Re: [RFC PATCH 2/3] powerpc/lib: Initialize a temporary mm for code patching
From: Christophe Leroy <hidden>
Date: 2020-03-24 16:15:12
Le 23/03/2020 à 05:52, Christopher M. Riedl a écrit :
quoted hunk ↗ jump to hunk
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 ?
+
+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 ?
+ + /* + * 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 ?
+ 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