smb: client: validate absolute native symlink targets before NT fixups

With symlinkroot unset, an absolute target is copied without conversion
to an NT drive path. Later code still assumes an NT prefix is present
when modifying the target and calculating the print name length.

For "/ab", this causes two failures: sym[5] and path[5] are written
past their allocations, and plen -= 2 * poff subtracts an assumed
8-byte prefix from a 6-byte UTF-16 target, wrapping u16 plen to 65534.
That underflow causes another overflow: memcpy() copies 65534 bytes
into a 24-byte buffer. A user with write access to a mounted share
can trigger these bugs with default settings.

Validate the NT drive prefix, including an ASCII drive letter, before
accessing fixed offsets or subtracting the prefix length.

Fixes: 3363da82e02f ("smb: client: fix native SMB symlink traversal")
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>

0 files changed, 0 insertions, 0 deletions