Re: [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount

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 [off-list ref] wrote:

quoted

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.

quoted

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 :)

quoted

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 <redacted>
---
 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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel