CVE-2024-47674

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

mm: avoid leaving partial pfn mappings around in error case

As Jann points out, PFN mappings are special, because unlike normal

memory mappings, there is no lifetime information associated with the

mapping - it is just a raw mapping of PFNs with no reference counting of

a 'struct page'.

That's all very much intentional, but it does mean that it's easy to

mess up the cleanup in case of errors. Yes, a failed mmap() will always

eventually clean up any partial mappings, but without any explicit

lifetime in the page table mapping itself, it's very easy to do the

error handling in the wrong order.

In particular, it's easy to mistakenly free the physical backing store

before the page tables are actually cleaned up and (temporarily) have

stale dangling PTE entries.

To make this situation less error-prone, just make sure that any partial

pfn mapping is torn down early, before any other error handling.