smb: client: fix off-by-8 bounds check in check_wsl_eas()

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

smb: client: fix off-by-8 bounds check in check_wsl_eas()

The bounds check uses (u8 *)ea + nlen + 1 + vlen as the end of the EA

name and value, but ea_data sits at offset sizeof(struct

smb2_file_full_ea_info) = 8 from ea, not at offset 0. The strncmp()

later reads ea->ea_data[0..nlen-1] and the value bytes follow at

ea_data[nlen+1..nlen+vlen], so the actual end is ea->ea_data + nlen + 1

+ vlen. Isn't pointer math fun?

The earlier check (u8 *)ea > end - sizeof(*ea) only guarantees the

8-byte header is in bounds, but since the last EA is placed within 8

bytes of the end of the response, the name and value bytes are read past

the end of iov.

Fix this mess all up by using ea->ea_data as the base for the bounds

check.

An "untrusted" server can use this to leak up to 8 bytes of kernel heap

into the EA name comparison and influence which WSL xattr the data is

interpreted as.