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Message-ID: <4b5382fd-e553-4fef-a7c7-a2d3130b948d@lucifer.local>
Date: Fri, 18 Oct 2024 17:17:56 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Suren Baghdasaryan <surenb@...gle.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Matthew Wilcox <willy@...radead.org>,
"Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>,
Richard Henderson <richard.henderson@...aro.org>,
Ivan Kokshaysky <ink@...assic.park.msu.ru>,
Matt Turner <mattst88@...il.com>,
Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
"James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...db.de>,
linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
linux-kselftest@...r.kernel.org,
Sidhartha Kumar <sidhartha.kumar@...cle.com>,
Jeff Xu <jeffxu@...omium.org>, Christoph Hellwig <hch@...radead.org>,
Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 0/4] implement lightweight guard pages
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@...r.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@...e.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@...gle.com>
> > Suggested-by: David Hildenbrand <david@...hat.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
>
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