linux-kernel - [PATCH v6 03/40] arm64: mte: Reset the page tag in page->flags

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Message-Id: <d31459cf316fff73874058b8753d2385a137e956.1603999489.git.andreyknvl@google.com>
Date:   Thu, 29 Oct 2020 20:25:24 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        kasan-dev@...glegroups.com, Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Elena Petrova <lenaptr@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: [PATCH v6 03/40] arm64: mte: Reset the page tag in page->flags

From: Vincenzo Frascino <vincenzo.frascino@....com>

The hardware tag-based KASAN for compatibility with the other modes
stores the tag associated to a page in page->flags.
Due to this the kernel faults on access when it allocates a page with an
initial tag and the user changes the tags.

Reset the tag associated by the kernel to a page in all the meaningful
places to prevent kernel faults on access.

Note: An alternative to this approach could be to modify page_to_virt().
This though could end up being racy, in fact if a CPU checks the
PG_mte_tagged bit and decides that the page is not tagged but another
CPU maps the same with PROT_MTE and becomes tagged the subsequent kernel
access would fail.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
---
Change-Id: I8451d438bb63364de2a3e68041e3a27866921d4e
---
 arch/arm64/kernel/mte.c  | 1 +
 arch/arm64/mm/copypage.c | 1 +
 arch/arm64/mm/mteswap.c  | 1 +
 3 files changed, 3 insertions(+)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 8f99c65837fd..06ba6c923ab7 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -34,6 +34,7 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 			return;
 	}
 
+	page_kasan_tag_reset(page);
 	mte_clear_page_tags(page_address(page));
 }
 
diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index 70a71f38b6a9..348f4627da08 100644
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -22,6 +22,7 @@ void copy_highpage(struct page *to, struct page *from)
 	copy_page(kto, kfrom);
 
 	if (system_supports_mte() && test_bit(PG_mte_tagged, &from->flags)) {
+		page_kasan_tag_reset(to);
 		set_bit(PG_mte_tagged, &to->flags);
 		mte_copy_page_tags(kto, kfrom);
 	}
diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index c52c1847079c..0e7eccbe598a 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -53,6 +53,7 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
 	if (!tags)
 		return false;
 
+	page_kasan_tag_reset(page);
 	mte_restore_page_tags(page_address(page), tags);
 
 	return true;
-- 
2.29.1.341.ge80a0c044ae-goog