From: "Christopher M. Riedl" <cmr@informatik.wtf>
To: linuxppc-dev@lists.ozlabs.org
Cc: kernel-hardening@lists.openwall.com
Subject: [PATCH v2 2/5] powerpc/lib: Initialize a temporary mm for code patching
Date: Wed, 8 Jul 2020 23:03:13 -0500 [thread overview]
Message-ID: <20200709040316.12789-3-cmr@informatik.wtf> (raw)
In-Reply-To: <20200709040316.12789-1-cmr@informatik.wtf>
When code patching a STRICT_KERNEL_RWX kernel the page containing the
address to be patched is temporarily mapped with permissive memory
protections. Currently, a per-cpu vmalloc patch area is used for this
purpose. While the patch area is per-cpu, the temporary page mapping is
inserted into the kernel page tables for the duration of the patching.
The mapping is exposed to CPUs other than the patching CPU - this is
undesirable from a hardening perspective.
Use the `poking_init` init hook to prepare a temporary mm and patching
address. Initialize the temporary mm by copying the init mm. Choose a
randomized patching address inside the temporary mm userspace address
portion. The next patch uses the temporary mm and patching address for
code patching.
Based on x86 implementation:
commit 4fc19708b165
("x86/alternatives: Initialize temporary mm for patching")
Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
---
arch/powerpc/lib/code-patching.c | 33 ++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 0a051dfeb177..8ae1a9e5fe6e 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -11,6 +11,8 @@
#include <linux/cpuhotplug.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
+#include <linux/sched/task.h>
+#include <linux/random.h>
#include <asm/tlbflush.h>
#include <asm/page.h>
@@ -44,6 +46,37 @@ int raw_patch_instruction(struct ppc_inst *addr, struct ppc_inst instr)
}
#ifdef CONFIG_STRICT_KERNEL_RWX
+
+static struct mm_struct *patching_mm __ro_after_init;
+static unsigned long patching_addr __ro_after_init;
+
+void __init poking_init(void)
+{
+ spinlock_t *ptl; /* for protecting pte table */
+ pte_t *ptep;
+
+ /*
+ * Some parts of the kernel (static keys for example) depend on
+ * successful code patching. Code patching under STRICT_KERNEL_RWX
+ * requires this setup - otherwise we cannot patch at all. We use
+ * BUG_ON() here and later since an early failure is preferred to
+ * buggy behavior and/or strange crashes later.
+ */
+ patching_mm = copy_init_mm();
+ BUG_ON(!patching_mm);
+
+ /*
+ * In hash we cannot go above DEFAULT_MAP_WINDOW easily.
+ * XXX: Do we want additional bits of entropy for radix?
+ */
+ patching_addr = (get_random_long() & PAGE_MASK) %
+ (DEFAULT_MAP_WINDOW - PAGE_SIZE);
+
+ ptep = get_locked_pte(patching_mm, patching_addr, &ptl);
+ BUG_ON(!ptep);
+ pte_unmap_unlock(ptep, ptl);
+}
+
static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
static int text_area_cpu_up(unsigned int cpu)
--
2.27.0
next prev parent reply other threads:[~2020-07-09 4:37 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-09 4:03 [PATCH 0/5] Use per-CPU temporary mappings for patching Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 1/5] powerpc/mm: Introduce temporary mm Christopher M. Riedl
2020-08-06 1:27 ` Daniel Axtens
2020-08-17 5:16 ` Christopher M. Riedl
2020-07-09 4:03 ` Christopher M. Riedl [this message]
2020-07-17 8:57 ` [PATCH v2 2/5] powerpc/lib: Initialize a temporary mm for code patching kernel test robot
2020-08-06 3:24 ` Daniel Axtens
2020-08-17 2:21 ` Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 3/5] powerpc/lib: Use " Christopher M. Riedl
2020-07-09 7:02 ` Christophe Leroy
2020-07-14 19:43 ` Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 4/5] powerpc/lib: Add LKDTM accessor for patching addr Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 5/5] powerpc: Add LKDTM test to hijack a patch mapping Christopher M. Riedl
2020-07-14 21:24 ` Kees Cook
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