From: Quentin Perret <qperret@google.com>
To: Marc Zyngier <maz@kernel.org>
Cc: kernel-team@android.com, Will Deacon <will@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount
Date: Mon, 4 Oct 2021 11:05:12 +0100 [thread overview]
Message-ID: <YVrR2M8OmU6ug1Yk@google.com> (raw)
In-Reply-To: <87bl45ru66.wl-maz@kernel.org>
Hey Marc,
On Monday 04 Oct 2021 at 10:55:13 (+0100), Marc Zyngier wrote:
> Hi Quentin,
>
> On Mon, 04 Oct 2021 10:03:13 +0100,
> Quentin Perret <qperret@google.com> wrote:
> >
> > The KVM page-table library refcounts the pages of concatenated stage-2
> > PGDs individually. However, the host's stage-2 PGD is currently managed
> > by EL2 as a single high-order compound page, which can cause the
> > refcount of the tail pages to reach 0 when they really shouldn't, hence
> > corrupting the page-table.
>
> nit: this comment only applies to the protected mode, right? As far as
> I can tell, 'classic' KVM is just fine.
Correct, this really only applies to the host stage-2, which implies
we're in protected mode. I'll make that a bit more explicit.
> > Fix this by introducing a new hyp_split_page() helper in the EL2 page
> > allocator (matching EL1's split_page() function), and make use of it
>
> uber nit: split_page() is not an EL1 function. more of a standard
> kernel function.
Fair enough :)
> > from host_s2_zalloc_page().
> >
> > Fixes: 1025c8c0c6ac ("KVM: arm64: Wrap the host with a stage 2")
> > Suggested-by: Will Deacon <will@kernel.org>
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> > arch/arm64/kvm/hyp/include/nvhe/gfp.h | 1 +
> > arch/arm64/kvm/hyp/nvhe/mem_protect.c | 6 +++++-
> > arch/arm64/kvm/hyp/nvhe/page_alloc.c | 14 ++++++++++++++
> > 3 files changed, 20 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > index fb0f523d1492..0a048dc06a7d 100644
> > --- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > +++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > @@ -24,6 +24,7 @@ struct hyp_pool {
> >
> > /* Allocation */
> > void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order);
> > +void hyp_split_page(struct hyp_page *page);
> > void hyp_get_page(struct hyp_pool *pool, void *addr);
> > void hyp_put_page(struct hyp_pool *pool, void *addr);
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > index bacd493a4eac..93a79736c283 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > @@ -35,7 +35,11 @@ const u8 pkvm_hyp_id = 1;
> >
> > static void *host_s2_zalloc_pages_exact(size_t size)
> > {
> > - return hyp_alloc_pages(&host_s2_pool, get_order(size));
> > + void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
> > +
> > + hyp_split_page(hyp_virt_to_page(addr));
>
> The only reason this doesn't lead to a subsequent memory leak is that
> concatenated page tables are always a power of two, right?
Indeed, and also because the host stage-2 is _never_ freed, so that's
not memory we're going to reclaim anyway -- we don't have an
implementation of ->free_pages_exact() in the host stage-2 mm_ops.
> If so, that deserves a comment, because I don't think this works in
> the general case unless you actively free the pages that are between
> size and (1 << order).
Ack, that'll probably confuse me too in a few weeks, so a comment won't
hurt. I'll re-spin shortly.
Thanks,
Quentin
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
next prev parent reply other threads:[~2021-10-04 10:05 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-04 9:03 [PATCH 0/2] A couple of EL2 refcounts fixes Quentin Perret
2021-10-04 9:03 ` [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount Quentin Perret
2021-10-04 9:55 ` Marc Zyngier
2021-10-04 10:05 ` Quentin Perret [this message]
2021-10-04 9:03 ` [PATCH 2/2] KVM: arm64: Report corrupted refcount at EL2 Quentin Perret
2021-10-04 9:39 ` [PATCH 0/2] A couple of EL2 refcounts fixes Will Deacon
2021-10-05 9:01 Quentin Perret
2021-10-05 9:01 ` [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount Quentin Perret
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=YVrR2M8OmU6ug1Yk@google.com \
--to=qperret@google.com \
--cc=catalin.marinas@arm.com \
--cc=kernel-team@android.com \
--cc=kvmarm@lists.cs.columbia.edu \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=maz@kernel.org \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).