Re: [syzbot] [mm?] WARNING: suspicious RCU usage in mas_walk (2)

[Matthew Wilcox]

On Tue, Jul 25, 2023 at 11:57:22PM -0700, syzbot wrote:
> syzbot has bisected this issue to:

FWIW, this is unrelated to the previous issue.  It's the caller of
mas_walk() that has violated the locking constraints, and mas_walk() is
simply reporting the issue.  Is there a way to get syzbot to understand
that it needs to unwind the call-stack further to decide who to blame?

> commit a52f58b34afe095ebc5823684eb264404dad6f7b
> Author: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Date:   Mon Jul 24 18:54:10 2023 +0000
> 
>     mm: handle faults that merely update the accessed bit under the VMA lock
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1783585ea80000
> start commit:   [unknown] 
> git tree:       linux-next
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=1443585ea80000
> console output: https://syzkaller.appspot.com/x/log.txt?x=1043585ea80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f481ab36ce878b84
> dashboard link: https://syzkaller.appspot.com/bug?extid=8645fe63c4d22c8d27b8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1697cec9a80000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1566986ea80000

#syz test

diff --git a/mm/memory.c b/mm/memory.c
index 20a2e9ed4aeb..57b271108bdc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3567,6 +3567,12 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		return 0;
 	}
 copy:
+	if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma->anon_vma) {
+		pte_unmap_unlock(vmf->pte, vmf->ptl);
+		vma_end_read(vmf->vma);
+		return VM_FAULT_RETRY;
+	}
+
 	/*
 	 * Ok, we need to copy. Oh, well..
 	 */