Re: [PATCH] mm: Check for SIGKILL inside dup_mmap() loop.

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

Andrew Morton wrote:
> On Sat, 7 Apr 2018 19:38:28 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> 
> > >From 31c863e57a4ab7dfb491b2860fe3653e1e8f593b Mon Sep 17 00:00:00 2001
> > From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > Date: Sat, 7 Apr 2018 19:29:30 +0900
> > Subject: [PATCH] mm: Check for SIGKILL inside dup_mmap() loop.
> > 
> > As a theoretical problem, an mm_struct with 60000+ vmas can loop with
> > potentially allocating memory, with mm->mmap_sem held for write by current
> > thread. This is bad if current thread was selected as an OOM victim, for
> > current thread will continue allocations using memory reserves while OOM
> > reaper is unable to reclaim memory.
> > 
> > As an actually observable problem, it is not difficult to make OOM reaper
> > unable to reclaim memory if the OOM victim is blocked at
> > i_mmap_lock_write() in this loop. Unfortunately, since nobody can explain
> > whether it is safe to use killable wait there, let's check for SIGKILL
> > before trying to allocate memory. Even without an OOM event, there is no
> > point with continuing the loop from the beginning if current thread is
> > killed.
> > 
> > ...
> >
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -441,6 +441,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> >  			continue;
> >  		}
> >  		charge = 0;
> > +		if (fatal_signal_pending(current)) {
> > +			retval = -EINTR;
> > +			goto out;
> > +		}
> >  		if (mpnt->vm_flags & VM_ACCOUNT) {
> >  			unsigned long len = vma_pages(mpnt);
> 
> Seems sane.  Has this been runtime tested?
> 

Yes, I tested with debug printk(). This patch should be safe
because we already fail if security_vm_enough_memory_mm() or
kmem_cache_alloc(GFP_KERNEL) fails and exit_mmap() handles it.

[  417.030691] ***** Aborting dup_mmap() due to SIGKILL *****
[  417.036129] ***** Aborting dup_mmap() due to SIGKILL *****
[  417.044544] ***** Aborting dup_mmap() due to SIGKILL *****
[  419.116445] ***** Aborting dup_mmap() due to SIGKILL *****
[  419.118401] ***** Aborting exit_mmap() due to NULL mmap *****
[  419.168917] ***** Aborting dup_mmap() due to SIGKILL *****
[  419.169064] ***** Aborting dup_mmap() due to SIGKILL *****
[  419.170913] ***** Aborting exit_mmap() due to NULL mmap *****
[  419.171411] ***** Aborting dup_mmap() due to SIGKILL *****
[  419.171417] ***** Aborting exit_mmap() due to NULL mmap *****
[  419.172804] ***** Aborting exit_mmap() due to NULL mmap *****
[  419.176253] ***** Aborting dup_mmap() due to SIGKILL *****
[  419.182676] ***** Aborting exit_mmap() due to NULL mmap *****

> I would like to see a comment here explaining why we're testing for
> this at this particualr place.
> 
Such comment goes to patch description.