[PATCH v2 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support

Daniel Axtens
2019-12-10T04:47:49Z

KASAN support on powerpc64 is challenging:

- We want to be able to support inline instrumentation so as to be
able to catch global and stack issues.

- We run some code in real mode after boot, most notably a lot of
KVM code. We'd like to be able to instrument this.

[For those not immersed in ppc64, in real mode, the top nibble or
2 bits (depending on radix/hash mmu) of the address is ignored. The
linear mapping is placed at 0xc000000000000000. This means that a
pointer to part of the linear mapping will work both in real mode,
where it will be interpreted as a physical address of the form
0x000..., and out of real mode, where it will go via the linear
mapping.]

- Inline instrumentation requires a fixed offset.

- Because of our running things in real mode, the offset has to
point to valid memory both in and out of real mode.

This makes finding somewhere to put the KASAN shadow region challenging.

One approach is just to give up on inline instrumentation and override
the address->shadow calculation. This way we can delay all checking
until after we get everything set up to our satisfaction. However,
we'd really like to do better.

What we can do - if we know _at compile time_ how much contiguous
physical memory we have - is to set aside the top 1/8th of the memory
and use that. This is a big hammer (hence the "heavyweight" name) and
comes with 3 big consequences:

- kernels will simply fail to boot on machines with less memory than
specified when compiling.

- kernels running on machines with more memory than specified when
compiling will simply ignore the extra memory.

- there's no nice way to handle physically discontiguous memory, so
you are restricted to the first physical memory block.

If you can bear all this, you get full support for KASAN.

Despite the limitations, it can still find bugs,
e.g. http://patchwork.ozlabs.org/patch/1103775/

The current implementation is Radix only.

Massive thanks to mpe, who had the idea for the initial design.

Signed-off-by: Daniel Axtens <dja@axtens.net>

---
Changes since v1:
- Landed kasan vmalloc support upstream
- Lots of feedback from Christophe.

Changes since the rfc:

- Boots real and virtual hardware, kvm works.

- disabled reporting when we're checking the stack for exception
frames. The behaviour isn't wrong, just incompatible with KASAN.

- Documentation!

- Dropped old module stuff in favour of KASAN_VMALLOC.

The bugs with ftrace and kuap were due to kernel bloat pushing
prom_init calls to be done via the plt. Because we did not have
a relocatable kernel, and they are done very early, this caused
everything to explode. Compile with CONFIG_RELOCATABLE!
---
Documentation/dev-tools/kasan.rst | 8 +-
Documentation/powerpc/kasan.txt | 102 +++++++++++++++++-
arch/powerpc/Kconfig | 3 +
arch/powerpc/Kconfig.debug | 21 ++++
arch/powerpc/Makefile | 11 ++
arch/powerpc/include/asm/kasan.h | 20 +++-
arch/powerpc/kernel/process.c | 8 ++
arch/powerpc/kernel/prom.c | 59 +++++++++-
arch/powerpc/mm/kasan/Makefile | 3 +-
.../mm/kasan/{kasan_init_32.c => init_32.c} | 0
arch/powerpc/mm/kasan/init_book3s_64.c | 67 ++++++++++++
11 files changed, 293 insertions(+), 9 deletions(-)
rename arch/powerpc/mm/kasan/{kasan_init_32.c => init_32.c} (100%)
create mode 100644 arch/powerpc/mm/kasan/init_book3s_64.c

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 4af2b5d2c9b4..d99dc580bc11 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -22,8 +22,9 @@ global variables yet.
Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later.

Currently generic KASAN is supported for the x86_64, arm64, xtensa and s390
-architectures. It is also supported on 32-bit powerpc kernels. Tag-based KASAN
-is supported only on arm64.
+architectures. It is also supported on powerpc, for 32-bit kernels, and for
+64-bit kernels running under the Radix MMU. Tag-based KASAN is supported only
+on arm64.

