Alexandre Ghiti <alexghiti@rivosinc.com> says:

Those 2 patches fix the set_memory_XX() and set_direct_map_XX() APIs, which
in turn fix STRICT_KERNEL_RWX and memfd_secret(). Those were broken since the
permission changes were not applied to the linear mapping because the linear
mapping is mapped using hugepages and walk_page_range_novma() does not split
such mappings.

To fix that, patch 1 disables PGD mappings in the linear mapping as it is
hard to propagate changes at this level in *all* the page tables, this has the
downside of disabling PMD mapping for sv32 and PUD (1GB) mapping for sv39 in
the linear mapping (for specific kernels, we could add a Kconfig to enable
ARCH_HAS_SET_DIRECT_MAP and STRICT_KERNEL_RWX if needed, I'm pretty sure we'll
discuss that).

patch 2 implements the split of the huge linear mappings so that
walk_page_range_novma() can properly apply the permissions. The whole split is
protected with mmap_sem in write mode, but I'm wondering if that's enough,
any opinion on that is appreciated.

* b4-shazam-merge:
  riscv: Fix set_memory_XX() and set_direct_map_XX() by splitting huge linear mappings
  riscv: Don't use PGD entries for the linear mapping

Link: https://lore.kernel.org/r/20231108075930.7157-1-alexghiti@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>