From: Al Viro <viro@ZenIV.linux.org.uk>
To: Eric Biggers <ebiggers3@gmail.com>
Cc: syzbot <syzbot+bacbe5d8791f30c9cee5@syzkaller.appspotmail.com>,
akpm@linux-foundation.org, aneesh.kumar@linux.vnet.ibm.com,
dan.j.williams@intel.com, james.morse@arm.com,
kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, mingo@kernel.org,
syzkaller-bugs@googlegroups.com
Subject: Re: possible deadlock in get_user_pages_unlocked
Date: Fri, 2 Feb 2018 05:46:26 +0000 [thread overview]
Message-ID: <20180202054626.GG30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180202053502.GB949@zzz.localdomain>
On Thu, Feb 01, 2018 at 09:35:02PM -0800, Eric Biggers wrote:
> Try starting up multiple instances of the program; that sometimes helps with
> these races that are hard to hit (since you may e.g. have a different number of
> CPUs than syzbot used). If I start up 4 instances I see the lockdep splat after
> around 2-5 seconds.
5 instances in parallel, 10 minutes into the run...
> This is on latest Linus tree (4bf772b1467). Also note the
> reproducer uses KVM, so if you're running it in a VM it will only work if you've
> enabled nested virtualization on the host (kvm_intel.nested=1).
cat /sys/module/kvm_amd/parameters/nested
1
on host
> Also it appears to go away if I revert ce53053ce378c21 ("kvm: switch
> get_user_page_nowait() to get_user_pages_unlocked()").
That simply prevents this reproducer hitting get_user_pages_unlocked()
instead of grab mmap_sem/get_user_pages/drop mmap_sem. I.e. does not
allow __get_user_pages_locked() to drop/regain ->mmap_sem.
The bug may be in the way we call get_user_pages_unlocked() in that
commit, but it might easily be a bug in __get_user_pages_locked()
exposed by that reproducer somehow.
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WARNING: multiple messages have this Message-ID (diff)
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Eric Biggers <ebiggers3@gmail.com>
Cc: syzbot <syzbot+bacbe5d8791f30c9cee5@syzkaller.appspotmail.com>,
akpm@linux-foundation.org, aneesh.kumar@linux.vnet.ibm.com,
dan.j.williams@intel.com, james.morse@arm.com,
kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, mingo@kernel.org,
syzkaller-bugs@googlegroups.com
Subject: Re: possible deadlock in get_user_pages_unlocked
Date: Fri, 2 Feb 2018 05:46:26 +0000 [thread overview]
Message-ID: <20180202054626.GG30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180202053502.GB949@zzz.localdomain>
On Thu, Feb 01, 2018 at 09:35:02PM -0800, Eric Biggers wrote:
> Try starting up multiple instances of the program; that sometimes helps with
> these races that are hard to hit (since you may e.g. have a different number of
> CPUs than syzbot used). If I start up 4 instances I see the lockdep splat after
> around 2-5 seconds.
5 instances in parallel, 10 minutes into the run...
> This is on latest Linus tree (4bf772b1467). Also note the
> reproducer uses KVM, so if you're running it in a VM it will only work if you've
> enabled nested virtualization on the host (kvm_intel.nested=1).
cat /sys/module/kvm_amd/parameters/nested
1
on host
> Also it appears to go away if I revert ce53053ce378c21 ("kvm: switch
> get_user_page_nowait() to get_user_pages_unlocked()").
That simply prevents this reproducer hitting get_user_pages_unlocked()
instead of grab mmap_sem/get_user_pages/drop mmap_sem. I.e. does not
allow __get_user_pages_locked() to drop/regain ->mmap_sem.
The bug may be in the way we call get_user_pages_unlocked() in that
commit, but it might easily be a bug in __get_user_pages_locked()
exposed by that reproducer somehow.
next prev parent reply other threads:[~2018-02-02 5:46 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-02 0:58 possible deadlock in get_user_pages_unlocked syzbot
2018-02-02 4:50 ` Al Viro
2018-02-02 4:50 ` Al Viro
2018-02-02 5:35 ` Eric Biggers
2018-02-02 5:35 ` Eric Biggers
2018-02-02 5:46 ` Al Viro [this message]
2018-02-02 5:46 ` Al Viro
2018-02-02 6:20 ` Al Viro
2018-02-02 6:20 ` Al Viro
2018-02-02 8:57 ` Dmitry Vyukov
2018-02-02 8:57 ` Dmitry Vyukov
2018-02-10 1:36 ` Al Viro
2018-02-10 1:36 ` Al Viro
2018-02-10 3:19 ` Eric Biggers
2018-02-10 3:19 ` Eric Biggers
2018-03-10 4:15 ` Eric Biggers
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