On Fri, Apr 12, 2013 at 07:31:32PM -0700, Michel Lespinasse wrote:
> Sorry for the earlier message getting sent before I was done typing it.
>
> On Fri, Apr 12, 2013 at 11:13 AM, Vivek Goyal <vgo...@redhat.com> wrote:
> > Hi,
> >
> > I am writing some code where I lock down a process memory at exec() time.
> > My patches were working fine till 3.9-rc4 and suddendly things broke down
> > in 3.9-rc5.
> >
> > Whenever I tried to exec() a process with memory locked down, my bash
> > session hangs and after a while I get following warning.
> >
> > login: [ 174.669002] INFO: rcu_sched self-detected stall on CPU { 2}
> > (t=60000 jiffies g=2580 c=2579 q=1085)
> > [ 174.669002] Pid: 4894, comm: kexec Not tainted 3.9.0-rc6+ #243
> > [ 174.669002] Call Trace:
> > [ 174.669002] <IRQ> [<ffffffff810c413a>] rcu_check_callbacks+0x21a/0x760
> > [ 174.669002] [<ffffffff810c7c0c>] ? acct_account_cputime+0x1c/0x20
> > [ 174.669002] [<ffffffff8104fd08>] update_process_times+0x48/0x80
> > [ 174.669002] [<ffffffff810913dd>] tick_sched_handle+0x3d/0x50
> > [ 174.669002] [<ffffffff810915e5>] tick_sched_timer+0x45/0x70
> > [ 174.669002] [<ffffffff81066951>] __run_hrtimer+0x81/0x220
> > [ 174.669002] [<ffffffff810915a0>] ? tick_nohz_handler+0xa0/0xa0
> > [ 174.669002] [<ffffffff8108ae0c>] ? ktime_get_update_offsets+0x4c/0xd0
> > [ 174.669002] [<ffffffff81067297>] hrtimer_interrupt+0xf7/0x250
> > [ 174.669002] [<ffffffff81886739>] smp_apic_timer_interrupt+0x69/0x99
> > [ 174.669002] [<ffffffff818859ca>] apic_timer_interrupt+0x6a/0x70
> > [ 174.669002] <EOI> [<ffffffff8111e557>] ?
> > __mlock_vma_pages_range+0x57/0x70
> > [ 174.669002] [<ffffffff8111e568>] ? __mlock_vma_pages_range+0x68/0x70
> > [ 174.669002] [<ffffffff8111ea01>] __mm_populate+0x71/0x140
> > [ 174.669002] [<ffffffff81121b5f>] vm_brk+0x7f/0xa0
> > [ 174.669002] [<ffffffff81199633>] load_elf_binary+0x1a73/0x1b10
> > [ 174.669002] [<ffffffff812d25a5>] ? ima_bprm_check+0x55/0x70
> > [ 174.669002] [<ffffffff8114890a>] search_binary_handler+0x12a/0x3b0
> > [ 174.669002] [<ffffffff81197bc0>] ? load_elf_library+0x210/0x210
> > [ 174.669002] [<ffffffff8114aa00>] do_execve_common+0x500/0x5c0
> > [ 174.669002] [<ffffffff8114aaf7>] do_execve+0x37/0x40
> > [ 174.669002] [<ffffffff8114ad9d>] sys_execve+0x3d/0x60
> > [ 174.669002] [<ffffffff81885379>] stub_execve+0x69/0xa0
> >
> > I did a git bisect and bisection says that following is first bad
> > commit.
> >
> > commit 09a9f1d27892255cfb9c91203f19476765e2d8d1
> > Author: Michel Lespinasse <wal...@google.com>
> > Date: Thu Mar 28 16:26:23 2013 -0700
> >
> > Revert "mm: introduce VM_POPULATE flag to better deal with racy
> > userspace pr
> >
> > This reverts commit 186930500985 ("mm: introduce VM_POPULATE flag to
> > better deal with racy userspace programs").
> >
> > I reverted above commit and problem gets fixed.
> >
> > Following is my simple patch to lock down a selected process memory.
> >
> > Index: linux-2.6/fs/binfmt_elf.c
> > ===================================================================
> > --- linux-2.6.orig/fs/binfmt_elf.c 2013-04-13 01:50:26.380184101
> > -0400
> > +++ linux-2.6/fs/binfmt_elf.c 2013-04-13 01:50:49.827184821 -0400
> > @@ -721,6 +721,10 @@ static int load_elf_binary(struct linux_
> >
> > /* OK, This is the point of no return */
> > current->mm->def_flags = def_flags;
> > + if (!strcmp(bprm->filename, "/sbin/kexec")) {
> > + printk("Memlocking /sbin/kexec\n");
> > + current->mm->def_flags |= VM_LOCKED;
> > + }
> >
> > /* Do this immediately, since STACK_TOP as used in setup_arg_pages
> > may depend on the personality. */
> >
> >
> > Do you have any thoughts on what's going on. I am wondering if it indicates
> > a bigger problem which can then be triggered from other paths too.
> >
> > Thanks
> > Vivek
>
> Based on your patch, it looks like from 3.9-rc1 to 3.9-rc5 your change
> wouldn't actually cause pages to get mlocked during exec - for this
> range of kernels, mlockall would need to set both VM_LOCKED and
> VM_POPULATE. I suspect you would see the same crash if you included
> VM_POPULATE in your change, too.
Ok. I specified VM_POPULATE and I see the same problem.
>
> That said, I am not sure immediately what's wrong. It looks like a
> deadlock situation, does CONFIG_LOCKDEP help here ?
I turned on CONFIG_LOCKDEP and nothing shows up.
>
> My first guess would be that mmap_sem is held during exec, so you
> can't have __mm_populate() try holding it recursively.
I think it is not mmap_sem as even with VM_LOCKED, we take mmap_sem
and things are fine.
So things work till 3.8 and break in 3.8-rc1 (with both VM_LOCKED and
VM_POPULATE specifed). I will do git bisect and try to figure out which
is first commit which has the issue.
Thanks
Vivek
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