[PATCH v3 2/3] mm: prevent vm_area_struct::anon_name refcount saturation

[Suren Baghdasaryan <surenb@google.com>]

A deep process chain with many vmas could grow really high.  With default
sysctl_max_map_count (64k) and default pid_max (32k) the max number of
vmas in the system is 2147450880 and the refcounter has headroom of
1073774592 before it reaches REFCOUNT_SATURATED (3221225472).

Therefore it's unlikely that an anonymous name refcounter will overflow
with these defaults.  Currently the max for pid_max is PID_MAX_LIMIT
(4194304) and for sysctl_max_map_count it's INT_MAX (2147483647).  In this
configuration anon_vma_name refcount overflow becomes theoretically
possible (that still require heavy sharing of that anon_vma_name between
processes).

kref refcounting interface used in anon_vma_name structure will detect a
counter overflow when it reaches REFCOUNT_SATURATED value but will only
generate a warning and freeze the ref counter. This would lead to the
refcounted object never being freed. A determined attacker could leak
memory like that but it would be rather expensive and inefficient way
to do so.

To ensure anon_vma_name refcount does not overflow, stop anon_vma_name
sharing when the refcount reaches REFCOUNT_MAX (2147483647), which still
leaves INT_MAX/2 (1073741823) values before the counter reaches
REFCOUNT_SATURATED.  This should provide enough headroom for raising the
refcounts temporarily.

Link: https://lkml.kernel.org/r/20220223153613.835563-2-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
---
changes in v3:
- Amended description with what happens when refcount saturates,
per Michal Hocko
- Added Ack, per Michal Hocko

 include/linux/mm_inline.h | 18 ++++++++++++++----
 mm/madvise.c              |  3 +--
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index dd3accaa4e6d..cf90b1fa2c60 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -161,15 +161,25 @@ static inline void anon_vma_name_put(struct anon_vma_name *anon_name)
 		kref_put(&anon_name->kref, anon_vma_name_free);
 }
 
+static inline
+struct anon_vma_name *anon_vma_name_reuse(struct anon_vma_name *anon_name)
+{
+	/* Prevent anon_name refcount saturation early on */
+	if (kref_read(&anon_name->kref) < REFCOUNT_MAX) {
+		anon_vma_name_get(anon_name);
+		return anon_name;
+
+	}
+	return anon_vma_name_alloc(anon_name->name);
+}
+
 static inline void dup_anon_vma_name(struct vm_area_struct *orig_vma,
 				     struct vm_area_struct *new_vma)
 {
 	struct anon_vma_name *anon_name = anon_vma_name(orig_vma);
 
-	if (anon_name) {
-		anon_vma_name_get(anon_name);
-		new_vma->anon_name = anon_name;
-	}
+	if (anon_name)
+		new_vma->anon_name = anon_vma_name_reuse(anon_name);
 }
 
 static inline void free_anon_vma_name(struct vm_area_struct *vma)
diff --git a/mm/madvise.c b/mm/madvise.c
index 081b1cded21e..1f2693dccf7b 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -113,8 +113,7 @@ static int replace_anon_vma_name(struct vm_area_struct *vma,
 	if (anon_vma_name_eq(orig_name, anon_name))
 		return 0;
 
-	anon_vma_name_get(anon_name);
-	vma->anon_name = anon_name;
+	vma->anon_name = anon_vma_name_reuse(anon_name);
 	anon_vma_name_put(orig_name);
 
 	return 0;
-- 
2.35.1.574.g5d30c73bfb-goog