On Sun, Mar 06, 2022 at 10:37:21AM +0100, gregkh@xxxxxxxxxxxxxxxxxxx wrote:
The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 96403e11283def1d1c465c8279514c9a504d8630 Mon Sep 17 00:00:00 2001
From: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Date: Fri, 4 Mar 2022 20:28:55 -0800
Subject: [PATCH] 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.
I think that this patch depends on 78db3412833d ("mm: add anonymous vma
name refcounting") which we don't have in any of the stable trees. (is
this why it wasn't tagged for stable?).
--
Thanks,
Sasha
