Re: ppc elf_map breakage with MAP_FIXED_NOREPLACE

[Michal Hocko]

On Mon 29-01-18 11:02:09, Anshuman Khandual wrote:
> On 01/29/2018 08:17 AM, Anshuman Khandual wrote:
> > On 01/26/2018 07:34 PM, Michal Hocko wrote:
> >> On Fri 26-01-18 18:04:27, Anshuman Khandual wrote:
> >> [...]
> >>> I tried to instrument mmap_region() for a single instance of 'sed'
> >>> binary and traced all it's VMA creation. But there is no trace when
> >>> that 'anon' VMA got created which suddenly shows up during subsequent
> >>> elf_map() call eventually failing it. Please note that the following
> >>> VMA was never created through call into map_region() in the process
> >>> which is strange.
> >>
> >> Could you share your debugging patch?
> > 
> > Please find the debug patch at the end.
> > 
> >>
> >>> =================================================================
> >>> [    9.076867] Details for VMA[3] c000001fce42b7c0
> >>> [    9.076925] vma c000001fce42b7c0 start 0000000010030000 end 0000000010040000
> >>> next c000001fce42b580 prev c000001fce42b880 mm c000001fce40fa00
> >>> prot 8000000000000104 anon_vma           (null) vm_ops           (null)
> >>> pgoff 1003 file           (null) private_data           (null)
> >>> flags: 0x100073(read|write|mayread|maywrite|mayexec|account)
> >>> =================================================================
> >>
> >> Isn't this vdso or some other special mapping? It is not really an
> >> anonymous vma. Please hook into __install_special_mapping
> > 
> > Yeah, will do. Its not an anon mapping as it does not have a anon_vma
> > structure ?
> 
> Okay, this colliding VMA seems to be getting loaded from load_elf_binary()
> function as well.
> 
> [    9.422410] vma c000001fceedbc40 start 0000000010030000 end 0000000010040000
> next c000001fceedbe80 prev c000001fceedb700 mm c000001fceea8200
> prot 8000000000000104 anon_vma           (null) vm_ops           (null)
> pgoff 1003 file           (null) private_data           (null)
> flags: 0x100073(read|write|mayread|maywrite|mayexec|account)
> [    9.422576] CPU: 46 PID: 7457 Comm: sed Not tainted 4.14.0-dirty #158
> [    9.422610] Call Trace:
> [    9.422623] [c000001fdc4f79b0] [c000000000b17ac0] dump_stack+0xb0/0xf0 (unreliable)
> [    9.422670] [c000001fdc4f79f0] [c0000000002dafb8] do_brk_flags+0x2d8/0x440
> [    9.422708] [c000001fdc4f7ac0] [c0000000002db3d0] vm_brk_flags+0x80/0x130
> [    9.422747] [c000001fdc4f7b20] [c0000000003d23a4] set_brk+0x80/0xdc
> [    9.422785] [c000001fdc4f7b60] [c0000000003d1f24] load_elf_binary+0x1304/0x158c
> [    9.422830] [c000001fdc4f7c80] [c00000000035d3e0] search_binary_handler+0xd0/0x270
> [    9.422881] [c000001fdc4f7d10] [c00000000035f338] do_execveat_common.isra.31+0x658/0x890
> [    9.422926] [c000001fdc4f7df0] [c00000000035f980] SyS_execve+0x40/0x50
> [    9.423588] [c000001fdc4f7e30] [c00000000000b220] system_call+0x58/0x6c
> 
> which is getting hit after adding some more debug.

Voila! So your binary simply overrides brk by elf segments. That sounds
like the exactly the thing that the patch is supposed to protect from.
Why this is the case I dunno. It is just clear that either brk or
elf base are not put to the proper place. Something to get fixed. You
are probably just lucky that brk allocations do not spil over to elf
mappings.

> @@ -2949,6 +2997,13 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
>         if (flags & VM_LOCKED)
>                 mm->locked_vm += (len >> PAGE_SHIFT);
>         vma->vm_flags |= VM_SOFTDIRTY;
> +
> +       if (!strcmp(current->comm, "sed")) {
> +               if (just_init && (mm_ptr == vma->vm_mm)) {
> +                       dump_vma(vma);
> +                       dump_stack();
> +               }
> +       }
>         return 0;
>  }
> 
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-- 
Michal Hocko
SUSE Labs