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Message-ID: <05a91777-3b95-14a9-c959-a12b25a9b26f@arm.com>
Date: Wed, 13 Feb 2019 14:38:37 +0000
From: Robin Murphy <robin.murphy@....com>
To: Michal Hocko <mhocko@...nel.org>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Fix __dump_page() for poisoned pages
On 13/02/2019 14:23, Michal Hocko wrote:
> On Wed 13-02-19 13:40:49, Robin Murphy wrote:
>> Evaluating page_mapping() on a poisoned page ends up dereferencing junk
>> and making PF_POISONED_CHECK() considerably crashier than intended. Fix
>> that by not inspecting the mapping until we've determined that it's
>> likely to be valid.
>
> Has this ever triggered? I am mainly asking because there is no usage of
> mapping so I would expect that the compiler wouldn't really call
> page_mapping until it is really used.
A function call is a sequence point, so any compiler that did that would
be totally broken.
The crash looks like this (now from an explicit dump_page() call before
it happens naturally deep within pfn_to_nid()):
-----
[ 107.147056] Unable to handle kernel NULL pointer dereference at
virtual address 0000000000000006
[ 107.155774] Mem abort info:
[ 107.158546] ESR = 0x96000005
[ 107.161572] Exception class = DABT (current EL), IL = 32 bits
[ 107.167437] SET = 0, FnV = 0
[ 107.170460] EA = 0, S1PTW = 0
[ 107.173568] Data abort info:
[ 107.176419] ISV = 0, ISS = 0x00000005
[ 107.180218] CM = 0, WnR = 0
[ 107.183151] user pgtable: 4k pages, 39-bit VAs, pgdp = 00000000c2f6ac38
[ 107.189702] [0000000000000006] pgd=0000000000000000, pud=0000000000000000
[ 107.196430] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[ 107.201942] Modules linked in:
[ 107.204962] CPU: 2 PID: 491 Comm: bash Not tainted 5.0.0-rc1+ #1
[ 107.210903] Hardware name: ARM LTD ARM Juno Development Platform/ARM
Juno Development Platform, BIOS EDK II Dec 17 2018
[ 107.221576] pstate: 00000005 (nzcv daif -PAN -UAO)
[ 107.226321] pc : page_mapping+0x18/0x118
[ 107.230200] lr : __dump_page+0x1c/0x398
[ 107.233990] sp : ffffff8011a53c30
[ 107.237265] x29: ffffff8011a53c30 x28: ffffffc039b6ec00
[ 107.242520] x27: 0000000000000000 x26: 0000000000000000
[ 107.247775] x25: 0000000056000000 x24: 0000000000000015
[ 107.253029] x23: ffffff80114d8b18 x22: 0000000000000022
[ 107.258283] x21: ffffffc03538ec38 x20: ffffff8011082e78
[ 107.263537] x19: ffffffbf20000000 x18: 0000000000000000
[ 107.268790] x17: 0000000000000000 x16: 0000000000000000
[ 107.274044] x15: 0000000000000000 x14: 0000000000000000
[ 107.279297] x13: 0000000000000000 x12: 0000000000000030
[ 107.284550] x11: 0000000000000030 x10: 0101010101010101
[ 107.289804] x9 : ff7274615e68726c x8 : 7f7f7f7f7f7f7f7f
[ 107.295057] x7 : feff64756e6c6471 x6 : 0000000000008080
[ 107.300310] x5 : 0000000000000000 x4 : 0000000000000000
[ 107.305564] x3 : ffffffc039b6ec00 x2 : fffffffffffffffe
[ 107.310817] x1 : ffffffffffffffff x0 : fffffffffffffffe
[ 107.316072] Process bash (pid: 491, stack limit = 0x000000004ebd4ecd)
[ 107.322442] Call trace:
[ 107.324858] page_mapping+0x18/0x118
[ 107.328392] __dump_page+0x1c/0x398
[ 107.331840] dump_page+0xc/0x18
[ 107.334945] remove_store+0xbc/0x120
[ 107.338479] dev_attr_store+0x18/0x28
[ 107.342103] sysfs_kf_write+0x40/0x50
[ 107.345722] kernfs_fop_write+0x130/0x1d8
[ 107.349687] __vfs_write+0x30/0x180
[ 107.353134] vfs_write+0xb4/0x1a0
[ 107.356410] ksys_write+0x60/0xd0
[ 107.359686] __arm64_sys_write+0x18/0x20
[ 107.363565] el0_svc_common+0x94/0xf8
[ 107.367184] el0_svc_handler+0x68/0x70
[ 107.370890] el0_svc+0x8/0xc
[ 107.373737] Code: f9400401 d1000422 f240003f 9a801040 (f9400402)
[ 107.379766] ---[ end trace cdb5eb5bf435cecb ]---
-----
While after this patch, DEBUG_VM works as intended:
-----
[ 46.835963] page:ffffffbf20000000 is uninitialized and poisoned
[ 46.835970] raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff
ffffffffffffffff
[ 46.849520] raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff
ffffffffffffffff
[ 46.857194] page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
[ 46.863170] ------------[ cut here ]------------
[ 46.867736] kernel BUG at ./include/linux/mm.h:1006!
[ 46.872646] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[ 46.878071] Modules linked in:
[ 46.881092] CPU: 1 PID: 483 Comm: bash Not tainted 5.0.0-rc1+ #3
[ 46.887032] Hardware name: ARM LTD ARM Juno Development Platform/ARM
Juno Development Platform, BIOS EDK II Dec 17 2018
[ 46.897704] pstate: 40000005 (nZcv daif -PAN -UAO)
[ 46.902449] pc : remove_store+0xbc/0x120
...
-----
Robin.
>> Fixes: 1c6fb1d89e73 ("mm: print more information about mapping in __dump_page")
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
>> mm/debug.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/debug.c b/mm/debug.c
>> index 0abb987dad9b..1611cf00a137 100644
>> --- a/mm/debug.c
>> +++ b/mm/debug.c
>> @@ -44,7 +44,7 @@ const struct trace_print_flags vmaflag_names[] = {
>>
>> void __dump_page(struct page *page, const char *reason)
>> {
>> - struct address_space *mapping = page_mapping(page);
>> + struct address_space *mapping;
>> bool page_poisoned = PagePoisoned(page);
>> int mapcount;
>>
>> @@ -58,6 +58,8 @@ void __dump_page(struct page *page, const char *reason)
>> goto hex_only;
>> }
>>
>> + mapping = page_mapping(page);
>> +
>> /*
>> * Avoid VM_BUG_ON() in page_mapcount().
>> * page->_mapcount space in struct page is used by sl[aou]b pages to
>> --
>> 2.20.1.dirty
>>
>
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