Date:   Fri, 4 Aug 2017 09:42:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Wenwei Tao <wenwei.tww@...baba-inc.com>,
        Oleg Nesterov <oleg@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, oom: fix potential data corruption when oom_reaper
 races with writer

On Fri 04-08-17 15:46:46, Tetsuo Handa wrote:
> Michal Hocko wrote:
> >                          So there is a race window when some threads
> > won't have fatal_signal_pending while the oom_reaper could start
> > unmapping the address space. generic_perform_write could then write
> > zero page to the page cache and corrupt data.
> 
> Oh, simple generic_perform_write() ?
> 
> > 
> > The race window is rather small and close to impossible to happen but it
> > would be better to have it covered.
> 
> OK, I confirmed that this problem is easily reproducible using below reproducer.

Yeah, I can imagine this could be triggered artificially. I am somehow
more skeptical about real life oom scenarios to trigger this though.
Anyway, thanks for your test case!
 
> Applying your patch seems to avoid this problem, but as far as I tested
> your patch seems to trivially trigger something lock related problem.
> Is your patch really safe?

> ----------
> [   58.539455] Out of memory: Kill process 1056 (a.out) score 603 or sacrifice child
> [   58.543943] Killed process 1056 (a.out) total-vm:4268108kB, anon-rss:2246048kB, file-rss:0kB, shmem-rss:0kB
> [   58.544245] a.out (1169) used greatest stack depth: 11664 bytes left
> [   58.557471] DEBUG_LOCKS_WARN_ON(depth <= 0)
> [   58.557480] ------------[ cut here ]------------
> [   58.564407] WARNING: CPU: 6 PID: 1339 at kernel/locking/lockdep.c:3617 lock_release+0x172/0x1e0
> [   58.569076] Modules linked in: nf_conntrack_netbios_ns nf_conntrack_broadcast ip6t_rpfilter ipt_REJECT nf_reject_ipv4 ip6t_REJECT nf_reject_ipv6 xt_conntrack ip_set nfnetlink ebtable_nat ebtable_broute bridge stp llc ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_raw iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_raw ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter coretemp ppdev pcspkr vmw_balloon vmw_vmci shpchp sg i2c_piix4 parport_pc parport ip_tables xfs libcrc32c sr_mod sd_mod cdrom ata_generic pata_acpi serio_raw mptspi scsi_transport_spi mptscsih ahci e1000 libahci ata_piix mptbase libata
> [   58.599401] CPU: 6 PID: 1339 Comm: a.out Not tainted 4.13.0-rc3-next-20170803+ #142
> [   58.604126] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/02/2015
> [   58.609790] task: ffff9d90df888040 task.stack: ffffa07084854000
> [   58.613944] RIP: 0010:lock_release+0x172/0x1e0
> [   58.617622] RSP: 0000:ffffa07084857e58 EFLAGS: 00010082
> [   58.621533] RAX: 000000000000001f RBX: ffff9d90df888040 RCX: 0000000000000000
> [   58.626074] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffffa30d4ba4
> [   58.630572] RBP: ffffa07084857e98 R08: 0000000000000000 R09: 0000000000000001
> [   58.635016] R10: 0000000000000000 R11: 000000000000001f R12: ffffa07084857f58
> [   58.639694] R13: ffff9d90f60d6cd0 R14: 0000000000000000 R15: ffffffffa305cb6e
> [   58.644200] FS:  00007fb932730740(0000) GS:ffff9d90f9f80000(0000) knlGS:0000000000000000
> [   58.648989] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   58.652903] CR2: 000000000040092f CR3: 0000000135229000 CR4: 00000000000606e0
> [   58.657280] Call Trace:
> [   58.659989]  up_read+0x1a/0x40
> [   58.662825]  __do_page_fault+0x28e/0x4c0
> [   58.665946]  do_page_fault+0x30/0x80
> [   58.668911]  page_fault+0x28/0x30

OK, I know what is going on here. The page fault must have returned with
VM_FAULT_RETRY when the caller drops mmap_sem. My patch overwrites the
this error code so the page fault path doesn't know that the lock is no
longer held and releases is unconditionally. This is a preexisting
problem introduced by 3f70dc38cec2 ("mm: make sure that kthreads will
not refault oom reaped memory"). I should have considered this option.

