Re: [PATCH] mm/page_alloc: fix a crash in free_pages_prepare()

[Andrew Morton]

On Fri, 27 Sep 2019 15:47:03 -0400 Qian Cai <cai@xxxxxx> 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?

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?