From: Ard Biesheuvel <[email protected]> One of the reasons the lack of randomization of the linear map on arm64 is considered problematic is the fact that bootloaders adhering to the original arm64 boot protocol (i.e., a substantial fraction of all Android phones) may place the kernel at the base of DRAM, and therefore at the base of the non-randomized linear map. This puts a writable alias of the kernel's data and bss regions at a predictable location, removing the need for an attacker to guess where KASLR mapped the kernel.
Let's unmap this linear, writable alias entirely, so that knowing the location of the linear alias does not give write access to the kernel's data and bss regions. Changes since v5: - Reorder series in ascending order of impact, so that the first few can be merged earlier if desired. This also makes the patch that remaps the data/bss linear alias as tagged redundant, which is therefore dropped. - Add patch #3 to address an existing issue spotted by Sashiko - Fix thinko in contiguous region check (#5), where the whole region needs to be considered and not only the first entry (dropped Rb as well) - this addresses the kfence issue Sashiko reported on v5 [0] - Update commit log on #6 to clarify that changing permission bits on PTE_CONT entries is safe as long as PTE_CONT itself does not change - Likewise, drop hunk that adds the PTE_CONT bit to the 'permitted' mask in pgattr_change_is_safe(), as changing it is not safe. (#8) - Move kasan's additional page table to pgdir BSS as well - Use (NOLOAD) on the .pgdir.bss section so it does not get emitted into vmlinux - Add powerpc and SuperH patches to deal with empty_zero_page[] being made const Changes since v4: - Update the correct [early] mapping in patch #1 - Make empty_zero_page[] const instead of __ro_after_init - Drop patches that remap the fixmap page tables r/o for now - Don't force page mappings for the data/bss linear alias, as it is no longer needed for set_memory_valid() - Add acks Changes since v3: - Drop bogus patch adding hierarchical PXN to the fixmap mapping, which breaks the KPTI trampoline (thanks to Sashiko) - Add generic patch to move the empty_zero_page to __ro_after_init, as it now lives in generic code. - Add patches to remap the linear aliases of the fixmap page tables read-only too - these live at an a priori known offset in the linear map if physical KASLR was omitted, and control a priori known addresses in the virtual kernel space. - Rebase onto v7.1-rc1 Changes since v2: - Keep bm_pte[] in the region that is remapped r/o or unmapped, as it is only manipulated via its kernel alias - Drop check that prohibits any manipulation of descriptors with the CONT bit set - Add Ryan's ack to a couple of patches - Rebase onto v7.0-rc4 Changes since v1: - Put zero page patch at the start of the series - Tweak __map_memblock() API to respect existing table and contiguous mappings, so that the logic to map the kernel alias can be simplified - Stop abusing the MEMBLOCK_NOMAP flag to initially omit the kernel linear alias from the linear map - Some additional cleanup patches - Use proper API [set_memory_valid()] to (un)map the linear alias of data/bss. Cc: Ryan Roberts <[email protected]> Cc: Anshuman Khandual <[email protected]> Cc; Kevin Brodsky <[email protected]> Cc: Liz Prucka <[email protected]> Cc: Seth Jenkins <[email protected]> Cc: Kees Cook <[email protected]> Cc: Mike Rapoport <[email protected]> Cc: David Hildenbrand <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Jann Horn <[email protected]> Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] [0] https://sashiko.dev/#/patchset/20260519151616.2557018-15-ardb%2Bgit%40google.com Ard Biesheuvel (15): arm64: mm: Remove bogus stop condition from map_mem() loop arm64: mm: Drop redundant pgd_t* argument from map_mem() arm64: mm: Check for pud_/pmd_set_huge() failures on kernel mappings arm64: mm: Preserve existing table mappings when mapping DRAM arm64: mm: Preserve non-contiguous descriptors when mapping DRAM arm64: mm: Permit contiguous descriptors to be manipulated arm64: kfence: Avoid NOMAP tricks when mapping the early pool arm64: mm: Permit contiguous attribute for preliminary mappings arm64: Move fixmap and kasan page tables to end of kernel image arm64: mm: Don't abuse memblock NOMAP to check for overlaps arm64: mm: Map the kernel data/bss read-only in the linear map powerpc/code-patching: Avoid r/w mapping of the zero page sh: cast away constness from the zero page when flushing it from the cache mm: Make empty_zero_page[] const arm64: mm: Unmap kernel data/bss entirely from the linear map arch/arm64/include/asm/mmu.h | 2 + arch/arm64/include/asm/pgtable.h | 4 + arch/arm64/kernel/vmlinux.lds.S | 8 +- arch/arm64/mm/fixmap.c | 6 +- arch/arm64/mm/kasan_init.c | 2 +- arch/arm64/mm/mmu.c | 153 ++++++++++++-------- arch/powerpc/lib/code-patching.c | 52 +------ arch/sh/mm/init.c | 2 +- include/linux/pgtable.h | 2 +- mm/mm_init.c | 2 +- 10 files changed, 111 insertions(+), 122 deletions(-) base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731 -- 2.54.0.794.g4f17f83d09-goog
