[PATCH v2] arm64/mm: Check the requested PFN range during memory removal

[Richard Cheng]

prevent_memory_remove_notifier() advances pfn while scanning the requested
range for early memory. When the loop completes, pfn is at or beyond
end_pfn. Passing it to can_unmap_without_split() therefore checks a range
after the one being offlined.

Consequently, a valid request can be rejected based on the following
range, while a request that would split a leaf mapping can be accepted if
the shifted range can be unmapped without a split. This was observed with
CXL DAX memory, where the final memory block was incorrectly allowed to
be offlined.

Pass arg->start_pfn into can_unmap_without_split() so it checks the
requested range.

Fixes: 95a58852b0e5 ("arm64/mm: Reject memory removal that splits a kernel leaf mapping")
Signed-off-by: Richard Cheng <icheng at nvidia.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual at arm.com>
---
Changelog:

V1->v2:
    - Fix the typo in the subject
    - Clarify how pfn advances and which range was incorrectly checked


The bug occurred on a machine with CXL Type-3 device.
Branch: cxl-next

Before 95a58852b0e5:

"""
$ sudo echo offline > memory557056/state
write error: Operation not permitted
$ sudo dmesg
---[snip]---
[440008000000 440010000000] splits a leaf entry in linear map

$ sudo echo offline > memory558079/state
$ cat memory558079/state
offline
"""

After:
"""
$ sudo echo offline > memory557056/state
write error: Operation not permitted
[440000000000 440008000000] splits a leaf entry in linear map

$ sudo echo offline > memory558079/state
write error: Operation not permitted
[441ff8000000 442000000000] splits a leaf entry in linear map
"""

My fix corrects the checked range and prevents the false acceptance.

Best regards,
Richard Cheng.

---
 arch/arm64/mm/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index a25d8beacc83..18a8b0d3714e 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -2194,7 +2194,7 @@ static int prevent_memory_remove_notifier(struct notifier_block *nb,
 		}
 	}
 
-	if (!can_unmap_without_split(pfn, arg->nr_pages))
+	if (!can_unmap_without_split(arg->start_pfn, arg->nr_pages))
 		return NOTIFY_BAD;
 
 	return NOTIFY_OK;

base-commit: 5ca04f3ba91f1773bbd5da6d9c654ccc1ba7831d
-- 
2.43.0