linux-kernel - [PATCH v10 1/6] arm64: mte: Sync tags for pages where PTE is untagged

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Message-Id: <20210312151902.17853-2-steven.price@arm.com>
Date:   Fri, 12 Mar 2021 15:18:57 +0000
From:   Steven Price <steven.price@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>
Cc:     Steven Price <steven.price@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: [PATCH v10 1/6] arm64: mte: Sync tags for pages where PTE is untagged

A KVM guest could store tags in a page even if the VMM hasn't mapped
the page with PROT_MTE. So when restoring pages from swap we will
need to check to see if there are any saved tags even if !pte_tagged().

However don't check pages which are !pte_valid_user() as these will
not have been swapped out.

Signed-off-by: Steven Price <steven.price@....com>
---
 arch/arm64/include/asm/pgtable.h |  2 +-
 arch/arm64/kernel/mte.c          | 16 ++++++++++++----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index e17b96d0e4b5..84166625c989 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 		__sync_icache_dcache(pte);
 
 	if (system_supports_mte() &&
-	    pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
+	    pte_present(pte) && pte_valid_user(pte) && !pte_special(pte))
 		mte_sync_tags(ptep, pte);
 
 	__check_racy_pte_update(mm, ptep, pte);
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index b3c70a612c7a..e016ab57ea36 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -26,17 +26,23 @@ u64 gcr_kernel_excl __ro_after_init;
 
 static bool report_fault_once = true;
 
-static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
+static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap,
+			       bool pte_is_tagged)
 {
 	pte_t old_pte = READ_ONCE(*ptep);
 
 	if (check_swap && is_swap_pte(old_pte)) {
 		swp_entry_t entry = pte_to_swp_entry(old_pte);
 
-		if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
+		if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
+			set_bit(PG_mte_tagged, &page->flags);
 			return;
+		}
 	}
 
+	if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags))
+		return;
+
 	page_kasan_tag_reset(page);
 	/*
 	 * We need smp_wmb() in between setting the flags and clearing the
@@ -54,11 +60,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
 	struct page *page = pte_page(pte);
 	long i, nr_pages = compound_nr(page);
 	bool check_swap = nr_pages == 1;
+	bool pte_is_tagged = pte_tagged(pte);
 
 	/* if PG_mte_tagged is set, tags have already been initialised */
 	for (i = 0; i < nr_pages; i++, page++) {
-		if (!test_and_set_bit(PG_mte_tagged, &page->flags))
-			mte_sync_page_tags(page, ptep, check_swap);
+		if (!test_bit(PG_mte_tagged, &page->flags))
+			mte_sync_page_tags(page, ptep, check_swap,
+					   pte_is_tagged);
 	}
 }
 
-- 
2.20.1