Usage
-----
@@ -256,7 +257,8 @@ CONFIG_KASAN_VMALLOC
~~~~~~~~~~~~~~~~~~~~

With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
-cost of greater memory usage. Currently this is only supported on x86.
+cost of greater memory usage. Currently this is optional on x86, and
+required on 64-bit powerpc.

This works by hooking into vmalloc and vmap, and dynamically
allocating real shadow memory to back the mappings.
diff --git a/Documentation/powerpc/kasan.txt b/Documentation/powerpc/kasan.txt
index a85ce2ff8244..d6e7a415195c 100644
--- a/Documentation/powerpc/kasan.txt
+++ b/Documentation/powerpc/kasan.txt
@@ -1,4 +1,4 @@
-KASAN is supported on powerpc on 32-bit only.
+KASAN is supported on powerpc on 32-bit and 64-bit Radix only.

32 bit support
==============
@@ -10,3 +10,103 @@ fixmap area and occupies one eighth of the total kernel virtual memory space.

Instrumentation of the vmalloc area is not currently supported, but modules
are.
+
+64 bit support
+==============
+
+Currently, only the radix MMU is supported. There have been versions for Book3E
+processors floating around on the mailing list, but nothing has been merged.
+
+KASAN support on Book3S is a bit tricky to get right:
+
+ - We want to be able to support inline instrumentation so as to be able to
+ catch global and stack issues.
+
+ - Inline instrumentation requires a fixed offset.
+
+ - We run a lot of code in real mode. Most notably a lot of KVM runs in real
+ mode, and we'd like to be able to instrument it.
+
+ - Because we run code in real mode after boot, the offset has to point to
+ valid memory both in and out of real mode.
+
+One approach is just to give up on inline instrumentation. This way we can
+delay all checks until after we get everything set up correctly. However, we'd
+really like to do better.
+
+If we know _at compile time_ how much contiguous physical memory we have, we
+can set aside the top 1/8th of the first block of physical memory and use
+that. This is a big hammer and comes with 3 big consequences:
+
+ - there's no nice way to handle physically discontiguous memory, so
+ you are restricted to the first physical memory block.
+
+ - kernels will simply fail to boot on machines with less memory than specified
+ when compiling.
+
+ - kernels running on machines with more memory than specified when compiling
+ will simply ignore the extra memory.
+
+If you can live with this, you get full support for KASAN.
+
+Tips
+----
+
+ - Compile with CONFIG_RELOCATABLE.
+
+ In development, we found boot hangs when building with ftrace and KUAP
+ on. These ended up being due to kernel bloat pushing prom_init calls to be
+ done via the PLT. Because we did not have a relocatable kernel, and they are
+ done very early, this caused us to jump off into somewhere invalid. Enabling
+ relocation fixes this.
+
+NUMA/discontiguous physical memory
+----------------------------------
+
+We currently cannot really deal with discontiguous physical memory. You are
+restricted to the physical memory that is contiguous from physical address
+zero, and must specify the size of that memory, not total memory, when
+configuring your kernel.
+
+Discontiguous memory can occur when you have a machine with memory spread
+across multiple nodes. For example, on a Talos II with 64GB of RAM:
+
+ - 32GB runs from 0x0 to 0x0000_0008_0000_0000,
+ - then there's a gap,
+ - then the final 32GB runs from 0x0000_2000_0000_0000 to 0x0000_2008_0000_0000
+
+This can create _significant_ issues:
+
+ - If we try to treat the machine as having 64GB of _contiguous_ RAM, we would
+ assume that ran from 0x0 to 0x0000_0010_0000_0000. We'd then reserve the
+ last 1/8th - 0x0000_000e_0000_0000 to 0x0000_0010_0000_0000 as the shadow
+ region. But when we try to access any of that, we'll try to access pages
+ that are not physically present.
+
+ - If we try to base the shadow region size on the top address, we'll need to
+ reserve 0x2008_0000_0000 / 8 = 0x0401_0000_0000 bytes = 4100 GB of memory,
