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From:   Sasha Levin <sashal@kernel.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com,
        Amir Goldstein <amir73il@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Colin Walters <walters@verbum.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sasha Levin <sashal@kernel.org>, linux-mm@kvack.org
Subject: [PATCH AUTOSEL 5.8 62/62] hugetlbfs: prevent filesystem stacking of hugetlbfs
Date:   Fri, 21 Aug 2020 12:14:23 -0400
Message-Id: <20200821161423.347071-62-sashal@kernel.org>
In-Reply-To: <20200821161423.347071-1-sashal@kernel.org>
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From: Mike Kravetz <mike.kravetz@oracle.com>

[ Upstream commit 15568299b7d9988063afce60731df605ab236e2a ]

syzbot found issues with having hugetlbfs on a union/overlay as reported
in [1].  Due to the limitations (no write) and special functionality of
hugetlbfs, it does not work well in filesystem stacking.  There are no
know use cases for hugetlbfs stacking.  Rather than making modifications
to get hugetlbfs working in such environments, simply prevent stacking.

[1] https://lore.kernel.org/linux-mm/000000000000b4684e05a2968ca6@google.com/

Reported-by: syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com
Suggested-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Colin Walters <walters@verbum.org>
Link: http://lkml.kernel.org/r/80f869aa-810d-ef6c-8888-b46cee135907@oracle.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/hugetlbfs/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index ef5313f9c78fe..f936bcf02cce7 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -1364,6 +1364,12 @@ hugetlbfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_magic = HUGETLBFS_MAGIC;
 	sb->s_op = &hugetlbfs_ops;
 	sb->s_time_gran = 1;
+
+	/*
+	 * Due to the special and limited functionality of hugetlbfs, it does
+	 * not work well as a stacking filesystem.
+	 */
+	sb->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
 	sb->s_root = d_make_root(hugetlbfs_get_root(sb, ctx));
 	if (!sb->s_root)
 		goto out_free;
-- 
2.25.1