CVE-2025-38691: pNFS: Fix uninited ptr deref in block/scsi layout

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

pNFS: Fix uninited ptr deref in block/scsi layout

The error occurs on the third attempt to encode extents. When function exttreepreparecommit() reallocates a larger buffer to retry encoding extents, the "layoutupdatepages" page array is initialized only after the retry loop. But exttreefreecommitdata() is called on every iteration and tries to put pages in the array, thus dereferencing uninitialized pointers.

An additional problem is that there is no limit on the maximum possible buffersize. When there are too many extents, the client may create a layoutcommit that is larger than the maximum possible RPC size accepted by the server.

During testing, we observed two typical scenarios. First, one memory page for extents is enough when we work with small files, append data to the end of the file, or preallocate extents before writing. But when we fill a new large file without preallocating, the number of extents can be huge, and counting the number of written extents in exttreeencodecommit() does not help much. Since this number increases even more between unlocking and locking of exttree, the reallocated buffer may not be large enough again and again.