linux-kernel - Re: [PATCH] mm: fix racing of vb->va when kasan enabled

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Date:   Tue, 21 Jun 2022 16:29:06 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        "zhaoyang.huang" <zhaoyang.huang@...soc.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ke Wang <ke.wang@...soc.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] mm: fix racing of vb->va when kasan enabled

> On Tue, Jun 21, 2022 at 5:27 PM Uladzislau Rezki <urezki@...il.com> wrote:
> >
> > > On Mon, Jun 20, 2022 at 6:44 PM Uladzislau Rezki <urezki@...il.com> wrote:
> > > >
> > > > > > >
> > > > > > Is it easy to reproduce? If so could you please describe the steps? As i see
> > > > > > the freeing of the "vb" is RCU safe whereas vb->va is not. But from the first
> > > > > > glance i do not see how it can accessed twice. Hm..
> > > > > It was raised from a monkey test on A13_k515 system and got 1/20 pcs
> > > > > failed. IMO, vb->va which out of vmap_purge_lock protection could race
> > > > > with a concurrent ra freeing within __purge_vmap_area_lazy.
> > > > >
> > > > Do you have exact steps how you run "monkey" test?
> > > There are about 30+ kos inserted during startup which could be a
> > > specific criteria for reproduction. Do you have doubts about the test
> > > result or the solution?
> > > >
> > I do not have any doubt about your test results, so if you can trigger it
> > then there is an issue at least on the 5.4.161-android12 kernel.
> >
> > 1. With your fix we get expanded mutex range, thus the worst case of vmalloc
> > allocation can be increased when it fails and repeat. Because it also invokes
> > the purge_vmap_area_lazy() that access the same mutex.
> I am not sure I get your point. _vm_unmap_aliases calls
> _purge_vmap_area_lazy instead of purge_vmap_area_lazy. Do you have any
> other solutions? I really don't think my patch is the best way as I
> don't have a full view of vmalloc mechanism.
>
Yep, but it holds the mutex:

<snip>
mutex_lock(&vmap_purge_lock);
purge_fragmented_blocks_allcpus();
if (!__purge_vmap_area_lazy(start, end) && flush)
	flush_tlb_kernel_range(start, end);
mutex_unlock(&vmap_purge_lock);
<snip>

I do not have a solution yet. I am trying still to figure out how you can
trigger it. 

<snip>
	rcu_read_lock();
	list_for_each_entry_rcu(vb, &vbq->free, free_list) {
		spin_lock(&vb->lock);
		if (vb->dirty && vb->dirty != VMAP_BBMAP_BITS) {
			unsigned long va_start = vb->va->va_start;
<snip>

so you say that "vb->va->va_start" can be accessed twice. I do not see
how it can happen. The purge_fragmented_blocks() removes "vb" from the 
free_list and set vb->dirty to the VMAP_BBMAP_BITS to prevent purging
it again. It is protected by the spin_lock(&vb->lock):

<snip>
spin_lock(&vb->lock);
if (vb->free + vb->dirty == VMAP_BBMAP_BITS && vb->dirty != VMAP_BBMAP_BITS) {
	vb->free = 0; /* prevent further allocs after releasing lock */
	vb->dirty = VMAP_BBMAP_BITS; /* prevent purging it again */
	vb->dirty_min = 0;
	vb->dirty_max = VMAP_BBMAP_BITS;
<snip>

so the VMAP_BBMAP_BITS is set under spinlock. The _vm_unmap_aliases() checks it:

<snip>
list_for_each_entry_rcu(vb, &vbq->free, free_list) {
	spin_lock(&vb->lock);
	if (vb->dirty && vb->dirty != VMAP_BBMAP_BITS) {
		unsigned long va_start = vb->va->va_start;
		unsigned long s, e;
<snip>

if the "vb->dirty != VMAP_BBMAP_BITS". I am missing your point here?

> >
> > 2. You run 5.4.161-android12 kernel what is quite old. Could you please
> > retest with latest kernel? I am asking because on the latest kernel with
> > CONFIG_KASAN i am not able to reproduce it.
> >
> > I do a lot of: vm_map_ram()/vm_unmap_ram()/vmalloc()/vfree() in parallel
> > by 64 kthreads on my 64 CPUs test system.
> The failure generates at 20s from starting up, I think it is a rare timing.
> >
> > Could you please confirm that you can trigger an issue on the latest kernel?
> Sorry, I don't have an available latest kernel for now.
>
Can you do: "gdb ./vmlinux", execute "l *_vm_unmap_aliases+0x164" and provide
output?

--
Uladzislau Rezki