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/[email protected]
Signed-off-by: Suren Baghdasaryan <[email protected]>
Suggested-by: Michal Hocko <[email protected]>
Acked-by: Michal Hocko <[email protected]>
---
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