arm64: mm: Check for pud_/pmd_set_huge() failures on kernel mappings

arm64: mm: Check for pud_/pmd_set_huge() failures on kernel mappings

Sashiko reports:

| If pmd_set_huge() rejects an unsafe page table transition (such as
| mapping a different physical address over an existing block mapping),
| it returns 0 and leaves the page table entry unmodified.
|
| Because *pmdp remains unmodified, READ_ONCE(pmd_val(*pmdp)) will equal
| pmd_val(old_pmd). The transition from old_pmd to old_pmd is evaluated
| as safe by pgattr_change_is_safe(), so the BUG_ON never triggers.
|
| This allows invalid and unsafe mapping updates to be silently dropped
| instead of panicking, leaving stale memory mappings active while the
| caller assumes the update was successful.

The same applies to pud_set_huge() in alloc_init_pud().

Given how it is generally preferred to limp on rather than blow up the
system if an unexpected condition such as this one occurs, and the fact
that there are no known cases where this disparity results in real
problems, let's WARN on these failures rather than BUG, allowing the
system to survive to the point where it can actually report them.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>