On Fri, 23 Sep 2022, Mike Kravetz wrote: > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > 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. Thanks Mike. Yes, it looks like userfaultfd's successful returns from shmem_get_folio_gfp() have (correctly?) filled in vmf->page directly, and are not setting *foliop at all. So this would affect any userfaultfd usage of shmem, not just an odd syzbot corner case. I do think shmem_fault() ought at least to initialize its folio pointer (as shmem_file_read_iter() correctly does); and if it does that, then we don't need to weed out the userfaultfd cases as such, but just avoid folio_file_page() when folio is still NULL. I've neither tried the syzbot test case (but see its .c does involve userfaultld), nor tried userfaultfd shmem: Peter, could you give this a try - we'd expect userfaultfd shmem to crash on linux-next without the patch below. And it implies that nobody has been trying userfaultfd shmem on recent linux-next, so whether vmf->page is then set correctly we're not sure. But me, I cannot even get the latest linux-next to boot: next job. Hugh --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2060,7 +2060,7 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf) struct vm_area_struct *vma = vmf->vma; struct inode *inode = file_inode(vma->vm_file); gfp_t gfp = mapping_gfp_mask(inode->i_mapping); - struct folio *folio; + struct folio *folio = NULL; int err; vm_fault_t ret = VM_FAULT_LOCKED; @@ -2127,7 +2127,8 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf) gfp, vma, vmf, &ret); if (err) return vmf_error(err); - vmf->page = folio_file_page(folio, vmf->pgoff); + if (folio) + vmf->page = folio_file_page(folio, vmf->pgoff); return ret; }
