[PATCH v3 5/6] powerpc: Initialize a temporary mm for code patching
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STALE2241d
From: Christopher M. Riedl <hidden>
Date: 2020-08-27 08:14:39
Subsystem:
linux for powerpc (32-bit and 64-bit), the rest · Maintainers:
Madhavan Srinivasan, Linus Torvalds
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 | 40 ++++++++++++++++++++++++++++++++
1 file changed, 40 insertions(+)
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 89b37ece6d2f..051d7ae6d8ee 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>
@@ -109,6 +111,44 @@ static inline void unuse_temporary_mm(struct temp_mm *temp_mm) } } +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); + + /* + * 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 in hash. + */ + patching_addr = PAGE_SIZE + ((get_random_long() & PAGE_MASK) + % (DEFAULT_MAP_WINDOW - 2 * PAGE_SIZE)); + + /* + * PTE allocation uses GFP_KERNEL which means we need to pre-allocate + * the PTE here. We cannot do the allocation during patching with IRQs + * disabled (ie. "atomic" context). + */ + 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); #ifdef CONFIG_LKDTM
--
2.28.0