[PATCH] f2fs: fix a dead loop in f2fs_fiemap() Content-Type:

[Colin King]

From: Colin Ian King <colin.king at canonical.com>

CVE-2017-18257

A dead loop can be triggered in f2fs_fiemap() using the test case
as below:

        ...
        fd = open();
        fallocate(fd, 0, 0, 4294967296);
        ioctl(fd, FS_IOC_FIEMAP, fiemap_buf);
        ...

It's caused by an overflow in __get_data_block():
        ...
        bh->b_size = map.m_len << inode->i_blkbits;
        ...
map.m_len is an unsigned int, and bh->b_size is a size_t which is 64 bits
on 64 bits archtecture, type conversion from an unsigned int to a size_t
will result in an overflow.

In the above-mentioned case, bh->b_size will be zero, and f2fs_fiemap()
will call get_data_block() at block 0 again an again.

Fix this by adding a force conversion before left shift.

Signed-off-by: Wei Fang <fangwei1 at huawei.com>
Acked-by: Chao Yu <yuchao0 at huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk at kernel.org>
(backported from upstream commit b86e33075ed1909d8002745b56ecf73b833db143)
Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 fs/f2fs/data.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index aa3438c..884bccc 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -455,7 +455,7 @@ static int get_data_block_ro(struct inode *inode, sector_t iblock,
 				!= (dn.data_blkaddr + i)) || maxblocks == i)
 				break;
 		map_bh(bh_result, inode->i_sb, dn.data_blkaddr);
-		bh_result->b_size = (i << blkbits);
+		bh_result->b_size = ((u64)i << blkbits);
 	}
 	f2fs_put_dnode(&dn);
 	trace_f2fs_get_data_block(inode, iblock, bh_result, 0);
-- 
1.9.1