x86/mm: Ensure input to pfn_to_kaddr() is treated as a 64-bit type

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: x86/mm: Ensure input to pfn_to_kaddr() is treated as a 64-bit type On 64-bit platforms, the pfn_to_kaddr() macro requires that the input value is 64 bits in order to ensure that valid address bits don't get lost when shifting that input by PAGE_SHIFT to calculate the physical address to provide a virtual address for. One such example is in pvalidate_pages() (used by SEV-SNP guests), where the GFN in the struct used for page-state change requests is a 40-bit bit-field, so attempts to pass this GFN field directly into pfn_to_kaddr() ends up causing guest crashes when dealing with addresses above the 1TB range due to the above. Fix this issue with SEV-SNP guests, as well as any similar cases that might cause issues in current/future code, by using an inline function, instead of a macro, so that the input is implicitly cast to the expected 64-bit input type prior to performing the shift operation. While it might be argued that the issue is on the caller side, other archs/macros have taken similar approaches to deal with instances like this, such as ARM explicitly casting the input to phys_addr_t: e48866647b48 ("ARM: 8396/1: use phys_addr_t in pfn_to_kaddr()") A C inline function is even better though. [ mingo: Refined the changelog some more & added __always_inline. ]

References

• https://git.kernel.org/stable/c/325956b0173f11e98f90462be4829a8b8b0682ce
• https://git.kernel.org/stable/c/7e1471888a5e6e846e9b4d306e5327db2b58e64e
• https://git.kernel.org/stable/c/814305b5c23cb815ada68d43019f39050472b25f
• https://git.kernel.org/stable/c/8e5647a723c49d73b9f108a8bb38e8c29d3948ea