CVE-2025-38691

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

ext_tree_prepare_commit() reallocates a larger buffer to retry encoding

extents, the "layoutupdate_pages" page array is initialized only after the

retry loop. But ext_tree_free_commitdata() 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

buffer_size. 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 ext_tree_encode_commit()

does not help much. Since this number increases even more between

unlocking and locking of ext_tree, the reallocated buffer may not be

large enough again and again.