[PATCH 0/4] arm64: mm: support dynamic vmalloc/pmd configuration

Maxwell Bland
2024-02-20T20:33:42Z

Reworks ARM's virtual memory allocation infrastructure to support
dynamic enforcement of page middle directory PXNTable restrictions
rather than only during the initial memory mapping. Runtime enforcement
of this bit prevents write-then-execute attacks, where malicious code is
staged in vmalloc'd data regions, and later the page table is changed to
make this code executable.

Previously the entire region from VMALLOC_START to VMALLOC_END was
vulnerable, but now the vulnerable region is restricted to the 2GB
reserved by module_alloc, a region which is generally read-only and more
difficult to inject staging code into, e.g., data must pass the BPF
verifier. These changes also set the stage for other systems, such as
KVM-level (EL2) changes to mark page tables immutable and code page
verification changes, forging a path toward complete mitigation of
kernel exploits on ARM.

Implementing this required minimal changes to the generic vmalloc
interface in the kernel to allow architecture overrides of some vmalloc
wrapper functions, refactoring vmalloc calls to use a standard interface
in the generic kernel, and passing the address parameter already passed
into PTE allocation to the pte_allocate child function call.

The new arm64 vmalloc wrapper functions ensure vmalloc data is not
allocated into the region reserved for module_alloc. arm64 BPF and
kprobe code also see a two-line-change ensuring their allocations abide
by the segmentation of code from data. Finally, arm64's pmd_populate
function is modified to set the PXNTable bit appropriately.

Signed-off-by: Maxwell Bland <mbland@motorola.com>

---

After Mark Rutland's feedback last week on my more minimal patch, see

<CAP5Mv+ydhk=Ob4b40ZahGMgT-5+-VEHxtmA=-LkJiEOOU+K6hw@mail.gmail.com>

I adopted a more sweeping and more correct overhaul of ARM's virtual
memory allocation infrastructure to support these changes. This patch
guarantees our ability to write future systems with a strong and
accessible distinction between code and data at the page allocation
layer, bolstering the guarantees of complementary contributions, i.e.
W^X and kCFI.

The current patch minimally reduces available vmalloc space, removing
the 2GB that should be reserved for code allocations regardless, and I
feel really benefits the kernel by making several memory allocation
interfaces more uniform, and providing hooks for non-ARM architectures
to follow suit.

I have done some minimal runtime testing using Torvald's test-tlb script
on a QEMU VM, but maybe more extensive benchmarking is needed?

Size: Before Patch -> After Patch
4k: 4.09ns 4.15ns 4.41ns 4.43ns -> 3.68ns 3.73ns 3.67ns 3.73ns
8k: 4.22ns 4.19ns 4.30ns 4.15ns -> 3.99ns 3.89ns 4.12ns 4.04ns
16k: 3.97ns 4.31ns 4.30ns 4.28ns -> 4.03ns 3.98ns 4.06ns 4.06ns
32k: 3.82ns 4.51ns 4.25ns 4.31ns -> 3.99ns 4.09ns 4.07ns 5.17ns
64k: 4.50ns 5.59ns 6.13ns 6.14ns -> 4.23ns 4.26ns 5.91ns 5.93ns
128k: 5.06ns 4.47ns 6.75ns 6.69ns -> 4.47ns 4.71ns 6.54ns 6.44ns
256k: 4.83ns 4.43ns 6.62ns 6.21ns -> 4.39ns 4.62ns 6.71ns 6.65ns
512k: 4.45ns 4.75ns 6.19ns 6.65ns -> 4.86ns 5.26ns 7.77ns 6.68ns
1M: 4.72ns 4.73ns 6.74ns 6.47ns -> 4.29ns 4.45ns 6.87ns 6.59ns
2M: 4.66ns 4.86ns 14.49ns 15.00ns -> 4.53ns 4.57ns 15.91ns 15.90ns
4M: 4.85ns 4.95ns 15.90ns 15.98ns -> 4.48ns 4.74ns 17.27ns 17.36ns
6M: 4.94ns 5.03ns 17.19ns 17.31ns -> 4.70ns 4.93ns 18.02ns 18.23ns
8M: 5.05ns 5.18ns 17.49ns 17.64ns -> 4.96ns 5.07ns 18.84ns 18.72ns
16M: 5.55ns 5.79ns 20.99ns 23.70ns -> 5.46ns 5.72ns 22.76ns 26.51ns
32M: 8.54ns 9.06ns 124.61ns 125.07ns -> 8.43ns 8.59ns 116.83ns 138.83ns
64M: 8.42ns 8.63ns 196.17ns 204.52ns -> 8.26ns 8.43ns 193.49ns 203.85ns
128M: 8.31ns 8.58ns 230.46ns 242.63ns -> 8.22ns 8.39ns 227.99ns 240.29ns
256M: 8.80ns 8.80ns 248.24ns 261.68ns -> 8.35ns 8.55ns 250.18ns 262.20ns

