From 337546e83fc7e50917f44846beee936abb9c9f1f Mon Sep 17 00:00:00 2001
From: Suren Baghdasaryan <surenb@google.com>
Date: Thu, 28 Oct 2021 14:36:14 -0700
Subject: [PATCH] mm/oom_kill.c: prevent a race between process_mrelease and
exit_mmap
Race between process_mrelease and exit_mmap, where free_pgtables is
called while __oom_reap_task_mm is in progress, leads to kernel crash
during pte_offset_map_lock call. oom-reaper avoids this race by setting
MMF_OOM_VICTIM flag and causing exit_mmap to take and release
mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock.
Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to
fix this race, however that would be considered a hack. Fix this race
by elevating mm->mm_users and preventing exit_mmap from executing until
process_mrelease is finished. Patch slightly refactors the code to
adapt for a possible mmget_not_zero failure.
This fix has considerable negative impact on process_mrelease
performance and will likely need later optimization.
Link: https://lkml.kernel.org/r/20211022014658.263508-1-surenb@google.com
Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Christian Brauner <christian@brauner.io>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
mm/oom_kill.c | 23 ++++++++++++-----------
1 file changed, 12 insertions(+), 11 deletions(-)
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 831340e7ad8b4..989f35a2bbb1d 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1150,7 +1150,7 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
struct task_struct *task;
struct task_struct *p;
unsigned int f_flags;
- bool reap = true;
+ bool reap = false;
struct pid *pid;
long ret = 0;
@@ -1177,15 +1177,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
goto put_task;
}
- mm = p->mm;
- mmgrab(mm);
-
- /* If the work has been done already, just exit with success */
- if (test_bit(MMF_OOM_SKIP, &mm->flags))
- reap = false;
- else if (!task_will_free_mem(p)) {
- reap = false;
- ret = -EINVAL;
+ if (mmget_not_zero(p->mm)) {
+ mm = p->mm;
+ if (task_will_free_mem(p))
+ reap = true;
+ else {
+ /* Error only if the work has not been done already */
+ if (!test_bit(MMF_OOM_SKIP, &mm->flags))
+ ret = -EINVAL;
+ }
}
task_unlock(p);
@@ -1201,7 +1201,8 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
mmap_read_unlock(mm);
drop_mm:
- mmdrop(mm);
+ if (mm)
+ mmput(mm);
put_task:
put_task_struct(task);
put_pid:
--
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