CVE-2026-99083: ksmbd: fix heap out-of-bounds write in krb5_authenticate()

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

ksmbd: fix heap out-of-bounds write in krb5authenticate()

In krb5authenticate(), outlen is calculated to determine available headroom in responsebuf for the incoming Kerberos AP-REP token. However, when SMB2SESSIONSETUP is processed as a non-first element of a compounded SMB2 request, the calculation omits work->nextsmb2rsphdroff.

This causes outlen to overstate remaining buffer headroom by the cumulative size of prior responses in the compound chain. Consequently, the length check in ksmbdkrb5authenticate() (outlen <= resp->spnegobloblen) passes erroneously, allowing memcpy() to write the AP-REP blob past the end of responsebuf into adjacent kernel heap memory.

Fix this by subtracting work->nextsmb2rsphdroff when computing outlen, ensuring it accurately reflects physical remaining buffer space.