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Date: Thu, 21 Oct 2021 18:46:58 -0700 From: Suren Baghdasaryan <surenb@...gle.com> To: akpm@...ux-foundation.org Cc: mhocko@...nel.org, mhocko@...e.com, rientjes@...gle.com, willy@...radead.org, hannes@...xchg.org, guro@...com, riel@...riel.com, minchan@...nel.org, christian@...uner.io, hch@...radead.org, oleg@...hat.com, david@...hat.com, jannh@...gle.com, shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com, fweimer@...hat.com, jengelh@...i.de, linux-api@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com, surenb@...gle.com Subject: [PATCH 1/1] mm: 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. Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call") Signed-off-by: Suren Baghdasaryan <surenb@...gle.com> --- 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 831340e7ad8b..989f35a2bbb1 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: -- 2.33.0.1079.g6e70778dc9-goog
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