Re: [BUG] Null pointer dereference when freeing pages on 4.6-rc6

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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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