CVE-2026-52957

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

libceph: Fix potential null-ptr-deref in decodechooseargs()

A message of type CEPHMSGOSDMAP contains an OSD map that itself contains a CRUSH map. When decoding this CRUSH map in crushdecode(), an array of maxbuckets CRUSH buckets is decoded, where some indices may not refer to actual buckets and are therefore set to NULL. The received CRUSH map may optionally contain chooseargs that get decoded in decodechooseargs(). When decoding a crushchooseargmap, a series of chooseargs for different buckets is decoded, with the bucketindex being read from the incoming message. It is only checked that the bucket index does not exceed maxbuckets, but not that it doesn't point to an index with a NULL bucket. If a (potentially corrupted) message contains a crushchooseargmap including such a bucketindex, a null pointer dereference may occur in the subsequent processing when attempting to access the bucket with the given index.

This patch fixes the issue by extending the affected check. Now, it is only attempted to access the bucket if it is not NULL.