Le 31/03/2020 à 05:19, Christopher M Riedl a écrit :
On March 24, 2020 11:10 AM Christophe Leroy <christophe.le...@c-s.fr> wrote:
Le 23/03/2020 à 05:52, Christopher M. Riedl a écrit :
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 <c...@informatik.wtf>
---
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.
+
+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():
But the system can continue running just fine after this failure.
Only the things that make use of code patching will fail (ftrace, kgdb, ...)
Checkpatch tells: "Avoid crashing the kernel - try using WARN_ON &
recovery code rather than BUG() or BUG_ON()"
All vital code patching has already been done previously, so I think a
WARN_ON() should be enough, plus returning non 0 to indicate that the
late_initcall failed.
/*
* 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.
+
+ /*
+ * 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.
Here as well, a WARN_ON() should be enough, the system will continue
running after that.
+ 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
Christophe
