Subject: Re: general protection fault in __handle_mm_fault

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 09 Oct 2018 14:23:23 +0530

Willem de Bruijn <[email-masked]> writes:

> On Mon, Oct 8, 2018 at 12:10 PM Willem de Bruijn
> <[email-masked]> wrote:
>>
>> On Fri, Oct 5, 2018 at 6:27 PM syzbot
>> <[email-masked]> wrote:
>> >
>> > Hello,
>> >
>> > syzbot found the following crash on:
>> >
>> > HEAD commit:    25bcda3e8b9f Add linux-next specific files for 20181004
>> > git tree:       linux-next
>> > console output: https://syzkaller.appspot.com/x/log.txt?x=130e3bf1400000
>> > kernel config:  https://syzkaller.appspot.com/x/.config?x=603d7f9140c3368a
>> > dashboard link: https://syzkaller.appspot.com/bug?extid=1577fbe983d20fe2e88f
>> > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=127e88d6400000
>> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13cdb67e400000
>>
>> > RIP: 0010:copy_user_enhanced_fast_string+0xe/0x20
>> > arch/x86/lib/copy_user_64.S:180
>> > Code: 89 d1 c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 31 c0 0f 1f 00 c3 0f 1f
>> > 80 00 00 00 00 0f 1f 00 83 fa 40 0f 82 70 ff ff ff 89 d1 <f3> a4 31 c0 0f
>> > 1f 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 83
>> > RSP: 0018:ffff8801bbe675b8 EFLAGS: 00010202
>> > RAX: 0000000000000000 RBX: 0000000000007a50 RCX: 0000000000001b40
>> > RDX: 0000000000007a50 RSI: 0000000020077000 RDI: ffff8801ce615f10
>> > RBP: ffff8801bbe675f0 R08: ffffed0039cc2f4a R09: ffffed0039cc2f4a
>> > R10: ffffed0039cc2f49 R11: ffff8801ce617a4f R12: 0000000020078b40
>> > R13: 00000000200710f0 R14: ffff8801ce610000 R15: 00007ffffffff000
>> >   _copy_from_iter_full+0x263/0xc20 lib/iov_iter.c:724
>> >   copy_from_iter_full include/linux/uio.h:124 [inline]
>> >   skb_do_copy_data_nocache include/net/sock.h:1951 [inline]
>> >   skb_copy_to_page_nocache include/net/sock.h:1977 [inline]
>> >   tcp_sendmsg_locked+0x159e/0x3f90 net/ipv4/tcp.c:1338
>>
>> This started on next-20181004. It still happens as of next-20181008.
>>
>> It does not trigger on next 20181003. It does not occur if
>> CONFIG_DEBUG_KOBJECT is disabled.
>
> Bisected to commit e4d0c281a4c9 ("mm/memory.c: recheck page table
> entry with page table lock held").
>
> Verified to not trigger on next-20181008 after reverting that commit.

Can you check with this patch

diff --git a/mm/memory.c b/mm/memory.c
index fa8894c70575..15c417e8e31d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3505,14 +3505,17 @@ static vm_fault_t do_fault(struct vm_fault *vmf)
 	 * The VMA was not fully populated on mmap() or missing VM_DONTEXPAND
 	 */
 	if (!vma->vm_ops->fault) {
-
 		/*
-		 * pmd entries won't be marked none during a R/M/W cycle.
+		 * If we find a migration pmd entry or a none pmd entry, which
+		 * should never happen, return SIGBUS
 		 */
-		if (unlikely(pmd_none(*vmf->pmd)))
+		if (unlikely(!pmd_present(*vmf->pmd)))
 			ret = VM_FAULT_SIGBUS;
 		else {
-			vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
+			vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm,
+						       vmf->pmd,
+						       vmf->address,
+						       &vmf->ptl);
 			/*
 			 * Make sure this is not a temporary clearing of pte
 			 * by holding ptl and checking again. A R/M/W update
@@ -3520,12 +3523,12 @@ static vm_fault_t do_fault(struct vm_fault *vmf)
 			 * we don't have concurrent modification by hardware
 			 * followed by an update.
 			 */
-			spin_lock(vmf->ptl);
 			if (unlikely(pte_none(*vmf->pte)))
 				ret = VM_FAULT_SIGBUS;
 			else
 				ret = VM_FAULT_NOPAGE;
-			spin_unlock(vmf->ptl);
+
+			pte_unmap_unlock(vmf->pte, vmf->ptl);
 		}
 	} else if (!(vmf->flags & FAULT_FLAG_WRITE))
 		ret = do_read_fault(vmf);