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@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Baoquan He <bhe@xxxxxxxxxx>
Cc: Christoph von Recklinghausen <crecklin@xxxxxxxxxx>
Cc: Don Dutile <ddutile@xxxxxxxxxx>
Cc: Herton R. Krzesinski <herton@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Joel Savitz <jsavitz@xxxxxxxxxx>
Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
Co-developed-by: Joel Savitz <jsavitz@xxxxxxxxxx>
Signed-off-by: Joel Savitz <jsavitz@xxxxxxxxxx>
Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
---
include/linux/oom.h | 3 ++-
mm/mmap.c | 3 ++-
mm/oom_kill.c | 14 +++++++++++---
3 files changed, 15 insertions(+), 5 deletions(-)
diff --git a/include/linux/oom.h b/include/linux/oom.h
index 2db9a1432511..580c95a0541d 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -106,7 +106,8 @@ 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, struct robust_list_head
+ __user *robust_list);
long oom_badness(struct task_struct *p,
unsigned long totalpages);
diff --git a/mm/mmap.c b/mm/mmap.c
index 3aa839f81e63..c14fe6f8e9a5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3126,7 +3126,8 @@ 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, current->robust_list);
+
set_bit(MMF_OOM_SKIP, &mm->flags);
}
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 7ec38194f8e1..727cfc3bd284 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -509,9 +509,11 @@ 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, struct robust_list_head
+ __user *robust_list)
{
struct vm_area_struct *vma;
+ unsigned long head = (unsigned long) robust_list;
bool ret = true;
/*
@@ -526,6 +528,11 @@ bool __oom_reap_task_mm(struct mm_struct *mm)
if (vma->vm_flags & (VM_HUGETLB|VM_PFNMAP))
continue;
+ if (vma->vm_start <= head && vma->vm_end > head) {
+ pr_info("oom_reaper: skipping vma, contains robust_list");
+ continue;
+ }
+
/*
* Only anonymous pages have a good chance to be dropped
* without additional steps which we cannot afford as we
@@ -587,7 +594,7 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
trace_start_task_reaping(tsk->pid);
/* failed to reap part of the address space. Try again later */
- ret = __oom_reap_task_mm(mm);
+ ret = __oom_reap_task_mm(mm, tsk->robust_list);
if (!ret)
goto out_finish;
@@ -1190,7 +1197,8 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
* 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, p->robust_list))
ret = -EAGAIN;
mmap_read_unlock(mm);
--
2.35.1
