[PATCH v2] vma: Detect infinite loop in vma tree

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>

There have been no reported infinite loops in the tree, but checking the
detection of an infinite loop during validation is simple enough.  Add
the detection to the validate_mm() function so that error reports are
clear and don't just report stalls.

This does not protect against internal maple tree issues, but it does
detect too many vmas being returned from the tree.

The variance of +10 is to allow for the debugging output to be more useful for
nearly correct counts.  In the event of more than 10 over the map_count, the
count will be set to -1 for easier identification of a potential infinite loop.

Note that the mmap lock is held to ensure a consistent tree state during the
validation process.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jann Horn <jannh@google.com>
---
 mm/vma.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/vma.c b/mm/vma.c
index 68138e8c153e..283e6bc4884f 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -615,7 +615,10 @@ void validate_mm(struct mm_struct *mm)
 			anon_vma_unlock_read(anon_vma);
 		}
 #endif
-		i++;
+		if (++i > mm->map_count + 10) {
+			i = -1;
+			break;
+		}
 	}
 	if (i != mm->map_count) {
 		pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i);
-- 
2.43.0