[PATCH v5 7/8] powerpc/64s: Initialize and use a temporary mm for patching on Radix
Christopher M. Riedl
cmr at linux.ibm.com
Tue Jul 13 15:31:12 AEST 2021
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 two new functions, map_patch() and unmap_patch(), to
respectively create and remove the 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 linux.ibm.com>
---
v5: * Only support Book3s64 Radix MMU for now.
* Use a per-cpu datastructure to hold the patching_addr and
patching_mm to avoid the need for a synchronization lock/mutex.
v4: * In the previous series this was two separate patches: one to init
the temporary mm in poking_init() (unused in powerpc at the time)
and the other to use it for patching (which removed all the
per-cpu vmalloc code). Now that we use poking_init() in the
existing per-cpu vmalloc approach, that separation doesn't work
as nicely anymore so I just merged the two patches into one.
* Preload the SLB entry and hash the page for the patching_addr
when using Hash on book3s64 to avoid taking an SLB and Hash fault
during patching. The previous implementation was a hack which
changed current->mm to allow the SLB and Hash fault handlers to
work with the temporary mm since both of those code-paths always
assume mm == current->mm.
* Also (hmm - seeing a trend here) with the book3s64 Hash MMU we
have to manage the mm->context.active_cpus counter and mm cpumask
since they determine (via mm_is_thread_local()) if the TLB flush
in pte_clear() is local or not - it should always be local when
we're using the temporary mm. On book3s64's Radix MMU we can
just call local_flush_tlb_mm().
* Use HPTE_USE_KERNEL_KEY on Hash to avoid costly lock/unlock of
KUAP.
---
arch/powerpc/lib/code-patching.c | 132 +++++++++++++++++++++++++++++--
1 file changed, 125 insertions(+), 7 deletions(-)
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 9f2eba9b70ee4..027dabd42b8dd 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>
@@ -103,6 +104,7 @@ static inline void unuse_temporary_mm(struct temp_mm *temp_mm)
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);
#if IS_BUILTIN(CONFIG_LKDTM)
unsigned long read_cpu_patching_addr(unsigned int cpu)
@@ -133,6 +135,51 @@ 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; /* for protecting pte table */
+ pte_t *ptep;
+ struct mm_struct *patching_mm;
+ unsigned long patching_addr;
+
+ for_each_possible_cpu(cpu) {
+ /*
+ * 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);
+
+ 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);
+ BUG_ON(!ptep);
+ pte_unmap_unlock(ptep, ptl);
+ }
+}
+
/*
* Although BUG_ON() is rude, in this case it should only happen if ENOMEM, and
* we judge it as being preferable to a kernel that will crash later when
@@ -140,6 +187,11 @@ static int text_area_cpu_down(unsigned int cpu)
*/
void __init poking_init(void)
{
+ if (radix_enabled()) {
+ __poking_init_temp_mm();
+ return;
+ }
+
BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
"powerpc/text_poke:online", text_area_cpu_up,
text_area_cpu_down));
@@ -213,30 +265,96 @@ static inline int unmap_patch_area(void)
return -EINVAL;
}
+struct patch_mapping {
+ spinlock_t *ptl; /* for protecting pte table */
+ pte_t *ptep;
+ struct temp_mm temp_mm;
+};
+
+/*
+ * This can be called for kernel text or a module.
+ */
+static int map_patch(const void *addr, struct patch_mapping *patch_mapping)
+{
+ struct page *page;
+ pte_t pte;
+ pgprot_t pgprot;
+ struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm);
+ unsigned long patching_addr = __this_cpu_read(cpu_patching_addr);
+
+ if (is_vmalloc_or_module_addr(addr))
+ page = vmalloc_to_page(addr);
+ else
+ page = virt_to_page(addr);
+
+ patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr,
+ &patch_mapping->ptl);
+ if (unlikely(!patch_mapping->ptep)) {
+ pr_warn("map patch: failed to allocate pte for patching\n");
+ return -1;
+ }
+
+ pgprot = PAGE_KERNEL;
+ pte = mk_pte(page, pgprot);
+ pte = pte_mkdirty(pte);
+ set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, pte);
+
+ init_temp_mm(&patch_mapping->temp_mm, patching_mm);
+ use_temporary_mm(&patch_mapping->temp_mm);
+
+ return 0;
+}
+
+static void unmap_patch(struct patch_mapping *patch_mapping)
+{
+ struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm);
+ unsigned long patching_addr = __this_cpu_read(cpu_patching_addr);
+
+ pte_clear(patching_mm, patching_addr, patch_mapping->ptep);
+
+ local_flush_tlb_mm(patching_mm);
+
+ pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl);
+
+ unuse_temporary_mm(&patch_mapping->temp_mm);
+}
+
static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
{
int err, rc = 0;
u32 *patch_addr = NULL;
unsigned long flags;
+ struct patch_mapping patch_mapping;
/*
- * 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 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))
- return raw_patch_instruction(addr, instr);
+ if (radix_enabled()) {
+ if (!this_cpu_read(cpu_patching_mm))
+ return raw_patch_instruction(addr, instr);
+ } else {
+ if (!this_cpu_read(text_poke_area))
+ return raw_patch_instruction(addr, instr);
+ }
local_irq_save(flags);
- err = map_patch_area(addr);
+ if (radix_enabled())
+ err = map_patch(addr, &patch_mapping);
+ else
+ err = map_patch_area(addr);
if (err)
goto out;
patch_addr = (u32 *)(__this_cpu_read(cpu_patching_addr) | offset_in_page(addr));
rc = __patch_instruction(addr, instr, patch_addr);
- err = unmap_patch_area();
+ if (radix_enabled())
+ unmap_patch(&patch_mapping);
+ else
+ err = unmap_patch_area();
out:
local_irq_restore(flags);
--
2.26.1
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