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 <c...@codefail.de>
---
arch/powerpc/lib/code-patching.c | 40 ++++++++++++++++++++++++++++++++
1 file changed, 40 insertions(+)
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 89b37ece6d2f..051d7ae6d8ee 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>
@@ -109,6 +111,44 @@ static inline void unuse_temporary_mm(struct temp_mm
*temp_mm)
}
}
+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);
+
+ /*
+ * Choose a randomized, page-aligned address from the range:
+ * [PAGE_SIZE, DEFAULT_MAP_WINDOW - PAGE_SIZE]
+ * The lower address bound is PAGE_SIZE to avoid the zero-page.
+ * The upper address bound is DEFAULT_MAP_WINDOW - PAGE_SIZE to stay
+ * under DEFAULT_MAP_WINDOW in hash.
+ */
+ patching_addr = PAGE_SIZE + ((get_random_long() & PAGE_MASK)
+ % (DEFAULT_MAP_WINDOW - 2 * PAGE_SIZE));
+
+ /*
+ * PTE allocation uses GFP_KERNEL which means we need to pre-allocate
+ * the PTE here. We cannot do the allocation during patching with IRQs
+ * disabled (ie. "atomic" context).
+ */
+ 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);
#ifdef CONFIG_LKDTM
--
2.28.0