+ which will clearly not work on a system with 64GB of RAM.
+
+Therefore, you are restricted to the memory in the node starting at 0x0. For
+this system, that's 32GB. If you specify a contiguous physical memory size
+greater than the size of the first contiguous region of memory, the system will
+be unable to boot or even print an error message warning you.
+
+You can determine the layout of your system's memory by observing the messages
+that the Radix MMU prints on boot. The Talos II discussed earlier has:
+
+radix-mmu: Mapped 0x0000000000000000-0x0000000040000000 with 1.00 GiB pages (exec)
+radix-mmu: Mapped 0x0000000040000000-0x0000000800000000 with 1.00 GiB pages
+radix-mmu: Mapped 0x0000200000000000-0x0000200800000000 with 1.00 GiB pages
+
+As discussed, you'd configure this system for 32768 MB.
+
+Another system prints:
+
+radix-mmu: Mapped 0x0000000000000000-0x0000000040000000 with 1.00 GiB pages (exec)
+radix-mmu: Mapped 0x0000000040000000-0x0000002000000000 with 1.00 GiB pages
+radix-mmu: Mapped 0x0000200000000000-0x0000202000000000 with 1.00 GiB pages
+
+This machine has more memory: 0x0000_0040_0000_0000 total, but only
+0x0000_0020_0000_0000 is physically contiguous from zero, so we'd configure the
+kernel for 131072 MB of physically contiguous memory.
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 1ec34e16ed65..f68650f14e61 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -173,6 +173,9 @@ config PPC
select HAVE_ARCH_HUGE_VMAP if PPC_BOOK3S_64 && PPC_RADIX_MMU
select HAVE_ARCH_JUMP_LABEL
select HAVE_ARCH_KASAN if PPC32
+ select HAVE_ARCH_KASAN if PPC_BOOK3S_64 && PPC_RADIX_MMU
+ select HAVE_ARCH_KASAN_VMALLOC if PPC_BOOK3S_64
+ select KASAN_VMALLOC if KASAN && PPC_BOOK3S_64
select HAVE_ARCH_KGDB
select HAVE_ARCH_MMAP_RND_BITS
select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index 4e1d39847462..90bb48455cb8 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug
@@ -394,6 +394,27 @@ config PPC_FAST_ENDIAN_SWITCH
help
If you're unsure what this is, say N.

+config PHYS_MEM_SIZE_FOR_KASAN
+ int "Contiguous physical memory size for KASAN (MB)" if KASAN && PPC_BOOK3S_64
+ default 0
+ help
+
+ To get inline instrumentation support for KASAN on 64-bit Book3S
+ machines, you need to know how much contiguous physical memory your
+ system has. A shadow offset will be calculated based on this figure,
+ which will be compiled in to the kernel. KASAN will use this offset
+ to access its shadow region, which is used to verify memory accesses.
+
+ If you attempt to boot on a system with less memory than you specify
+ here, your system will fail to boot very early in the process. If you
+ boot on a system with more memory than you specify, the extra memory
+ will wasted - it will be reserved and not used.
+
+ For systems with discontiguous blocks of physical memory, specify the
+ size of the block starting at 0x0. You can determine this by looking
+ at the memory layout info printed to dmesg by the radix MMU code
+ early in boot. See Documentation/powerpc/kasan.txt.
+
config KASAN_SHADOW_OFFSET
hex
depends on KASAN
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index f35730548e42..eff693527462 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -230,6 +230,17 @@ ifdef CONFIG_476FPE_ERR46
-T $(srctree)/arch/powerpc/platforms/44x/ppc476_modules.lds
endif

+ifdef CONFIG_PPC_BOOK3S_64
+# The KASAN shadow offset is such that linear map (0xc000...) is shadowed by
+# the last 8th of linearly mapped physical memory. This way, if the code uses
+# 0xc addresses throughout, accesses work both in in real mode (where the top
+# 2 bits are ignored) and outside of real mode.
+#
+# 0xc000000000000000 >> 3 = 0xa800000000000000 = 12105675798371893248
+KASAN_SHADOW_OFFSET = $(shell echo 7 \* 1024 \* 1024 \* $(CONFIG_PHYS_MEM_SIZE_FOR_KASAN) / 8 + 12105675798371893248 | bc)
+KBUILD_CFLAGS += -DKASAN_SHADOW_OFFSET=$(KASAN_SHADOW_OFFSET)UL
+endif
+
# No AltiVec or VSX instructions when building kernel
KBUILD_CFLAGS += $(call cc-option,-mno-altivec)