Note I also chose to enforce PXNTable at the PMD layer only (for now),
since the 194 descriptors which are affected by this change on my
testing setup are not sufficient to warrant enforcement at a coarser
granularity.

The architecture-independent changes (I term "generic") can be
classified only as refactoring, but I feel are also major improvements
in that they standardize most uses of the vmalloc interface across the
kernel.

Note this patch reduces the arm64 allocated region for BPF and kprobes,
but only to match with the existing allocation choices made by the
generic kernel. I will admit I do not understand why BPF JIT allocation
code was duplicated into arm64, but I also feel that this was either an
artifact or that these overrides for generic allocation should require a
specific KConfig as they trade off between security and space. That
said, I have chosen not to wrap this patch in a KConfig interface, as I
feel the changes provide significant benefit to the arm64 kernel's
baseline security, though a KConfig could certainly be added if the
maintainers see the need.

Maxwell Bland (4):
mm/vmalloc: allow arch-specific vmalloc_node overrides
mm: pgalloc: support address-conditional pmd allocation
arm64: separate code and data virtual memory allocation
arm64: dynamic enforcement of pmd-level PXNTable

arch/arm/kernel/irq.c | 2 +-
arch/arm64/include/asm/pgalloc.h | 11 +++++-
arch/arm64/include/asm/vmalloc.h | 8 ++++
arch/arm64/include/asm/vmap_stack.h | 2 +-
arch/arm64/kernel/efi.c | 2 +-
arch/arm64/kernel/module.c | 7 ++++
arch/arm64/kernel/probes/kprobes.c | 2 +-
arch/arm64/mm/Makefile | 3 +-
arch/arm64/mm/trans_pgd.c | 2 +-
arch/arm64/mm/vmalloc.c | 57 +++++++++++++++++++++++++++++
arch/arm64/net/bpf_jit_comp.c | 5 ++-
arch/powerpc/kernel/irq.c | 2 +-
arch/riscv/include/asm/irq_stack.h | 2 +-
arch/s390/hypfs/hypfs_diag.c | 2 +-
arch/s390/kernel/setup.c | 6 +--
arch/s390/kernel/sthyi.c | 2 +-
include/asm-generic/pgalloc.h | 18 +++++++++
include/linux/mm.h | 4 +-
include/linux/vmalloc.h | 15 +++++++-
kernel/bpf/syscall.c | 4 +-
kernel/fork.c | 4 +-
kernel/scs.c | 3 +-
lib/objpool.c | 2 +-
lib/test_vmalloc.c | 6 +--
mm/hugetlb_vmemmap.c | 4 +-
mm/kasan/init.c | 22 ++++++-----
mm/memory.c | 4 +-
mm/percpu.c | 2 +-
mm/pgalloc-track.h | 3 +-
mm/sparse-vmemmap.c | 2 +-
mm/util.c | 3 +-
mm/vmalloc.c | 39 +++++++-------------
32 files changed, 176 insertions(+), 74 deletions(-)
create mode 100644 arch/arm64/mm/vmalloc.c

base-commit: b401b621758e46812da61fa58a67c3fd8d91de0d
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2.39.2