Re: [PATCH v7] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head

[Nico Pache <npache@redhat.com>]

On 4/7/22 23:12, Andrew Morton wrote:
> On Thu,  7 Apr 2022 23:01:37 -0400 Nico Pache <npache@redhat.com> 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
>>
>> [1] https://elixir.bootlin.com/glibc/latest/source/nptl/allocatestack.c#L370
>>
>> ...
>>
>> --- 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 *robust_list);
>>  
>>  long oom_badness(struct task_struct *p,
>>  		unsigned long totalpages);
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 3aa839f81e63..d5af1b83cbb2 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;
>> +
>> +#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);
> 
> uninitialized var warning when CONFIG_FUTEX=n?
> 
>> +
>>  		set_bit(MMF_OOM_SKIP, &mm->flags);
>>  	}
>>  
>> --- 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 *robust_list)
> 
> Well, this is no longer necessarily a robust_list*.  It's just an
> address to skip and the name should reflect that?
ok fair point. I left it as robust list to be more clean what is being skipped
and renamed the head variable to skip_vma so its more verbose. But I dont have
that strong of an opinion on it so I'll change it real quick.

> 
>>  {
>>  	struct vm_area_struct *vma;
>> +	unsigned long skip_vma = (unsigned long) robust_list;
>>  	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;
>>  
>>  	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);
> 
> unintialized var when CONFIG_FUTEX=n?
> 
>>  	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;
>>  
>>  	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))
> 
> again
Good catch. Ill fix those real quick.
> 
>>  		ret = -EAGAIN;
>>  	mmap_read_unlock(mm);
>