[PATCH] Null check to prevent null-ptr-deref bug

[Prince Kumar Maurya]

sb_getblk(inode->i_sb, parent) return a null ptr and taking lock on
that leads to the null-ptr-deref bug.

Signed-off-by: Prince Kumar Maurya <princekumarmaurya06@xxxxxxxxx>
---
Change since v1: update the commit message.
The bug was reproducible using the reproducer code and assets found in
bug report:https://syzkaller.appspot.com/bug?extid=aad58150cbc64ba41bdc
I used qemu to reproduce the bug and after the code fix I rebooted the 
qemu with updated bzImage containing the fix.

qemu-system-x86_64 -m 4G -nographic -drive \
file=./asset/disk-4d6d4c7f.raw,format=raw \
-enable-kvm -net nic -net user,hostfwd=tcp::2222-:22

 fs/sysv/itree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/sysv/itree.c b/fs/sysv/itree.c
index b22764fe669c..3a6b66e719fd 100644
--- a/fs/sysv/itree.c
+++ b/fs/sysv/itree.c
@@ -145,6 +145,8 @@ static int alloc_branch(struct inode *inode,
 		 */
 		parent = block_to_cpu(SYSV_SB(inode->i_sb), branch[n-1].key);
 		bh = sb_getblk(inode->i_sb, parent);
+		if (!bh)
+			break;
 		lock_buffer(bh);
 		memset(bh->b_data, 0, blocksize);
 		branch[n].bh = bh;
-- 
2.40.1