Re: [syzbot] [mm?] kernel BUG in vma_replace_policy

[Matthew Wilcox]

On Tue, Sep 05, 2023 at 06:03:49PM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    a47fc304d2b6 Add linux-next specific files for 20230831
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=16502ddba80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6ecd2a74f20953b9
> dashboard link: https://syzkaller.appspot.com/bug?extid=b591856e0f0139f83023
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=120e7d70680000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1523f9c0680000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/b2e8f4217527/disk-a47fc304.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/ed6cdcc09339/vmlinux-a47fc304.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/bd9b2475bf5a/bzImage-a47fc304.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b591856e0f0139f83023@xxxxxxxxxxxxxxxxxxxxxxxxx

#syz test

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 42b5567e3773..90ad5fe60824 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1342,6 +1342,7 @@ static long do_mbind(unsigned long start, unsigned long len,
 	vma_iter_init(&vmi, mm, start);
 	prev = vma_prev(&vmi);
 	for_each_vma_range(vmi, vma, end) {
+		vma_start_write(vma);
 		err = mbind_range(&vmi, vma, &prev, start, end, new);
 		if (err)
 			break;