Re: [PATCH v5 7/8] powerpc/64s: Initialize and use a temporary mm for patching on Radix

On Thu Aug 5, 2021 at 4:48 AM CDT, Christophe Leroy wrote:

Le 13/07/2021 à 07:31, Christopher M. Riedl a écrit :

quoted

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 <redacted>

---

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);

Read
https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on
and
https://github.com/linuxppc/issues/issues/88

Avoid BUG_ON()s thanks.

Fine, @mpe's reply on the GH issue says the check is probably redundant:

"In general we don't need to BUG_ON(!ptr), the MMU will catch NULL
pointer dereferences for us."

quoted

+
+		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));

% should be at the end of first line and the second line alignment
should match open parenthesis in
first line.

Ok - thanks!

quoted

+
+		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);

Avoid BUG_ON() please

Yup, I'll remove these in the next spin.

quoted

+		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);

Not sure you need to split this line, checkpatch now allows 100 chars
per line.

I prefer sticking to 80 columns unless readability *really* improves by
going over that limit.

quoted

+	if (unlikely(!patch_mapping->ptep)) {
+		pr_warn("map patch: failed to allocate pte for patching\n");

That's a lot better than all above BUG_ONs

quoted

+		return -1;
+	}
+
+	pgprot = PAGE_KERNEL;
+	pte = mk_pte(page, pgprot);
+	pte = pte_mkdirty(pte);

I'm sure you can do

pte = pte_mkdirty(mk_pte(page, PAGE_KERNEL));

And indeed PAGE_KERNEL already includes _PAGE_DIRTY, so all you should
need is

pte = mk_pte(page, PAGE_KERNEL);

Or even just

set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, mk_pte(page,
PAGE_KERNEL));

Ok, I'll consolidate this in the next spin. Thanks!

quoted

+	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);

Shouldn't you stop using it before unmapping/unlocking it ?

Yes I think you're right - IIRC I had to do this for the Hash MMU (which
we don't support w/ this verion of the series anymore anyways). I'll
revisit this for the next spin.

quoted

+}
+
  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);

Maybe call it map_patch_mm() or map_patch_mapping() ?

Yes that does sound better.

quoted

+	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);

No err ? Would be better if it could return something, allthough always
0.

Ok I'll do that.

And same comment about naming.

Yes I'll use your suggested names.

quoted

+	else
+		err = unmap_patch_area();
  
  out:
  	local_irq_restore(flags);