I believe the easiest way around this is the following patch
---
>From dd31779f763bbe2aa86100f804656ac680c49d35 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Fri, 4 Aug 2017 09:36:34 +0200
Subject: [PATCH] mm: fix double mmap_sem unlock on MMF_UNSTABLE enforced
 SIGBUS

Tetsuo Handa has noticed that MMF_UNSTABLE SIGBUS path in
handle_mm_fault causes a lockdep splat
[   58.539455] Out of memory: Kill process 1056 (a.out) score 603 or sacrifice child
[   58.543943] Killed process 1056 (a.out) total-vm:4268108kB, anon-rss:2246048kB, file-rss:0kB, shmem-rss:0kB
[   58.544245] a.out (1169) used greatest stack depth: 11664 bytes left
[   58.557471] DEBUG_LOCKS_WARN_ON(depth <= 0)
[   58.557480] ------------[ cut here ]------------
[   58.564407] WARNING: CPU: 6 PID: 1339 at kernel/locking/lockdep.c:3617 lock_release+0x172/0x1e0
[   58.599401] CPU: 6 PID: 1339 Comm: a.out Not tainted 4.13.0-rc3-next-20170803+ #142
[   58.604126] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/02/2015
[   58.609790] task: ffff9d90df888040 task.stack: ffffa07084854000
[   58.613944] RIP: 0010:lock_release+0x172/0x1e0
[   58.617622] RSP: 0000:ffffa07084857e58 EFLAGS: 00010082
[   58.621533] RAX: 000000000000001f RBX: ffff9d90df888040 RCX: 0000000000000000
[   58.626074] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffffa30d4ba4
[   58.630572] RBP: ffffa07084857e98 R08: 0000000000000000 R09: 0000000000000001
[   58.635016] R10: 0000000000000000 R11: 000000000000001f R12: ffffa07084857f58
[   58.639694] R13: ffff9d90f60d6cd0 R14: 0000000000000000 R15: ffffffffa305cb6e
[   58.644200] FS:  00007fb932730740(0000) GS:ffff9d90f9f80000(0000) knlGS:0000000000000000
[   58.648989] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   58.652903] CR2: 000000000040092f CR3: 0000000135229000 CR4: 00000000000606e0
[   58.657280] Call Trace:
[   58.659989]  up_read+0x1a/0x40
[   58.662825]  __do_page_fault+0x28e/0x4c0
[   58.665946]  do_page_fault+0x30/0x80
[   58.668911]  page_fault+0x28/0x30

The reason is that the page fault path might have dropped the mmap_sem
and returned with VM_FAULT_RETRY. MMF_UNSTABLE check however rewrites
the error path to VM_FAULT_SIGBUS and we always expect mmap_sem taken in
that path. Fix this by taking mmap_sem when VM_FAULT_RETRY is held in
the MMF_UNSTABLE path. We cannot simply add VM_FAULT_SIGBUS to the
existing error code because all arch specific page fault handlers and
g-u-p would have to learn a new error code combination.

Reported-by: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Fixes: 3f70dc38cec2 ("mm: make sure that kthreads will not refault oom reaped memory")
Cc: stable # 4.9+
Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 mm/memory.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0e517be91a89..4fe5b6254688 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3881,8 +3881,18 @@ int handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 	 * further.
 	 */
 	if (unlikely((current->flags & PF_KTHREAD) && !(ret & VM_FAULT_ERROR)
-				&& test_bit(MMF_UNSTABLE, &vma->vm_mm->flags)))
+				&& test_bit(MMF_UNSTABLE, &vma->vm_mm->flags))) {
+
+		/*
+		 * We are going to enforce SIGBUS but the PF path might have
+		 * dropped the mmap_sem already so take it again so that
+		 * we do not break expectations of all arch specific PF paths
+		 * and g-u-p
+		 */
+		if (ret & VM_FAULT_RETRY)
+			down_read(&vma->vm_mm->mmap_sem);
 		ret = VM_FAULT_SIGBUS;
+	}
 
 	return ret;
 }
-- 
2.13.2

-- 
Michal Hocko
SUSE Labs

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