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SubjectRe: [PATCH] mm/page_alloc: fix a crash in free_pages_prepare()
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On Fri, 2019-09-27 at 14:02 -0700, Andrew Morton wrote:
> On Fri, 27 Sep 2019 15:47:03 -0400 Qian Cai <cai@lca.pw> wrote:
>
> > On architectures like s390, arch_free_page() could mark the page unused
> > (set_page_unused()) and any access later would trigger a kernel panic.
> > Fix it by moving arch_free_page() after all possible accessing calls.
> >
> > Hardware name: IBM 2964 N96 400 (z/VM 6.4.0)
> > Krnl PSW : 0404e00180000000 0000000026c2b96e
> > (__free_pages_ok+0x34e/0x5d8)
> > R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
> > Krnl GPRS: 0000000088d43af7 0000000000484000 000000000000007c
> > 000000000000000f
> > 000003d080012100 000003d080013fc0 0000000000000000
> > 0000000000100000
> > 00000000275cca48 0000000000000100 0000000000000008
> > 000003d080010000
> > 00000000000001d0 000003d000000000 0000000026c2b78a
> > 000000002717fdb0
> > Krnl Code: 0000000026c2b95c: ec1100b30659 risbgn %r1,%r1,0,179,6
> > 0000000026c2b962: e32014000036 pfd 2,1024(%r1)
> > #0000000026c2b968: d7ff10001000 xc 0(256,%r1),0(%r1)
> > >0000000026c2b96e: 41101100 la %r1,256(%r1)
> > 0000000026c2b972: a737fff8 brctg %r3,26c2b962
> > 0000000026c2b976: d7ff10001000 xc 0(256,%r1),0(%r1)
> > 0000000026c2b97c: e31003400004 lg %r1,832
> > 0000000026c2b982: ebff1430016a asi 5168(%r1),-1
> > Call Trace:
> > __free_pages_ok+0x16a/0x5d8)
> > memblock_free_all+0x206/0x290
> > mem_init+0x58/0x120
> > start_kernel+0x2b0/0x570
> > startup_continue+0x6a/0xc0
> > INFO: lockdep is turned off.
> > Last Breaking-Event-Address:
> > __free_pages_ok+0x372/0x5d8
> > Kernel panic - not syncing: Fatal exception: panic_on_oops
> > 00: HCPGIR450W CP entered; disabled wait PSW 00020001 80000000 00000000
> > 26A2379C
> >
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1175,11 +1175,11 @@ static __always_inline bool free_pages_prepare(struct page *page,
> > debug_check_no_obj_freed(page_address(page),
> > PAGE_SIZE << order);
> > }
> > - arch_free_page(page, order);
> > if (want_init_on_free())
> > kernel_init_free_pages(page, 1 << order);
> >
> > kernel_poison_pages(page, 1 << order, 0);
> > + arch_free_page(page, order);
> > if (debug_pagealloc_enabled())
> > kernel_map_pages(page, 1 << order, 0);
>
> AFAICT the meticulously undocumented s390 set_page_unused() will cause
> there to be a fault if anyone tries to access the page contents, yes?

Yes.

>
> So I think you've moved the arch_free_page() to be after the final
> thing which can access page contents, yes? If so, we should have a
> comment in free_pages_prepare() to attmept to prevent this problem from
> reoccurring as the code evolves?

Right, something like this above arch_free_page() there?

/*
* It needs to be just above kernel_map_pages(), as s390 could mark those
* pages unused and then trigger a fault when accessing.
*/

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