Re: general protection fault in khugepaged

[Hillf Danton <hdanton@sina.com>]

Wed, 08 Jul 2020 23:43:19 -0700
> syzbot found the following crash on:
> 
> HEAD commit:    e44f65fd xen-netfront: remove redundant assignment to vari..
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=15de54a7100000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=829871134ca5e230
> dashboard link: https://syzkaller.appspot.com/bug?extid=ed318e8b790ca72c5ad0
> compiler:       gcc (GCC) 10.1.0-syz 20200507
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=113406a7100000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=175597d3100000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+ed318e8b790ca72c5ad0@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: 1155 Comm: khugepaged Not tainted 5.8.0-rc2-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:anon_vma_lock_write include/linux/rmap.h:120 [inline]
> RIP: 0010:collapse_huge_page mm/khugepaged.c:1110 [inline]
> RIP: 0010:khugepaged_scan_pmd mm/khugepaged.c:1349 [inline]
> RIP: 0010:khugepaged_scan_mm_slot mm/khugepaged.c:2110 [inline]
> RIP: 0010:khugepaged_do_scan mm/khugepaged.c:2193 [inline]
> RIP: 0010:khugepaged+0x3bba/0x5a10 mm/khugepaged.c:2238
> Code: 01 00 00 48 8d bb 88 00 00 00 48 89 f8 48 c1 e8 03 42 80 3c 30 00 0f 85 fa 0f 00 00 48 8b 9b 88 00 00 00 48 89 d8 48 c1 e8 03 <42> 80 3c 30 00 0f 85 d4 0f 00 00 48 8b 3b 48 83 c7 08 e8 9f ff 30
> RSP: 0018:ffffc90004be7c80 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff81a72d8b
> RDX: ffff8880a69d8100 RSI: ffffffff81b7606b RDI: ffff88809f0577c0
> RBP: 0000000000000000 R08: 0000000000000000 R09: ffff8881ff213a7f
> R10: 0000000000000080 R11: 0000000000000000 R12: ffffffff8aae6110
> R13: ffffc90004be7de0 R14: dffffc0000000000 R15: 0000000020000000
> FS:  0000000000000000(0000) GS:ffff8880ae700000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 00000001fe0cf000 CR4: 00000000001406e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  kthread+0x3b5/0x4a0 kernel/kthread.c:291
>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")

--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -435,7 +435,8 @@ static inline int khugepaged_test_exit(s
 }
 
 static bool hugepage_vma_check(struct vm_area_struct *vma,
-			       unsigned long vm_flags)
+			       unsigned long vm_flags,
+			       bool anon)
 {
 	if ((!(vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
 	    (vm_flags & VM_NOHUGEPAGE) ||
@@ -446,6 +447,8 @@ static bool hugepage_vma_check(struct vm
 	    (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
 	     vma->vm_file &&
 	     (vm_flags & VM_DENYWRITE))) {
+		if (anon)
+			return false;
 		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
 				HPAGE_PMD_NR);
 	}
@@ -499,7 +502,7 @@ int khugepaged_enter_vma_merge(struct vm
 	 * khugepaged does not yet work on special mappings. And
 	 * file-private shmem THP is not supported.
 	 */
-	if (!hugepage_vma_check(vma, vm_flags))
+	if (!hugepage_vma_check(vma, vm_flags, false))
 		return 0;
 
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
@@ -956,7 +959,7 @@ static int hugepage_vma_revalidate(struc
 	hend = vma->vm_end & HPAGE_PMD_MASK;
 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
 		return SCAN_ADDRESS_RANGE;
-	if (!hugepage_vma_check(vma, vma->vm_flags))
+	if (!hugepage_vma_check(vma, vma->vm_flags, true))
 		return SCAN_VMA_CHECK;
 	return 0;
 }
@@ -1426,7 +1429,7 @@ void collapse_pte_mapped_thp(struct mm_s
 	 * the valid THP. Add extra VM_HUGEPAGE so hugepage_vma_check()
 	 * will not fail the vma for missing VM_HUGEPAGE
 	 */
-	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
+	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE, false))
 		return;
 
 	pmd = mm_find_pmd(mm, haddr);
@@ -2071,7 +2074,7 @@ static unsigned int khugepaged_scan_mm_s
 			progress++;
 			break;
 		}
-		if (!hugepage_vma_check(vma, vma->vm_flags)) {
+		if (!hugepage_vma_check(vma, vma->vm_flags, false)) {
 skip:
 			progress++;
 			continue;