Re: Hang on large copy_from_user with PREEMPT_NONE

[Sasha Levin <sasha.levin@oracle.com>]

On 04/06/2015 01:26 PM, Linus Torvalds wrote:
> On Sun, Apr 5, 2015 at 8:59 PM, Sasha Levin <sasha.levin@oracle.com> wrote:
>>
>> This is the result of getting copy_user_handle_tail to zero out a large block of
>> kernel memory very inefficiently:
> 
> Ugh.
> 
> Normally we should be able to just do
> 
>         if (zerorest)
>                 memset(to, 0, len);
> 
> and be done with it.
> 
> The only reason we don't do that actually looks like a buglet in
> 'copy_in_user()', which can have a source fault but should *not*
> necessarily try to clear the rest of the destination buffer. But it
> uses the shared "copy_user_generic()" logic, so it doesn't even
> realize that.
> 
> I call it a "buglet", because there's not necessarily anything
> horribly wrong with clearing the tail, it's just completely wasted
> work. And it makes the "oops, source is bad" case unnecessarily
> horribly slow.
> 
> In fact, the whole "zerorest" thing is garbage, I think. The
> copy_user_generic() code seems to always set it to just 'len', and
> it's because it doesn't even know or care about the actual direction.
> 
> The *real* test should just be "is the destination a kernel space
> buffer" (we have done the "access_ok()" things independently). And
> that test we can do without any 'zerorest' parameter.
> 
> So the attached (untested) patch should
> 
>  (a) remove the pointless 'zerorest' parameter
> 
>  (b) fix the 'copy_in_user()' buglet
> 
>  (c) make the kernel destination case be much more efficient with just
> a simple 'memset()'
> 
> Hmm? Comments? Sasha, do you have the initial random number state to
> recreate this easily?

Your patch just makes it hang in memset instead:

[  963.104556] NMI watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [trinity-c224:9845]
[  963.104556] Modules linked in:
[  963.104556] irq event stamp: 3773324
[  963.104556] hardirqs last  enabled at (3773323): [<ffffffffb7af8e3c>] restore_args+0x0/0x34
[  963.104556] hardirqs last disabled at (3773324): [<ffffffffb7af919e>] apic_timer_interrupt+0x6e/0x80
[  963.104556] softirqs last  enabled at (3773322): [<ffffffffad1f6589>] __do_softirq+0x709/0xd40
[  963.104556] softirqs last disabled at (3773317): [<ffffffffad1f746d>] irq_exit+0x29d/0x320
[  963.104556] CPU: 1 PID: 9845 Comm: trinity-c224 Not tainted 4.0.0-rc6-next-20150402-sasha-00039-ge0bdae3-dirty #2130
[  963.104556] task: ffff8802a3560000 ti: ffff8802a3568000 task.ti: ffff8802a3568000
[  963.104556] RIP: 0010:[<ffffffffaefbd173>]  [<ffffffffaefbd173>] memset_orig+0x33/0xb0
[  963.104556] RSP: 0000:ffff8802a356fdf0  EFLAGS: 00010212
[  963.104556] RAX: 0000000000000000 RBX: 000000007e777000 RCX: 000000000042f8bf
[  963.104556] RDX: 000000007e777000 RSI: 0000000000000000 RDI: ffffc9008f766000
[  963.104556] RBP: ffff8802a356fe18 R08: 000000000fceee00 R09: 0000000000000000
[  963.104556] R10: ffffc90021bd2000 R11: fffff52014069200 R12: 000000000fceee00
[  963.104556] R13: fffff520136892d8 R14: 000000000fceee00 R15: fffff520140691ff
[  963.104556] FS:  00007fd40c4fe700(0000) GS:ffff8800a2800000(0000) knlGS:0000000000000000
[  963.104556] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  963.104556] CR2: 00007fd40c018000 CR3: 00000002a351a000 CR4: 00000000000007a0
[  963.104556] DR0: 00007fd409b8f000 DR1: 0000000000000000 DR2: 0000000000000000
[  963.104556] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
[  963.104556] Stack:
[  963.104556]  ffffffffad63b4ac ffffffffb7afd69c 000000007e777000 ffff8802a356fe78
[  963.104556]  ffff8802a356ff18 ffff8802a356fe38 ffffffffaefbd759 0000000000010206
[  963.104556]  1ffff100546adfcb ffff8802a356ff48 ffffffffad3c5ef0 ffffc90020b49000
[  963.104556] Call Trace:
[  963.104556]  [<ffffffffad63b4ac>] ? memset+0x2c/0x40
[  963.104556]  [<ffffffffb7afd69c>] ? bad_to_user+0x66/0x1391
[  963.104556]  [<ffffffffaefbd759>] copy_user_handle_tail+0x69/0x80
[  963.104556]  [<ffffffffad3c5ef0>] SyS_init_module+0x150/0x210
[  963.104556]  [<ffffffffad3c5da0>] ? load_module+0x96a0/0x96a0
[  963.104556]  [<ffffffffaefbd3b8>] ? trace_hardirqs_on_thunk+0x17/0x19
[  963.104556]  [<ffffffffb7af83b6>] system_call_fastpath+0x16/0x84
[  963.104556] Code: b8 01 01 01 01 01 01 01 01 48 0f af c1 41 89 f9 41 83 e1 07 75 70 48 89 d1 48 c1 e9 06 74 39 66 0f 1f 84 00 00 00 00 00 48 ff c9 <48> 89 07 48 89 47 08 48 89 47 10 48 89 47 18 48 89 47 20 48 89

It's easy to reproduce it with trinity by adding -cinit_module, no need to touch the seed.


Thanks,
Sasha