KBUILD_CFLAGS += $(call cc-option,-mno-vsx)
diff --git a/arch/powerpc/include/asm/kasan.h b/arch/powerpc/include/asm/kasan.h
index 296e51c2f066..98d995bc9b5e 100644
--- a/arch/powerpc/include/asm/kasan.h
+++ b/arch/powerpc/include/asm/kasan.h
@@ -14,13 +14,20 @@

#ifndef __ASSEMBLY__

-#include <asm/page.h>
+#ifdef CONFIG_KASAN
+void kasan_init(void);
+#else
+static inline void kasan_init(void) { }
+#endif

#define KASAN_SHADOW_SCALE_SHIFT 3

#define KASAN_SHADOW_START (KASAN_SHADOW_OFFSET + \
(PAGE_OFFSET >> KASAN_SHADOW_SCALE_SHIFT))

+#ifdef CONFIG_PPC32
+#include <asm/page.h>
+
#define KASAN_SHADOW_OFFSET ASM_CONST(CONFIG_KASAN_SHADOW_OFFSET)

#define KASAN_SHADOW_END 0UL
@@ -30,11 +37,18 @@
#ifdef CONFIG_KASAN
void kasan_early_init(void);
void kasan_mmu_init(void);
-void kasan_init(void);
#else
-static inline void kasan_init(void) { }
static inline void kasan_mmu_init(void) { }
#endif
+#endif
+
+#ifdef CONFIG_PPC_BOOK3S_64
+#include <asm/pgtable.h>
+
+#define KASAN_SHADOW_SIZE ((u64)CONFIG_PHYS_MEM_SIZE_FOR_KASAN * \
+ 1024 * 1024 * 1 / 8)
+
+#endif /* CONFIG_PPC_BOOK3S_64 */

#endif /* __ASSEMBLY */
#endif
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 4df94b6e2f32..c60ff299f39b 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -2081,7 +2081,14 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
/*
* See if this is an exception frame.
* We look for the "regshere" marker in the current frame.
+ *
+ * KASAN may complain about this. If it is an exception frame,
+ * we won't have unpoisoned the stack in asm when we set the
+ * exception marker. If it's not an exception frame, who knows
+ * how things are laid out - the shadow could be in any state
+ * at all. Just disable KASAN reporting for now.
*/
+ kasan_disable_current();
if (validate_sp(sp, tsk, STACK_INT_FRAME_SIZE)
&& stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
struct pt_regs *regs = (struct pt_regs *)
@@ -2091,6 +2098,7 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
regs->trap, (void *)regs->nip, (void *)lr);
firstframe = 1;
}
+ kasan_enable_current();

sp = newsp;
} while (count++ < kstack_depth_to_print);
diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 6620f37abe73..b32036f61cad 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -72,6 +72,7 @@ unsigned long tce_alloc_start, tce_alloc_end;
u64 ppc64_rma_size;
#endif
static phys_addr_t first_memblock_size;
+static phys_addr_t top_phys_addr;
static int __initdata boot_cpu_count;

static int __init early_parse_mem(char *p)
@@ -449,6 +450,21 @@ static bool validate_mem_limit(u64 base, u64 *size)
{
u64 max_mem = 1UL << (MAX_PHYSMEM_BITS);

+#ifdef CONFIG_KASAN
+ /*
+ * To handle the NUMA/discontiguous memory case, don't allow a block
+ * to be added if it falls completely beyond the configured physical
+ * memory.
+ *
+ * See Documentation/powerpc/kasan.txt
+ */
+ if (base >= (u64)CONFIG_PHYS_MEM_SIZE_FOR_KASAN * 1024 * 1024) {
+ pr_warn("KASAN: not adding mem block at %llx (size %llx)",
+ base, *size);
+ return false;
+ }
+#endif
+
if (base >= max_mem)
return false;
if ((base + *size) > max_mem)
@@ -572,8 +588,11 @@ void __init early_init_dt_add_memory_arch(u64 base, u64 size)

/* Add the chunk to the MEMBLOCK list */
if (add_mem_to_memblock) {
- if (validate_mem_limit(base, &size))
+ if (validate_mem_limit(base, &size)) {
memblock_add(base, size);
+ if (base + size > top_phys_addr)
+ top_phys_addr = base + size;
+ }
}
}

@@ -613,6 +632,8 @@ static void __init early_reserve_mem_dt(void)
static void __init early_reserve_mem(void)
{
__be64 *reserve_map;
+ phys_addr_t kasan_shadow_start;
+ phys_addr_t kasan_memory_size;

reserve_map = (__be64 *)(((unsigned long)initial_boot_params) +
fdt_off_mem_rsvmap(initial_boot_params));
@@ -651,6 +672,42 @@ static void __init early_reserve_mem(void)
return;
}
#endif
+
+ if (IS_ENABLED(CONFIG_KASAN) && IS_ENABLED(CONFIG_PPC_BOOK3S_64)) {
+ kasan_memory_size =
+ ((phys_addr_t)CONFIG_PHYS_MEM_SIZE_FOR_KASAN << 20);
