Linux: smb/client: validate new EOF for insert range

In the Linux kernel, the following vulnerability has been resolved:

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