Re: [BUG] Null pointer dereference when freeing pages on 4.6-rc6
From: Shi, Yang
Date: Wed May 18 2016 - 14:41:42 EST
On 5/16/2016 5:44 AM, Vlastimil Babka wrote:
[+CC Joonsoo based on git blame]
On 05/05/2016 11:13 PM, Shi, Yang wrote:
Hi folks,
When I enable the below kernel configs on 4.6-rc6, I came across null
pointer deference issue in boot stage.
CONFIG_SPARSEMEM
CONFIG_DEFERRED_STRUCT_PAGE_INIT
CONFIG_DEBUG_PAGEALLOC
CONFIG_PAGE_EXTENSION
CONFIG_DEBUG_VM
The splat is:
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff8118934b>] page_is_buddy+0x7b/0xe0
PGD 0
Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
Modules linked in:
CPU: 3 PID: 106 Comm: pgdatinit1 Not tainted 4.6.0-rc6 #8
Hardware name: Intel Corporation S5520HC/S5520HC, BIOS
S5500.86B.01.10.0025.030220091519 03/02/2009
task: ffff88017c1d0040 ti: ffff88017c1d4000 task.ti: ffff88017c1d4000
RIP: 0010:[<ffffffff8118934b>] [<ffffffff8118934b>]
page_is_buddy+0x7b/0xe0
RSP: 0000:ffff88017c1d7bf0 EFLAGS: 00010046
RAX: 0000000000000000 RBX: ffffea0019810040 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffea0019810040 RDI: 0000000000660401
RBP: ffff88017c1d7c08 R08: 0000000000000001 R09: 0000000000000000
R10: 00000000000001af R11: 0000000000000001 R12: ffffea0019810000
R13: 0000000000000000 R14: 0000000000000009 R15: ffffea0019810000
FS: 0000000000000000(0000) GS:ffff88066cc40000(0000)
knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000002406000 CR4: 00000000000006e0
Stack:
0000000019810000 0000000000000000 ffff88066cfe6080 ffff88017c1d7c70
ffffffff8118bfea 0000000a00000000 0000160000000000 0000000000000001
0000000000000401 ffffea0019810040 0000000000000400 ffff88066cc5aca8
Call Trace:
[<ffffffff8118bfea>] __free_one_page+0x23a/0x450
[<ffffffff8118c586>] free_pcppages_bulk+0x136/0x360
[<ffffffff8118cae8>] free_hot_cold_page+0x168/0x1b0
[<ffffffff8118cd8c>] __free_pages+0x5c/0x90
[<ffffffff8260fbf2>] __free_pages_boot_core.isra.70+0x11a/0x14d
[<ffffffff8260ff09>] deferred_free_range+0x50/0x62
[<ffffffff8261013b>] deferred_init_memmap+0x220/0x3c3
[<ffffffff8260ff1b>] ? deferred_free_range+0x62/0x62
[<ffffffff8108afc8>] kthread+0xf8/0x110
[<ffffffff81c026b2>] ret_from_fork+0x22/0x40
[<ffffffff8108aed0>] ? kthread_create_on_node+0x200/0x200
Code: 75 7b 48 89 d8 8b 40 1c 85 c0 74 50 48 c7 c6 38 bd 0d 82 48 89 df
e8 25 e2 02 00 0f 0b 48 89 f7 89 55 ec e8 18 cb 07 00 8b 55 ec <48> 8b
00 a8 02 74 9d 3b 53 30 75 98 49 8b 14 24 48 8b 03 48 c1
RIP [<ffffffff8118934b>] page_is_buddy+0x7b/0xe0
RSP <ffff88017c1d7bf0>
CR2: 0000000000000000
---[ end trace e0c05a86b43d97f9 ]---
note: pgdatinit1[106] exited with preempt_count 1
I changed page_is_buddy and __free_one_page to non-inline to get more
accurate stack trace.
Then I did some investigation on it with printing the address of page
and buddy, please see the below log:
@@@@@@__free_one_page:715: page is at ffffea0005f05c00 buddy is at
ffffea0005f05c80, order is 1
@@@@@@__free_one_page:715: page is at ffffea0005f05c00 buddy is at
ffffea0005f05d00, order is 2
@@@@@@__free_one_page:715: page is at ffffea0005f05c00 buddy is at
ffffea0005f05e00, order is 3
@@@@@@__free_one_page:715: page is at ffffea0019810000 buddy is at
ffffea0019810040, order is 0
call trace splat
@@@@@@__free_one_page:715: page is at ffffea0005f05bc0 buddy is at
ffffea0005f05b80, order is 0
@@@@@@__free_one_page:715: page is at ffffea0005f05b80 buddy is at
ffffea0005f05bc0, order is 0
@@@@@@__free_one_page:715: page is at ffffea0005f05b80 buddy is at
ffffea0005f05b00, order is 1
@@@@@@__free_one_page:715: page is at ffffea0005f05b40 buddy is at
ffffea0005f05b00, order is 0
@@@@@@__free_one_page:715: page is at ffffea0005f05b00 buddy is at
ffffea0005f05b40, order is 0
It shows just before the call trace splat, the page address jumped to
ffffea0019810000 from ffffea0005f05xxx, not sure why this is happening.
Any hint is appreciated.
I think the page address didn't jump, it was that the previous call of
__free_one_page() finished at order=3 and now another one was invoked
that started at order=0 page ffffea0019810000 and immediately hit the
splat.
And, reading the code leads me to the below call path:
page_is_buddy()
--> page_is_guard()
--> lookup_page_ext()
From a quick decodecode I guess that's indeed the path that's crashed.
You could try verifying with e.g. addr2line on your vmlinux?
I tracked down there via gdb.
Then lookup_page_ext() just returns null due to the below code:
#if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PAGE_POISONING)
/*
* The sanity checks the page allocator does upon freeing a
* page can reach here before the page_ext arrays are
* allocated when feeding a range of pages to the allocator
* for the first time during bootup or memory hotplug.
*
* This check is also necessary for ensuring page poisoning
* works as expected when enabled
*/
if (!section->page_ext)
return NULL;
#endif
So, according to the comment, it looks there should be a WARN or BUG if
it returns NULL?
The comment reads like it's unlikely, but possible, so WARN/BUG doesn't
sounds right.
And, almost no codes check if the return pointer is
null or not after lookup_page_ext() is called.
Yep, the callers related to DEBUG_VM (includes page_is_guard()) and page
poisoning should probably check the return value. I hope Joonsoo knows
something more here :)
Also DEBUG_ALLOC since page_is_guard just returns false if DEBUG_ALLOC
is disabled.
So, page_is_guard should just return false if lookup_page_ext returns NULL?
Thanks,
Yang
Thanks,
Yang
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