CVE-2026-46119: libceph: Fix slab-out-of-bounds access in auth message processing

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

libceph: Fix slab-out-of-bounds access in auth message processing

If a (potentially corrupted) message of type CEPHMSGAUTHREPLY contains a positive value in its result field, it is treated as an error code by cephhandleauthreply() and returned to handleauthreply(). Thereafter, an attempt is made to send the preallocated message of type CEPHMSGAUTH, where the returned value is interpreted as the size of the front segment to send. If the result value in the message is greater than the size of the memory buffer allocated for the front segment, an out-of-bounds access occurs, and the content of the memory region beyond this buffer is sent out.

This patch fixes the issue by treating only negative values in the result field as errors. Positive values are therefore treated as success in the same way as a zero value. Additionally, a BUGON is added to sendpreparedauthrequest() comparing the len parameter to frontalloclen to prevent sending the message if it exceeds the bounds of the allocation and to make it easier to catch any logic flaws leading to this.