On March 24, 2020 11:10 AM Christophe Leroy [off-list ref] wrote:

 
Le 23/03/2020 à 05:52, 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 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 <redacted>
---
  arch/powerpc/lib/code-patching.c | 26 ++++++++++++++++++++++++++
  1 file changed, 26 insertions(+)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 3345f039a876..18b88ecfc5a8 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/pgtable.h>
  #include <asm/tlbflush.h>

@@ -39,6 +41,30 @@ int raw_patch_instruction(unsigned int *addr, unsigned int instr)
  }
  
  #ifdef CONFIG_STRICT_KERNEL_RWX
+
+__ro_after_init struct mm_struct *patching_mm;
+__ro_after_init unsigned long patching_addr;

Can we make those those static ?

Yes, makes sense to me.

quoted

+
+void __init poking_init(void)
+{
+	spinlock_t *ptl; /* for protecting pte table */
+	pte_t *ptep;
+
+	patching_mm = copy_init_mm();
+	BUG_ON(!patching_mm);

Does it needs to be a BUG_ON() ? Can't we fail gracefully with just a 
WARN_ON ?

I'm not sure what failing gracefully means here? The main reason this could
fail is if there is not enough memory to allocate the patching_mm. The
previous implementation had this justification for BUG_ON():

/*
 * Run as a late init call. This allows all the boot time patching to be done
 * simply by patching the code, and then we're called here prior to
 * mark_rodata_ro(), which happens after all init calls are run. 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 someone tries
 * to use patch_instruction().
 */
static int __init setup_text_poke_area(void)
{
        BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
                "powerpc/text_poke:online", text_area_cpu_up,
                text_area_cpu_down));

        return 0;
}
late_initcall(setup_text_poke_area);

I think the BUG_ON() is appropriate even if only to adhere to the previous
judgement call. I can add a similar comment explaining the reasoning if
that helps.

quoted

+
+	/*
+	 * 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);

Same here, can we fail gracefully instead ?

Same reasoning as above.

quoted

+	pte_unmap_unlock(ptep, ptl);
+}
+
  static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
  
  static int text_area_cpu_up(unsigned int cpu)

Christophe