[PATCH v7 5/5] powerpc/64s: Initialize and use a temporary mm for patching on Radix
Jordan Niethe
jniethe5 at gmail.com
Wed Nov 10 11:37:17 AEDT 2021
From: "Christopher M. Riedl" <cmr at bluescreens.de>
When code patching a STRICT_KERNEL_RWX kernel the page containing the
address to be patched is temporarily mapped as writeable. 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 patching. The mapping is exposed to CPUs
other than the patching CPU - this is undesirable from a hardening
perspective. Use a temporary mm instead which keeps the mapping local to
the CPU doing the patching.
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
space. The patching address is randomized between PAGE_SIZE and
DEFAULT_MAP_WINDOW-PAGE_SIZE.
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
The upper limit is DEFAULT_MAP_WINDOW due to how the Book3s64 Hash MMU
operates - by default the space above DEFAULT_MAP_WINDOW is not
available. Currently the Hash MMU does not use a temporary mm so
technically this upper limit isn't necessary; however, a larger
randomization range does not further "harden" this overall approach and
future work may introduce patching with a temporary mm on Hash as well.
Randomization occurs only once during initialization at boot for each
possible CPU in the system.
Introduce a new function, patch_instruction_mm(), to perform the
patching with a temporary mapping with write permissions at
patching_addr. Map the page with PAGE_KERNEL to set EAA[0] for the PTE
which ignores the AMR (so no need to unlock/lock KUAP) according to
PowerISA v3.0b Figure 35 on Radix.
Based on x86 implementation:
commit 4fc19708b165
("x86/alternatives: Initialize temporary mm for patching")
and:
commit b3fd8e83ada0
("x86/alternatives: Use temporary mm for text poking")
Signed-off-by: Christopher M. Riedl <cmr at bluescreens.de>
Signed-off-by: Jordan Niethe <jniethe5 at gmail.com>
---
v7: - Change to patch_instruction_mm() instead of map_patch_mm() and
unmap_patch_mm()
- include ptesync
---
arch/powerpc/lib/code-patching.c | 106 +++++++++++++++++++++++++++++--
1 file changed, 101 insertions(+), 5 deletions(-)
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index aa466e4930ec..7722dec4a914 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -11,6 +11,7 @@
#include <linux/cpuhotplug.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
+#include <linux/random.h>
#include <asm/tlbflush.h>
#include <asm/page.h>
@@ -76,6 +77,7 @@ static inline void stop_using_temp_mm(struct temp_mm_state prev_state)
static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
static DEFINE_PER_CPU(unsigned long, cpu_patching_addr);
+static DEFINE_PER_CPU(struct mm_struct *, cpu_patching_mm);
static int text_area_cpu_up(unsigned int cpu)
{
@@ -99,8 +101,48 @@ static int text_area_cpu_down(unsigned int cpu)
return 0;
}
+static __always_inline void __poking_init_temp_mm(void)
+{
+ int cpu;
+ spinlock_t *ptl;
+ pte_t *ptep;
+ struct mm_struct *patching_mm;
+ unsigned long patching_addr;
+
+ for_each_possible_cpu(cpu) {
+ patching_mm = copy_init_mm();
+ WARN_ON(!patching_mm);
+ per_cpu(cpu_patching_mm, cpu) = 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 with the Book3s64 Hash MMU.
+ */
+ patching_addr = PAGE_SIZE + ((get_random_long() & PAGE_MASK) %
+ (DEFAULT_MAP_WINDOW - 2 * PAGE_SIZE));
+ per_cpu(cpu_patching_addr, cpu) = patching_addr;
+
+ /*
+ * PTE allocation uses GFP_KERNEL which means we need to
+ * pre-allocate the PTE here because we cannot do the
+ * allocation during patching when IRQs are disabled.
+ */
+ ptep = get_locked_pte(patching_mm, patching_addr, &ptl);
+ WARN_ON(!ptep);
+ pte_unmap_unlock(ptep, ptl);
+ }
+}
+
void __init poking_init(void)
{
+ if (radix_enabled()) {
+ __poking_init_temp_mm();
+ return;
+ }
+
WARN_ON(cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
"powerpc/text_poke:online", text_area_cpu_up,
text_area_cpu_down) < 0);
@@ -167,6 +209,57 @@ static inline int unmap_patch_area(unsigned long addr)
return 0;
}
+/*
+ * This can be called for kernel text or a module.
+ */
+static int patch_instruction_mm(u32 *addr, struct ppc_inst instr)
+{
+ struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm);
+ unsigned long text_poke_addr;
+ u32 *patch_addr = NULL;
+ struct temp_mm_state prev;
+ unsigned long flags;
+ struct page *page;
+ spinlock_t *ptl;
+ int rc;
+ pte_t *ptep;
+
+ text_poke_addr = __this_cpu_read(cpu_patching_addr);
+
+ local_irq_save(flags);
+
+ if (is_vmalloc_or_module_addr(addr))
+ page = vmalloc_to_page(addr);
+ else
+ page = virt_to_page(addr);
+
+ ptep = get_locked_pte(patching_mm, text_poke_addr, &ptl);
+ if (unlikely(!ptep)) {
+ pr_warn("map patch: failed to allocate pte for patching\n");
+ return -1;
+ }
+
+ set_pte_at(patching_mm, text_poke_addr, ptep, pte_mkdirty(mk_pte(page, PAGE_KERNEL)));
+ asm volatile("ptesync": : :"memory");
+
+ prev = start_using_temp_mm(patching_mm);
+
+ patch_addr = (u32 *)(text_poke_addr | offset_in_page(addr));
+ rc = __patch_instruction(addr, instr, patch_addr);
+
+ pte_clear(patching_mm, text_poke_addr, ptep);
+
+ local_flush_tlb_mm(patching_mm);
+
+ stop_using_temp_mm(prev);
+ pte_unmap_unlock(ptep, ptl);
+
+ local_irq_restore(flags);
+ WARN_ON(!ppc_inst_equal(ppc_inst_read(addr), instr));
+
+ return rc;
+}
+
static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
{
int err, rc = 0;
@@ -175,16 +268,19 @@ static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
unsigned long text_poke_addr;
/*
- * During early early boot patch_instruction is called
- * when text_poke_area is not ready, but we still need
- * to allow patching. We just do the plain old patching
+ * During early boot patch_instruction is called when the
+ * patching_mm/text_poke_area is not ready, but we still need to allow
+ * patching. We just do the plain old patching.
*/
- if (!this_cpu_read(text_poke_area))
+ text_poke_addr = __this_cpu_read(cpu_patching_addr);
+ if (!text_poke_addr)
return raw_patch_instruction(addr, instr);
+ if (radix_enabled())
+ return patch_instruction_mm(addr, instr);
+
local_irq_save(flags);
- text_poke_addr = __this_cpu_read(cpu_patching_addr);
err = map_patch_area(addr, text_poke_addr);
if (err)
goto out;
--
2.25.1
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