Re: a racy access flag clearing warning when calling mmap system call

[Will Deacon <will.deacon@arm.com>]

On Fri, Dec 01, 2017 at 03:38:04PM +0800, chenjiankang wrote:
>    I find a warning by a syzkaller test;
> 
>    When the mmap syscall is called to create a virtual memory,
> firstly it delete a old huge page mapping area;   
>    Before splitting the huge page, the pmd of a huge page is set up.
> But The PTE_AF is zreo belonging to the current pmd of huge page.
>    I suspect that when the child process is created, the PTE_AF is cleared in copy_one_pte();
> So, the set_pte_at() can have a warning.
>    
>    There, whether the set_pte_at() should detect PTE_AF?
> 
> Thanks.
> 
> The log:
> 
>     set_pte_at: racy access flag clearing: 00e000009d200bd1 -> 01e000009d200bd1
> ------------[ cut here ]------------      
> WARNING: at ../../../../../kernel/linux-4.1/arch/arm64/include/asm/pgtable.h:211

Given that this is a fairly old 4.1 kernel, could you try to reproduce the
failure with something more recent, please? We've fixed many bugs since
then, some of them involving huge pages.

Thanks,

Will

> Modules linked in:                         
> CPU: 0 PID: 3665 Comm: syz-executor7 Not tainted 4.1.44+ #1
> Hardware name: linux,dummy-virt (DT)       
> task: ffffffc06a873fc0 ti: ffffffc05aefc000 task.ti: ffffffc05aefc000
> PC is at pmdp_splitting_flush+0x194/0x1b0
> LR is at pmdp_splitting_flush+0x194/0x1b0
> pc : [<ffffffc0000a5244>] lr : [<ffffffc0000a5244>] pstate: 80000145
> sp : ffffffc05aeff770                     
> x29: ffffffc05aeff770 x28: ffffffc05ae45800 
> x27: 0000000020200000 x26: ffffffc061fdf450 
> x25: 0000000000020000 x24: 0000000000000001 
> x23: ffffffc06333d9c8 x22: ffffffc0014ba000 
> x21: 01e000009d200bd1 x20: 00e000009d200bd1 
> x19: ffffffc05ae45800 x18: 0000000000000000 
> x17: 00000000004b4000 x16: ffffffc00017fdc0 
> x15: 00000000038ee280 x14: 3030653130203e2d 
> x13: 2031646230303264 x12: 3930303030306530 
> x11: 30203a676e697261 x10: 656c632067616c66 
> x9 : 2073736563636120 x8 : 79636172203a7461 
> x7 : ffffffc05aeff430 x6 : ffffffc00015f38c 
> x5 : 0000000000000003 x4 : 0000000000000000 
> x3 : 0000000000000003 x2 : 0000000000010000 
> x1 : ffffff9005a03000 x0 : 000000000000004b 
> Call trace:                                
> [<ffffffc0000a5244>] pmdp_splitting_flush+0x194/0x1b0
> [<ffffffc0002784c0>] split_huge_page_to_list+0x168/0xdb0
> [<ffffffc00027a0d8>] __split_huge_page_pmd+0x1b0/0x510
> [<ffffffc00027b5ac>] split_huge_page_pmd_mm+0x84/0x88
> [<ffffffc00027b674>] split_huge_page_address+0xc4/0xe8
> [<ffffffc00027b7f4>] __vma_adjust_trans_huge+0x15c/0x190
> [<ffffffc000238d5c>] vma_adjust+0x884/0x9f0
> [<ffffffc0002390c8>] __split_vma.isra.5+0x200/0x310
> [<ffffffc00023b7b8>] do_munmap+0x5e0/0x608 
> [<ffffffc00023c0d4>] mmap_region+0x12c/0x900
> [<ffffffc00023cd2c>] do_mmap_pgoff+0x484/0x540
> [<ffffffc000218118>] vm_mmap_pgoff+0x128/0x158
> [<ffffffc000239920>] SyS_mmap_pgoff+0x188/0x300
> [<ffffffc00008cce0>] sys_mmap+0x58/0x80   
>