On Thu, 7 Apr 2022 14:42:54 -0400 Nico Pache <npache@xxxxxxxxxx> wrote: > 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 Should this fix be backported into -stable kernels? > --- 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); Should explicitly include futex.h > 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) > { It's pretty sad to make such a low-level function aware of futex internals. How about making it a more general `void *skip_area'? > 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) { This check as you have it is making assumptions about the length of the area at *robust_list and about that area's relation to the area represented by the vma. So if this is to be made more generic, we'd also need skip_area_len so we can perform a full overlap check. I dunno, maybe not worth it at this time, what do others think. But the special-casing in here is pretty painful. > + 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
