Re: [PATCH v2 2/5] powerpc/lib: Initialize a temporary mm for code patching

On Thu Aug 6, 2020 at 8:24 AM CDT, Daniel Axtens wrote:

"Christopher M. Riedl" [off-list ref] writes:

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 | 33 ++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 0a051dfeb177..8ae1a9e5fe6e 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>

@@ -44,6 +46,37 @@ int raw_patch_instruction(struct ppc_inst *addr, struct ppc_inst instr)
 }
 
 #ifdef CONFIG_STRICT_KERNEL_RWX
+
+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);
+
+	/*
+	 * 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);

It took me a while to understand this calculation. I see that it's
calculating a base address for a page in which to do patching. It does
the following:

I will add a comment explaining the calulcation in the next spin.

- get a random long

- mask with PAGE_MASK so as to get a page aligned value

- make sure that the base address is at least one PAGE_SIZE below
DEFAULT_MAP_WINDOW so we have a clear page between the base and
DEFAULT_MAP_WINDOW.

On 64-bit Book3S with 64K pages, that works out to be

PAGE_SIZE = 0x0000 0000 0001 0000
PAGE_MASK = 0xFFFF FFFF FFFF 0000

DEFAULT_MAP_WINDOW = DEFAULT_MAP_WINDOW_USER64 = TASK_SIZE_128TB
= 0x0000_8000_0000_0000

DEFAULT_MAP_WINDOW - PAGE_SIZE = 0x0000 7FFF FFFF 0000

It took a while (and a conversation with my wife who studied pure
maths!) but I am convinced that the modulo preserves the page-alignement
of the patching address.

I am glad a proper mathematician agrees because my maths are decidedly
unpure :)

One thing I did realise is that patching_addr can be zero at the end of
this process. That seems dubious and slightly error-prone to me - is
the patching process robust to that or should we exclude it?

Good catch! I will fix this in the next spin.

Anyway, if I have the maths right, that there are 0x7fffffff or ~2
billion possible locations for the patching page, which is just shy of
31 bits of entropy.

I think this compares pretty favourably to most (K)ASLR implementations?

I will stress that I am not an expert here, but it looks like this does
compares favorably against other 64b ASLR [0].

[0]: https://www.cs.ucdavis.edu/~peisert/research/2017-SecDev-AnalysisASLR.pdf

What's the range if built with 4k pages?

Using the formula from my series coverletter, we should expect 34 bits
of entropy since DEFAULT_MAP_WINDOW_USER64 is 64TB for 4K pages:

	bits of entropy = log2(DEFAULT_MAP_WINDOW_USER64 / PAGE_SIZE)

	PAGE_SIZE=4K, DEFAULT_MAP_WINDOW_USER64=64TB
	bits of entropy = log2(64TB / 4K)
	bits of entropy = 34

Kind regards,
Daniel

quoted

+
+	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);
 
 static int text_area_cpu_up(unsigned int cpu)
-- 
2.27.0