Date: Thu, 27 Aug 2020 00:26:53 -0500 From: "Christopher M. Riedl" <cmr@...efail.de> To: linuxppc-dev@...ts.ozlabs.org Cc: kernel-hardening@...ts.openwall.com Subject: [PATCH v3 0/6] Use per-CPU temporary mappings for patching When compiled with CONFIG_STRICT_KERNEL_RWX, the kernel must create temporary mappings when patching itself. These mappings temporarily override the strict RWX text protections to permit a write. Currently, powerpc allocates a per-CPU VM area for patching. Patching occurs as follows: 1. Map page of text to be patched to per-CPU VM area w/ PAGE_KERNEL protection 2. Patch text 3. Remove the temporary mapping While the VM area is per-CPU, the mapping is actually inserted into the kernel page tables. Presumably, this could allow another CPU to access the normally write-protected text - either malicously or accidentally - via this same mapping if the address of the VM area is known. Ideally, the mapping should be kept local to the CPU doing the patching (or any other sensitive operations requiring temporarily overriding memory protections) . x86 introduced "temporary mm" structs which allow the creation of mappings local to a particular CPU . This series intends to bring the notion of a temporary mm to powerpc and harden powerpc by using such a mapping for patching a kernel with strict RWX permissions. The first, second, and third patches implement an LKDTM test "proof-of-concept" which exploits the potential vulnerability (ie. the mapping during patching is exposed in kernel page tables and accessible by other CPUS). The LKDTM test is somewhat "rough" in that it uses a brute-force approach - I am open to any suggestions and/or ideas to improve this. Currently, the LKDTM test passes with this series on POWER8 (hash) and POWER9 (radix, hash) and fails without this series (ie. the temporary mapping for patching is exposed to CPUs other than the patching CPU). The test is also implemented on x86_64 where it passes with a current tree and fails only when using a tree prior to the temporary mappings. I have such a tree here which intentionally fails: https://github.com/cmr-informatik/linux/tree/x86_64-non-percpu-lkdtm The fourth patch introduces the temporary mm struct and API for powerpc along with a new function to retrieve a current hw breakpoint. The fifth patch uses the `poking_init` init hook added by the x86 patches to initialize a temporary mm and patching address. The patching address is randomized between PAGE_SIZE and DEFAULT_MAP_WINDOW-PAGE_SIZE. The upper limit is necessary due to how the hash MMU operates - by default the space above DEFAULT_MAP_WINDOW is not available. For now, both hash and radix randomize inside this range. The number of possible random addresses is dependent on PAGE_SIZE and limited by DEFAULT_MAP_WINDOW. Bits of entropy with 64K page size on BOOK3S_64: bits of entropy = log2(DEFAULT_MAP_WINDOW_USER64 / PAGE_SIZE) PAGE_SIZE=64K, DEFAULT_MAP_WINDOW_USER64=128TB bits of entropy = log2(128TB / 64K) bits of entropy = 31 Randomization occurs only once during initialization at boot. The sixth patch replaces the VM area with the temporary mm in the patching code. The page for patching has to be mapped PAGE_SHARED with the hash MMU since hash prevents the kernel from accessing userspace pages with PAGE_PRIVILEGED bit set. On the radix MMU the page is mapped with PAGE_KERNEL which has the added benefit that we can skip KUAP. Tested on Blackbird (8-core POWER9) w/ Hash (`disable_radix`) and Radix MMUs, QEMU (TCG) POWER8 and POWER9, POWER8 VM. Tested LKDTM test (passing and failing situations) on QEMU x86_64. v3: * Rebase on linuxppc/next: commit 9123e3a74ec7 ("Linux 5.9-rc1") * Move temporary mm implementation into code-patching.c where it belongs * Implement LKDTM hijacker test on x86_64 (on IBM time oof) * Do not use address zero for the patching address in the temporary mm (thanks @dja for pointing this out!) * Wrap the LKDTM test w/ CONFIG_SMP as suggested by Christophe Leroy * Comments to clarify PTE pre-allocation and patching addr selection v2: * Rebase on linuxppc/next: commit 105fb38124a4 ("powerpc/8xx: Modify ptep_get()") * Always dirty pte when mapping patch * Use `ppc_inst_len` instead of `sizeof` on instructions * Declare LKDTM patching addr accessor in header where it belongs v1: * Rebase on linuxppc/next (4336b9337824) * Save and restore second hw watchpoint * Use new ppc_inst_* functions for patching check and in LKDTM test rfc-v2: * Many fixes and improvements mostly based on extensive feedback and testing by Christophe Leroy (thanks!). * Make patching_mm and patching_addr static and mode '__ro_after_init' to after the variable name (more common in other parts of the kernel) * Use 'asm/debug.h' header instead of 'asm/hw_breakpoint.h' to fix PPC64e compile * Add comment explaining why we use BUG_ON() during the init call to setup for patching later * Move ptep into patch_mapping to avoid walking page tables a second time when unmapping the temporary mapping * Use KUAP under non-radix, also manually dirty the PTE for patch mapping on non-BOOK3S_64 platforms * Properly return any error from __patch_instruction * Do not use 'memcmp' where a simple comparison is appropriate * Simplify expression for patch address by removing pointer maths * Add LKDTM test : https://github.com/linuxppc/issues/issues/224 : https://email@example.com/ Christopher M. Riedl (6): powerpc: Add LKDTM accessor for patching addr x86: Add LKDTM accessor for patching addr Add LKDTM test to hijack a patch mapping (powerpc,x86_64) powerpc: Introduce temporary mm powerpc: Initialize a temporary mm for code patching powerpc: Use a temporary mm for code patching arch/powerpc/include/asm/code-patching.h | 4 + arch/powerpc/include/asm/debug.h | 1 + arch/powerpc/kernel/process.c | 5 + arch/powerpc/lib/code-patching.c | 239 +++++++++++++++-------- arch/x86/include/asm/text-patching.h | 4 + arch/x86/kernel/alternative.c | 7 + drivers/misc/lkdtm/core.c | 1 + drivers/misc/lkdtm/lkdtm.h | 1 + drivers/misc/lkdtm/perms.c | 146 ++++++++++++++ 9 files changed, 322 insertions(+), 86 deletions(-) -- 2.28.0
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