+
+ if (top_phys_addr < kasan_memory_size) {
+ /*
+ * We are doomed. Attempts to call e.g. panic() are
+ * likely to fail because they call out into
+ * instrumented code, which will almost certainly
+ * access memory beyond the end of physical
+ * memory. Hang here so that at least the NIP points
+ * somewhere that will help you debug it if you look at
+ * it in qemu.
+ */
+ while (true)
+ ;
+ } else if (top_phys_addr > kasan_memory_size) {
+ /* print a biiiig warning in hopes people notice */
+ pr_err("===========================================\n"
+ "Physical memory exceeds compiled-in maximum!\n"
+ "This kernel was compiled for KASAN with %u MB physical memory.\n"
+ "The actual physical memory detected is %llu MB.\n"
+ "Memory above the compiled limit will not be used!\n"
+ "===========================================\n",
+ CONFIG_PHYS_MEM_SIZE_FOR_KASAN,
+ top_phys_addr / (1024 * 1024));
+ }
+
+ kasan_shadow_start = _ALIGN_DOWN(kasan_memory_size * 7 / 8,
+ PAGE_SIZE);
+ DBG("reserving %llx -> %llx for KASAN",
+ kasan_shadow_start, top_phys_addr);
+ memblock_reserve(kasan_shadow_start,
+ top_phys_addr - kasan_shadow_start);
+ }
}

#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
diff --git a/arch/powerpc/mm/kasan/Makefile b/arch/powerpc/mm/kasan/Makefile
index 6577897673dd..f02b15c78e4d 100644
--- a/arch/powerpc/mm/kasan/Makefile
+++ b/arch/powerpc/mm/kasan/Makefile
@@ -2,4 +2,5 @@

KASAN_SANITIZE := n

-obj-$(CONFIG_PPC32) += kasan_init_32.o
+obj-$(CONFIG_PPC32) += init_32.o
+obj-$(CONFIG_PPC_BOOK3S_64) += init_book3s_64.o
diff --git a/arch/powerpc/mm/kasan/kasan_init_32.c b/arch/powerpc/mm/kasan/init_32.c
similarity index 100%
rename from arch/powerpc/mm/kasan/kasan_init_32.c
rename to arch/powerpc/mm/kasan/init_32.c
diff --git a/arch/powerpc/mm/kasan/init_book3s_64.c b/arch/powerpc/mm/kasan/init_book3s_64.c
new file mode 100644
index 000000000000..43e9252c8bd3
--- /dev/null
+++ b/arch/powerpc/mm/kasan/init_book3s_64.c
@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KASAN for 64-bit Book3S powerpc
+ *
+ * Copyright (C) 2019 IBM Corporation
+ * Author: Daniel Axtens <dja@axtens.net>
+ */
+
+#define DISABLE_BRANCH_PROFILING
+
+#include <linux/kasan.h>
+#include <linux/printk.h>
+#include <linux/sched/task.h>
+#include <asm/pgalloc.h>
+
+void __init kasan_init(void)
+{
+ int i;
+ void *k_start = kasan_mem_to_shadow((void *)RADIX_KERN_VIRT_START);
+ void *k_end = kasan_mem_to_shadow((void *)RADIX_VMEMMAP_END);
+
+ pte_t pte = __pte(__pa(kasan_early_shadow_page) |
+ pgprot_val(PAGE_KERNEL) | _PAGE_PTE);
+
+ if (!early_radix_enabled())
+ panic("KASAN requires radix!");
+
+ for (i = 0; i < PTRS_PER_PTE; i++)
+ __set_pte_at(&init_mm, (unsigned long)kasan_early_shadow_page,
+ &kasan_early_shadow_pte[i], pte, 0);
+
+ for (i = 0; i < PTRS_PER_PMD; i++)
+ pmd_populate_kernel(&init_mm, &kasan_early_shadow_pmd[i],
+ kasan_early_shadow_pte);
+
+ for (i = 0; i < PTRS_PER_PUD; i++)
+ pud_populate(&init_mm, &kasan_early_shadow_pud[i],
+ kasan_early_shadow_pmd);
+
+ memset(kasan_mem_to_shadow((void *)PAGE_OFFSET), KASAN_SHADOW_INIT,
+ KASAN_SHADOW_SIZE);
+
+ kasan_populate_early_shadow(
+ kasan_mem_to_shadow((void *)RADIX_KERN_VIRT_START),
+ kasan_mem_to_shadow((void *)RADIX_VMALLOC_START));
+
+ /* leave a hole here for vmalloc */
+
+ kasan_populate_early_shadow(
+ kasan_mem_to_shadow((void *)RADIX_VMALLOC_END),
+ kasan_mem_to_shadow((void *)RADIX_VMEMMAP_END));
+
+ flush_tlb_kernel_range((unsigned long)k_start, (unsigned long)k_end);
+
+ /* mark early shadow region as RO and wipe */
+ pte = __pte(__pa(kasan_early_shadow_page) |
+ pgprot_val(PAGE_KERNEL_RO) | _PAGE_PTE);
+ for (i = 0; i < PTRS_PER_PTE; i++)
+ __set_pte_at(&init_mm, (unsigned long)kasan_early_shadow_page,
+ &kasan_early_shadow_pte[i], pte, 0);
+
+ memset(kasan_early_shadow_page, 0, PAGE_SIZE);
+
+ /* Enable error messages */
+ init_task.kasan_depth = 0;
+ pr_info("KASAN init done (64-bit Book3S heavyweight mode)\n");
+}
--
2.20.1