Re: [PATCH 0/4] implement lightweight guard pages

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Oct 18, 2024 at 06:10:37PM +0200, Vlastimil Babka wrote:
> +CC linux-api (also should on future revisions)
>

They're cc'd :) assuming Linux API <linux-api@vger.kernel.org> is correct
right?

> On 10/17/24 22:42, Lorenzo Stoakes wrote:
> > Userland library functions such as allocators and threading implementations
> > often require regions of memory to act as 'guard pages' - mappings which,
> > when accessed, result in a fatal signal being sent to the accessing
> > process.
> >
> > The current means by which these are implemented is via a PROT_NONE mmap()
> > mapping, which provides the required semantics however incur an overhead of
> > a VMA for each such region.
> >
> > With a great many processes and threads, this can rapidly add up and incur
> > a significant memory penalty. It also has the added problem of preventing
> > merges that might otherwise be permitted.
> >
> > This series takes a different approach - an idea suggested by Vlasimil
> > Babka (and before him David Hildenbrand and Jann Horn - perhaps more - the
> > provenance becomes a little tricky to ascertain after this - please forgive
> > any omissions!)  - rather than locating the guard pages at the VMA layer,
> > instead placing them in page tables mapping the required ranges.
> >
> > Early testing of the prototype version of this code suggests a 5 times
> > speed up in memory mapping invocations (in conjunction with use of
> > process_madvise()) and a 13% reduction in VMAs on an entirely idle android
> > system and unoptimised code.
> >
> > We expect with optimisation and a loaded system with a larger number of
> > guard pages this could significantly increase, but in any case these
> > numbers are encouraging.
> >
> > This way, rather than having separate VMAs specifying which parts of a
> > range are guard pages, instead we have a VMA spanning the entire range of
> > memory a user is permitted to access and including ranges which are to be
> > 'guarded'.
> >
> > After mapping this, a user can specify which parts of the range should
> > result in a fatal signal when accessed.
> >
> > By restricting the ability to specify guard pages to memory mapped by
> > existing VMAs, we can rely on the mappings being torn down when the
> > mappings are ultimately unmapped and everything works simply as if the
> > memory were not faulted in, from the point of view of the containing VMAs.
> >
> > This mechanism in effect poisons memory ranges similar to hardware memory
> > poisoning, only it is an entirely software-controlled form of poisoning.
> >
> > Any poisoned region of memory is also able to 'unpoisoned', that is, to
> > have its poison markers removed.
> >
> > The mechanism is implemented via madvise() behaviour - MADV_GUARD_POISON
> > which simply poisons ranges - and MADV_GUARD_UNPOISON - which clears this
> > poisoning.
> >
> > Poisoning can be performed across multiple VMAs and any existing mappings
> > will be cleared, that is zapped, before installing the poisoned page table
> > mappings.
> >
> > There is no concept of 'nested' poisoning, multiple attempts to poison a
> > range will, after the first poisoning, have no effect.
> >
> > Importantly, unpoisoning of poisoned ranges has no effect on non-poisoned
> > memory, so a user can safely unpoison a range of memory and clear only
> > poison page table mappings leaving the rest intact.
> >
> > The actual mechanism by which the page table entries are specified makes
> > use of existing logic - PTE markers, which are used for the userfaultfd
> > UFFDIO_POISON mechanism.
> >
> > Unfortunately PTE_MARKER_POISONED is not suited for the guard page
> > mechanism as it results in VM_FAULT_HWPOISON semantics in the fault
> > handler, so we add our own specific PTE_MARKER_GUARD and adapt existing
> > logic to handle it.
> >
> > We also extend the generic page walk mechanism to allow for installation of
> > PTEs (carefully restricted to memory management logic only to prevent
> > unwanted abuse).
> >
> > We ensure that zapping performed by, for instance, MADV_DONTNEED, does not
> > remove guard poison markers, nor does forking (except when VM_WIPEONFORK is
> > specified for a VMA which implies a total removal of memory
> > characteristics).
> >
> > It's important to note that the guard page implementation is emphatically
> > NOT a security feature, so a user can remove the poisoning if they wish. We
> > simply implement it in such a way as to provide the least surprising
> > behaviour.
> >
> > An extensive set of self-tests are provided which ensure behaviour is as
> > expected and additionally self-documents expected behaviour of poisoned
> > ranges.
> >
> > Suggested-by: Vlastimil Babka <vbabka@suze.cz>
>
> Please fix the domain typo (also in patch 3 :)
>

