From: Suren Baghdasaryan <surenb@xxxxxxxxxx> Subject: 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@xxxxxxxxxx Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call") Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Roman Gushchin <guro@xxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Christian Brauner <christian@xxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx> Cc: Florian Weimer <fweimer@xxxxxxxxxx> Cc: Jan Engelhardt <jengelh@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/oom_kill.c | 23 ++++++++++++----------- 1 file changed, 12 insertions(+), 11 deletions(-) --- a/mm/oom_kill.c~mm-prevent-a-race-between-process_mrelease-and-exit_mmap +++ a/mm/oom_kill.c @@ -1150,7 +1150,7 @@ SYSCALL_DEFINE2(process_mrelease, int, p 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, p 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, p mmap_read_unlock(mm); drop_mm: - mmdrop(mm); + if (mm) + mmput(mm); put_task: put_task_struct(task); put_pid: _
