Re: [syzbot] general protection fault in PageHeadHuge

[Mike Kravetz <mike.kravetz@oracle.com>]

On 09/23/22 09:05, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    483fed3b5dc8 Add linux-next specific files for 20220921
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=16f0a418880000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=849cb9f70f15b1ba
> dashboard link: https://syzkaller.appspot.com/bug?extid=152d76c44ba142f8992b
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12b97b64880000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11fb9040880000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/1cb3f4618323/disk-483fed3b.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/cc02cb30b495/vmlinux-483fed3b.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+152d76c44ba142f8992b@syzkaller.appspotmail.com
> 
> general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> CPU: 1 PID: 3617 Comm: syz-executor722 Not tainted 6.0.0-rc6-next-20220921-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2022
> RIP: 0010:PagePoisoned include/linux/page-flags.h:304 [inline]
> RIP: 0010:PageHead include/linux/page-flags.h:787 [inline]
> RIP: 0010:PageHeadHuge+0x1d/0x200 mm/hugetlb.c:1892
> Code: ff 66 66 2e 0f 1f 84 00 00 00 00 00 90 41 54 55 48 89 fd 53 e8 54 c9 b9 ff 48 89 ea 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 0f 85 a2 01 00 00 48 8b 5d 00 48 c7 c7 ff ff ff ff 48
> RSP: 0018:ffffc90003e7f5a0 EFLAGS: 00010246
> RAX: dffffc0000000000 RBX: ffffc90003e7f788 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffffffff81c2cb2c RDI: 0000000000000000
> RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000001 R12: ffffc90003e7f798
> R13: 0000000000000000 R14: 0000000000000000 R15: 00000000000003f4
> FS:  00007f5642262700(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000203f4ef0 CR3: 000000007adcc000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  folio_test_hugetlb include/linux/page-flags.h:831 [inline]
>  folio_file_page include/linux/pagemap.h:683 [inline]
>  shmem_fault+0x27c/0x8a0 mm/shmem.c:2130
>  __do_fault+0x107/0x600 mm/memory.c:4191
>  do_shared_fault mm/memory.c:4597 [inline]
>  do_fault mm/memory.c:4675 [inline]
>  handle_pte_fault mm/memory.c:4943 [inline]
>  __handle_mm_fault+0x2200/0x3a40 mm/memory.c:5085
>  handle_mm_fault+0x1c8/0x780 mm/memory.c:5206
>  do_user_addr_fault+0x475/0x1210 arch/x86/mm/fault.c:1428
>  handle_page_fault arch/x86/mm/fault.c:1519 [inline]
>  exc_page_fault+0x94/0x170 arch/x86/mm/fault.c:1575
>  asm_exc_page_fault+0x22/0x30 arch/x86/include/asm/idtentry.h:570
> RIP: 0010:__put_user_nocheck_4+0x3/0x11
> Code: 00 00 48 39 d9 73 54 0f 01 cb 66 89 01 31 c9 0f 01 ca c3 0f 1f 44 00 00 48 bb fd ef ff ff ff 7f 00 00 48 39 d9 73 34 0f 01 cb <89> 01 31 c9 0f 01 ca c3 66 0f 1f 44 00 00 48 bb f9 ef ff ff ff 7f
> RSP: 0018:ffffc90003e7fa00 EFLAGS: 00050293
> RAX: 0000000000000000 RBX: ffffc90003e7fdf4 RCX: 00000000203f4ef0
> RDX: ffff888020c51d40 RSI: ffffffff8726d52f RDI: 0000000000000005
> RBP: ffffc90003e7fdb0 R08: 0000000000000005 R09: 0000000000000000
> R10: 0000000000000002 R11: 0000000000000001 R12: 0000000000000000
> R13: 0000000000000002 R14: 00000000203f4ef0 R15: 0000000000000000
>  ____sys_recvmsg+0x3ba/0x610 net/socket.c:2714
>  ___sys_recvmsg+0xf2/0x180 net/socket.c:2743
>  do_recvmmsg+0x25e/0x6e0 net/socket.c:2837
>  __sys_recvmmsg net/socket.c:2916 [inline]
>  __do_sys_recvmmsg net/socket.c:2939 [inline]
>  __se_sys_recvmmsg net/socket.c:2932 [inline]
>  __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2932
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f56422dabb9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 91 18 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f5642262208 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
> RAX: ffffffffffffffda RBX: 00007f564235c4b8 RCX: 00007f56422dabb9
> RDX: 0000000000010106 RSI: 00000000200000c0 RDI: 0000000000000003
> RBP: 00007f564235c4b0 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000002 R11: 0000000000000246 R12: 00007f564235c4bc
> R13: 00007fffbde3618f R14: 00007f5642262300 R15: 0000000000022000
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---

While it is true that the addressing exception is happening in the hugetlb
routine PageHeadHuge(), the reason is because it is passed a NULL page
pointer.  This is via the call to folio_file_page() at the end of shmem_fault.

	err = shmem_get_folio_gfp(inode, vmf->pgoff, &folio, SGP_CACHE,
				  gfp, vma, vmf, &ret);
	if (err)
		return vmf_error(err);

	vmf->page = folio_file_page(folio, vmf->pgoff);
	return ret;

The code assumes that if a non-zero value is returned from shmem_get_folio_gfp,
then folio pointer will be set to a folio.  However, it looks like there are
a few places in shmem_get_folio_gfp where it will return zero and not set
folio.

In this specific case, it is the code block:

        if (vma && userfaultfd_missing(vma)) {
                *fault_type = handle_userfault(vmf, VM_UFFD_MISSING);
                return 0;
        }

I could try to sort this out, but I believe Matthew has the most context as
he has been changing this code recently.
-- 
Mike Kravetz