Damnnn it! I can't believe I left that in. Sorry about that! Will fix on
respin.

Hopefully not to suse.cs ;)

> Thanks for implementing this,
> Vlastimil

Thanks!

>
> > Suggested-by: Jann Horn <jannh@google.com>
> > Suggested-by: David Hildenbrand <david@redhat.com>
> >
> > v1
> > * Un-RFC'd as appears no major objections to approach but rather debate on
> >   implementation.
> > * Fixed issue with arches which need mmu_context.h and
> >   tlbfush.h. header imports in pagewalker logic to be able to use
> >   update_mmu_cache() as reported by the kernel test bot.
> > * Added comments in page walker logic to clarify who can use
> >   ops->install_pte and why as well as adding a check_ops_valid() helper
> >   function, as suggested by Christoph.
> > * Pass false in full parameter in pte_clear_not_present_full() as suggested
> >   by Jann.
> > * Stopped erroneously requiring a write lock for the poison operation as
> >   suggested by Jann and Suren.
> > * Moved anon_vma_prepare() to the start of madvise_guard_poison() to be
> >   consistent with how this is used elsewhere in the kernel as suggested by
> >   Jann.
> > * Avoid returning -EAGAIN if we are raced on page faults, just keep looping
> >   and duck out if a fatal signal is pending or a conditional reschedule is
> >   needed, as suggested by Jann.
> > * Avoid needlessly splitting huge PUDs and PMDs by specifying
> >   ACTION_CONTINUE, as suggested by Jann.
> >
> > RFC
> > https://lore.kernel.org/all/cover.1727440966.git.lorenzo.stoakes@oracle.com/
> >
> > Lorenzo Stoakes (4):
> >   mm: pagewalk: add the ability to install PTEs
> >   mm: add PTE_MARKER_GUARD PTE marker
> >   mm: madvise: implement lightweight guard page mechanism
> >   selftests/mm: add self tests for guard page feature
> >
> >  arch/alpha/include/uapi/asm/mman.h       |    3 +
> >  arch/mips/include/uapi/asm/mman.h        |    3 +
> >  arch/parisc/include/uapi/asm/mman.h      |    3 +
> >  arch/xtensa/include/uapi/asm/mman.h      |    3 +
> >  include/linux/mm_inline.h                |    2 +-
> >  include/linux/pagewalk.h                 |   18 +-
> >  include/linux/swapops.h                  |   26 +-
> >  include/uapi/asm-generic/mman-common.h   |    3 +
> >  mm/hugetlb.c                             |    3 +
> >  mm/internal.h                            |    6 +
> >  mm/madvise.c                             |  168 ++++
> >  mm/memory.c                              |   18 +-
> >  mm/mprotect.c                            |    3 +-
> >  mm/mseal.c                               |    1 +
> >  mm/pagewalk.c                            |  200 ++--
> >  tools/testing/selftests/mm/.gitignore    |    1 +
> >  tools/testing/selftests/mm/Makefile      |    1 +
> >  tools/testing/selftests/mm/guard-pages.c | 1168 ++++++++++++++++++++++
> >  18 files changed, 1564 insertions(+), 66 deletions(-)
> >  create mode 100644 tools/testing/selftests/mm/guard-pages.c
> >
> > --
> > 2.46.2
>