Re: [RFC PATCH v4 4/4] mseal:add documentation

[Jeff Xu <jeffxu@chromium.org>]

On Thu, Jan 4, 2024 at 3:47 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
>
>
> On 1/4/24 10:51, jeffxu@chromium.org wrote:
> > From: Jeff Xu <jeffxu@chromium.org>
> >
> > Add documentation for mseal().
> >
> > Signed-off-by: Jeff Xu <jeffxu@chromium.org>
> > ---
> >  Documentation/userspace-api/mseal.rst | 181 ++++++++++++++++++++++++++
> >  1 file changed, 181 insertions(+)
> >  create mode 100644 Documentation/userspace-api/mseal.rst
> >
> > diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst
> > new file mode 100644
> > index 000000000000..1700ce5af218
> > --- /dev/null
> > +++ b/Documentation/userspace-api/mseal.rst
> > @@ -0,0 +1,181 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +=====================
> > +Introduction of mseal
> > +=====================
> > +
> > +:Author: Jeff Xu <jeffxu@chromium.org>
> > +
> > +Modern CPUs support memory permissions such as RW and NX bits. The memory
> > +permission feature improves security stance on memory corruption bugs, i.e.
> > +the attacker can’t just write to arbitrary memory and point the code to it,
> > +the memory has to be marked with X bit, or else an exception will happen.
> > +
> > +Memory sealing additionally protects the mapping itself against
> > +modifications. This is useful to mitigate memory corruption issues where a
> > +corrupted pointer is passed to a memory management system. For example,
> > +such an attacker primitive can break control-flow integrity guarantees
> > +since read-only memory that is supposed to be trusted can become writable
> > +or .text pages can get remapped. Memory sealing can automatically be
> > +applied by the runtime loader to seal .text and .rodata pages and
> > +applications can additionally seal security critical data at runtime.
> > +
> > +A similar feature already exists in the XNU kernel with the
> > +VM_FLAGS_PERMANENT flag [1] and on OpenBSD with the mimmutable syscall [2].
> > +
> > +User API
> > +========
> > +Two system calls are involved in virtual memory sealing, mseal() and mmap().
> > +
> > +mseal()
> > +-----------
> > +The mseal() syscall has following signature:
> > +
> > +``int mseal(void addr, size_t len, unsigned long flags)``
> > +
> > +**addr/len**: virtual memory address range.
> > +
> > +The address range set by ``addr``/``len`` must meet:
> > +   - The start address must be in an allocated VMA.
> > +   - The start address must be page aligned.
> > +   - The end address (``addr`` + ``len``) must be in an allocated VMA.
> > +   - no gap (unallocated memory) between start and end address.
> > +
> > +The ``len`` will be paged aligned implicitly by the kernel.
>
> Does that mean that the <len> will be extended to be page aligned
> if it's not already page aligned?
>
Yes.
the code (do_mseal) calls PAGE_ALIGNED(len).
mprotect() also has this.

Two test cases cover this part.
test_seal_mprotect_unalign_len
test_seal_mprotect_unalign_len_variant_2

-Jeff

> --
> #Randy