smb/client: validate new EOF for insert range

smb3_insert_range() does not check if the new file size
(i_size + len) is valid. This allows FALLOC_FL_INSERT_RANGE to bypass
RLIMIT_FSIZE, exceed s_maxbytes, or produce a size outside the loff_t
range.

Use check_add_overflow() to calculate the new EOF. Validate it with
inode_newsize_ok() before modifying the file.

Reproducer, using a file on a CIFS mount:

	bash -c '
		FILE=/mnt/cifs/repro

		trap "" SIGXFSZ
		ulimit -f 3072		# RLIMIT_FSIZE = 3 MiB

		# A regular write is stopped at 3 MiB.
		dd if=/dev/zero of="$FILE" bs=1M count=4 status=none
		stat -c "size after write: %s" "$FILE"

		# Insert 2 MiB into a 2 MiB file.
		truncate -s 2M "$FILE"
		fallocate -i -o 0 -l 2M "$FILE"
		stat -c "size after insert: %s" "$FILE"
	'

Before this change, the regular write stops at the 3 MiB limit, but
insert range grows the file to 4 MiB:

	dd: error writing '/mnt/cifs/repro': File too large
	size after write: 3145728
	size after insert: 4194304

After this change, insert range also fails at the limit and leaves the
2 MiB file unchanged:

	dd: error writing '/mnt/cifs/repro': File too large
	size after write: 3145728
	fallocate: fallocate failed: File too large
	size after insert: 2097152

Fixes: 7fe6fe95b936 ("cifs: add FALLOC_FL_INSERT_RANGE support")
Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>

0 files changed, 0 insertions, 0 deletions