From 96403e11283def1d1c465c8279514c9a504d8630 Mon Sep 17 00:00:00 2001 From: Suren Baghdasaryan Date: Fri, 4 Mar 2022 20:28:55 -0800 Subject: mm: prevent vm_area_struct::anon_name refcount saturation 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 Suggested-by: Michal Hocko Acked-by: Michal Hocko Cc: Alexey Gladkov Cc: Chris Hyser Cc: Christian Brauner Cc: Colin Cross Cc: Cyrill Gorcunov Cc: Dave Hansen Cc: David Hildenbrand Cc: Davidlohr Bueso Cc: "Eric W. Biederman" Cc: Johannes Weiner Cc: Kees Cook Cc: "Kirill A. Shutemov" Cc: Matthew Wilcox Cc: Peter Collingbourne Cc: Sasha Levin Cc: Sumit Semwal Cc: Vlastimil Babka Cc: Xiaofeng Cao Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/madvise.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) (limited to 'mm/madvise.c') 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; -- cgit v1.2.3