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Date: Wed, 18 May 2016 11:41:35 -0700 From: "Shi, Yang" <yang.shi@...aro.org> To: Vlastimil Babka <vbabka@...e.cz>, LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org, Joonsoo Kim <iamjoonsoo.kim@....com> Subject: Re: [BUG] Null pointer dereference when freeing pages on 4.6-rc6 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 >> >> -- >> To unsubscribe, send a message with 'unsubscribe linux-mm' in >> the body to majordomo@...ck.org. For more info on Linux MM, >> see: http://www.linux-mm.org/ . >> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> >> >
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