromium.org wrote:
> > > From: Jeff Xu <jeffxu@chromium.org>
> > >
> > > Provide support for CONFIG_MSEAL_SYSTEM_MAPPINGS on x86-64,
> > > covering the vdso, vvar, vvar_vclock.
> > >
> > > Production release testing passes on Android and Chrome OS.
> > >
> > > Signed-off-by: Jeff Xu <jeffxu@chromium.org>
> > > ---
> > >  arch/x86/Kconfig          |  1 +
> > >  arch/x86/entry/vdso/vma.c | 16 ++++++++++------
> > >  2 files changed, 11 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index 87198d957e2f..8fa17032ca46 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -26,6 +26,7 @@ config X86_64
> > >       depends on 64BIT
> > >       # Options that are inherently 64-bit kernel only:
> > >       select ARCH_HAS_GIGANTIC_PAGE
> > > +     select ARCH_HAS_MSEAL_SYSTEM_MAPPINGS
> > >       select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
> > >       select ARCH_SUPPORTS_PER_VMA_LOCK
> > >       select ARCH_SUPPORTS_HUGE_PFNMAP if TRANSPARENT_HUGEPAGE
> > > diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
> > > index 39e6efc1a9ca..1b1c009f20a8 100644
> > > --- a/arch/x86/entry/vdso/vma.c
> > > +++ b/arch/x86/entry/vdso/vma.c
> > > @@ -247,6 +247,7 @@ static int map_vdso(const struct vdso_image *image, unsigned long addr)
> > >       struct mm_struct *mm = current->mm;
> > >       struct vm_area_struct *vma;
> > >       unsigned long text_start;
> > > +     unsigned long vm_flags;
> > >       int ret = 0;
> > >
> > >       if (mmap_write_lock_killable(mm))
> > > @@ -264,11 +265,12 @@ static int map_vdso(const struct vdso_image *image, unsigned long addr)
> > >       /*
> > >        * MAYWRITE to allow gdb to COW and set breakpoints
> > >        */
> > > +     vm_flags = VM_READ|VM_EXEC|VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC;
> > > +     vm_flags |= VM_SEALED_SYSMAP;
> > >       vma = _install_special_mapping(mm,
> > >                                      text_start,
> > >                                      image->size,
> > > -                                    VM_READ|VM_EXEC|
> > > -                                    VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
> > > +                                    vm_flags,
> > >                                      &vdso_mapping);
> > >
> > >       if (IS_ERR(vma)) {
> > > @@ -276,11 +278,12 @@ static int map_vdso(const struct vdso_image *image, unsigned long addr)
> > >               goto up_fail;
> > >       }
> > >
> > > +     vm_flags = VM_READ|VM_MAYREAD|VM_IO|VM_DONTDUMP|VM_PFNMAP;
> > > +     vm_flags |= VM_SEALED_SYSMAP;
> > >       vma = _install_special_mapping(mm,
> > >                                      addr,
> > >                                      (__VVAR_PAGES - VDSO_NR_VCLOCK_PAGES) * PAGE_SIZE,
> > > -                                    VM_READ|VM_MAYREAD|VM_IO|VM_DONTDUMP|
> > > -                                    VM_PFNMAP,
> > > +                                    vm_flags,
> > >                                      &vvar_mapping);
> >
> > This hunk (and the vvar mapping in the arm64 patch) will conflict with my
> > "Generic vDSO datapage" series.
> > That series is already part of the tip tree (branch timers/vdso) and scheduled
> > for the next merge window.
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/log/?h=timers/vdso
> >
> > The conflict resolution is fairly easy:
> > Move the new flag logic into lib/vdso/datastore.c
> >
> Thank you for bringing this to my attention.
> 
> In your change,  it seems lib/vdso/datastore.c implements a
> vdso_install_vvar_mapping(), then all the architectures call this
> function.

Correct.

At least all the architectures using the generic vDSO infrastructure,
which are the ones you care about.
Sparc for example has its own implementation.

> So merging conflict won't be as straightforward.

Wouldn't it be enough to unconditionally use VM_SEALED_SYSMAP in
vdso_install_vvar_mapping()?
The symbol is a noop on architectures or configurations where the new
functionality is not available or enabled.

> Maybe a better
> approach is that I continue resolving all the comments, based on the
> latest main. Then wait for your change to be merged and submit another
> version.

That would work, too. As you prefer.