Re: general protection fault in khugepaged

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Jul 22, 2020 at 11:53:11PM +0000, Song Liu wrote:
> 
> 
> > On Jul 20, 2020, at 2:08 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> > 
> > On Sat, Jul 18, 2020 at 04:54:09PM +0800, Hillf Danton wrote:
> >> 
> >> 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://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_log.txt-3Fx-3D15de54a7100000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=MQfhDjE9yj2_juw34scN2XTASkKaxHCfKtrY5UlwTx4&s=6MfBZJE4yUynBic_FV4udh1WdM1wXz8vdcsOfuR23y0&e= 
> >>> kernel config:  https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_.config-3Fx-3D829871134ca5e230&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=MQfhDjE9yj2_juw34scN2XTASkKaxHCfKtrY5UlwTx4&s=KvhWD3PT9mC8iTIUtQGrMj2UJpBMlsmJOv_wx1kxkQQ&e= 
> >>> dashboard link: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_bug-3Fextid-3Ded318e8b790ca72c5ad0&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=MQfhDjE9yj2_juw34scN2XTASkKaxHCfKtrY5UlwTx4&s=sLoGzpV5UcRyP33XaPaGp_NoxP6VSXh4jttnxne6n6I&e= 
> >>> compiler:       gcc (GCC) 10.1.0-syz 20200507
> >>> syz repro:      https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_repro.syz-3Fx-3D113406a7100000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=MQfhDjE9yj2_juw34scN2XTASkKaxHCfKtrY5UlwTx4&s=HXYlOJ8r_Xv84UDkW6MSoEiG122ATfKJdhQlv3J7mao&e= 
> >>> C reproducer:   https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_repro.c-3Fx-3D175597d3100000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=MQfhDjE9yj2_juw34scN2XTASkKaxHCfKtrY5UlwTx4&s=c5Vlm43NNztzFCNb0S6s8IKJuVUtEtm0yWwfV55CeAo&e= 
> >>> 
> >>> 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")
> > 
> > It looks sane, but I think it's easier to check in
> > hugepage_vma_revalidate(). Something like this (untested):
> > 
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index b043c40a21d4..700f5160f3e4 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -958,6 +958,9 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> > 		return SCAN_ADDRESS_RANGE;
> > 	if (!hugepage_vma_check(vma, vma->vm_flags))
> > 		return SCAN_VMA_CHECK;
> > +	/* Anon VMA expected */
> > +	if (!vma->anon_vma || vma->vm_ops)
> > +		return SCAN_VMA_CHECK;
> > 	return 0;
> > }
> 
> Maybe move the new check to earlier in the function, say right after 
> we get the vma? 

Maybe. But we only protect against the race and it's not going to make a
difference for performance.

-- 
 Kirill A. Shutemov