linux-kernel - [PATCH v2 08/17] kernel/events/uprobes: handle device-exclusive entries correctly in __replace

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Message-ID: <20250210193801.781278-9-david@redhat.com>
Date: Mon, 10 Feb 2025 20:37:50 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-doc@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	linux-mm@...ck.org,
	nouveau@...ts.freedesktop.org,
	linux-trace-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org,
	damon@...ts.linux.dev,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jérôme Glisse <jglisse@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Alex Shi <alexs@...nel.org>,
	Yanteng Si <si.yanteng@...ux.dev>,
	Karol Herbst <kherbst@...hat.com>,
	Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	SeongJae Park <sj@...nel.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Jann Horn <jannh@...gle.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Peter Xu <peterx@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Jason Gunthorpe <jgg@...dia.com>
Subject: [PATCH v2 08/17] kernel/events/uprobes: handle device-exclusive entries correctly in __replace_page()

Ever since commit b756a3b5e7ea ("mm: device exclusive memory access")
we can return with a device-exclusive entry from page_vma_mapped_walk().

__replace_page() is not prepared for that, so teach it about these
PFN swap PTEs. Note that device-private entries are so far not
applicable on that path, because GUP would never have returned such
folios (conversion to device-private happens by page migration, not
in-place conversion of the PTE).

There is a race between GUP and us locking the folio to look it up
using page_vma_mapped_walk(), so this is likely a fix (unless something
else could prevent that race, but it doesn't look like). pte_pfn() on
something that is not a present pte could give use garbage, and we'd
wrongly mess up the mapcount because it was already adjusted by calling
folio_remove_rmap_pte() when making the entry device-exclusive.

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 kernel/events/uprobes.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 2ca797cbe465f..cd6105b100325 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -173,6 +173,7 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	DEFINE_FOLIO_VMA_WALK(pvmw, old_folio, vma, addr, 0);
 	int err;
 	struct mmu_notifier_range range;
+	pte_t pte;
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, addr,
 				addr + PAGE_SIZE);
@@ -192,6 +193,16 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	if (!page_vma_mapped_walk(&pvmw))
 		goto unlock;
 	VM_BUG_ON_PAGE(addr != pvmw.address, old_page);
+	pte = ptep_get(pvmw.pte);
+
+	/*
+	 * Handle PFN swap PTES, such as device-exclusive ones, that actually
+	 * map pages: simply trigger GUP again to fix it up.
+	 */
+	if (unlikely(!pte_present(pte))) {
+		page_vma_mapped_walk_done(&pvmw);
+		goto unlock;
+	}
 
 	if (new_page) {
 		folio_get(new_folio);
@@ -206,7 +217,7 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 		inc_mm_counter(mm, MM_ANONPAGES);
 	}
 
-	flush_cache_page(vma, addr, pte_pfn(ptep_get(pvmw.pte)));
+	flush_cache_page(vma, addr, pte_pfn(pte));
 	ptep_clear_flush(vma, addr, pvmw.pte);
 	if (new_page)
 		set_pte_at(mm, addr, pvmw.pte,
-- 
2.48.1