[PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head

Nico Pache
2022-04-08T03:28:44Z

The pthread struct is allocated on PRIVATE|ANONYMOUS memory [1] which can
be targeted by the oom reaper. This mapping is used to store the futex
robust list head; the kernel does not keep a copy of the robust list and
instead references a userspace address to maintain the robustness during
a process death. A race can occur between exit_mm and the oom reaper that
allows the oom reaper to free the memory of the futex robust list before
the exit path has handled the futex death:

CPU1 CPU2
------------------------------------------------------------------------
page_fault
do_exit "signal"
wake_oom_reaper
oom_reaper
oom_reap_task_mm (invalidates mm)
exit_mm
exit_mm_release
futex_exit_release
futex_cleanup
exit_robust_list
get_user (EFAULT- can't access memory)

If the get_user EFAULT's, the kernel will be unable to recover the
waiters on the robust_list, leaving userspace mutexes hung indefinitely.

Use the robust_list address stored in the kernel to skip the VMA that holds
it, allowing a successful futex_cleanup.

Theoretically a failure can still occur if there are locks mapped as
PRIVATE|ANON; however, the robust futexes are a best-effort approach.
This patch only strengthens that best-effort.

The following case can still fail:
robust head (skipped) -> private lock (reaped) -> shared lock (skipped)

Reproducer: https://gitlab.com/jsavitz/oom_futex_reproducer

[1] https://elixir.bootlin.com/glibc/latest/source/nptl/allocatestack.c#L370

Fixes: 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christoph von Recklinghausen <crecklin@redhat.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Herton R. Krzesinski <herton@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Savitz <jsavitz@redhat.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: stable@kernel.org
Co-developed-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
include/linux/oom.h | 2 +-
mm/mmap.c | 8 +++++++-
mm/oom_kill.c | 31 ++++++++++++++++++++++++++++---
3 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/include/linux/oom.h b/include/linux/oom.h
index 2db9a1432511..46355ff97abb 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -106,7 +106,7 @@ static inline vm_fault_t check_stable_address_space(struct mm_struct *mm)
return 0;
}

-bool __oom_reap_task_mm(struct mm_struct *mm);
+bool __oom_reap_task_mm(struct mm_struct *mm, void *skip_area);

long oom_badness(struct task_struct *p,
unsigned long totalpages);
diff --git a/mm/mmap.c b/mm/mmap.c
index 3aa839f81e63..6dee7ea60a13 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3109,6 +3109,11 @@ void exit_mmap(struct mm_struct *mm)
struct mmu_gather tlb;
struct vm_area_struct *vma;
unsigned long nr_accounted = 0;
+ void *robust_list = NULL;
+
+#ifdef CONFIG_FUTEX
+ robust_list = current->robust_list;
+#endif

/* mm's last user has gone, and its about to be pulled down */
mmu_notifier_release(mm);
@@ -3126,7 +3131,8 @@ void exit_mmap(struct mm_struct *mm)
* to mmu_notifier_release(mm) ensures mmu notifier callbacks in
* __oom_reap_task_mm() will not block.
*/
- (void)__oom_reap_task_mm(mm);
+ (void)__oom_reap_task_mm(mm, robust_list);
+
set_bit(MMF_OOM_SKIP, &mm->flags);
}

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 7ec38194f8e1..2a7443a64909 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -509,9 +509,10 @@ static DECLARE_WAIT_QUEUE_HEAD(oom_reaper_wait);
static struct task_struct *oom_reaper_list;
static DEFINE_SPINLOCK(oom_reaper_lock);

-bool __oom_reap_task_mm(struct mm_struct *mm)
+bool __oom_reap_task_mm(struct mm_struct *mm, void *skip_area)
{
struct vm_area_struct *vma;
+ unsigned long skip_vma = (unsigned long) skip_area;
bool ret = true;

/*
@@ -526,6 +527,20 @@ bool __oom_reap_task_mm(struct mm_struct *mm)
if (vma->vm_flags & (VM_HUGETLB|VM_PFNMAP))
continue;

+#ifdef CONFIG_FUTEX
+ /*
+ * The OOM reaper runs concurrently with do_exit.
+ * The robust_list_head is stored in userspace and is required
+ * by the exit path to recover the robust futex waiters.
+ * Skip the VMA that contains the robust_list to allow for
+ * proper cleanup.
+ */
+ if (vma->vm_start <= skip_vma && vma->vm_end > skip_vma) {
+ pr_info("oom_reaper: skipping vma, contains robust_list");
+ continue;
+ }
+#endif
+
/*
* Only anonymous pages have a good chance to be dropped
* without additional steps which we cannot afford as we
@@ -567,6 +582,7 @@ bool __oom_reap_task_mm(struct mm_struct *mm)
static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
{
bool ret = true;
+ void *robust_list = NULL;

if (!mmap_read_trylock(mm)) {
trace_skip_task_reaping(tsk->pid);
@@ -586,8 +602,11 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)

trace_start_task_reaping(tsk->pid);

+#ifdef CONFIG_FUTEX
+ robust_list = tsk->robust_list;
+#endif
/* failed to reap part of the address space. Try again later */
- ret = __oom_reap_task_mm(mm);
+ ret = __oom_reap_task_mm(mm, robust_list);
if (!ret)
goto out_finish;

@@ -1149,6 +1168,7 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
unsigned int f_flags;
bool reap = false;
long ret = 0;
+ void *robust_list = NULL;

if (flags)
return -EINVAL;
@@ -1186,11 +1206,16 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
ret = -EINTR;
goto drop_mm;
}
+
+#ifdef CONFIG_FUTEX
+ robust_list = p->robust_list;
+#endif
/*
* Check MMF_OOM_SKIP again under mmap_read_lock protection to ensure
* possible change in exit_mmap is seen
*/
- if (!test_bit(MMF_OOM_SKIP, &mm->flags) && !__oom_reap_task_mm(mm))
+ if (!test_bit(MMF_OOM_SKIP, &mm->flags) &&
+ !__oom_reap_task_mm(mm, robust_list))
ret = -EAGAIN;
mmap_read_unlock(mm);

--
2.35.1