Re: [PATCH v4 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support

From: Christophe Leroy
Date: Thu Dec 19 2019 - 02:53:14 EST

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On 12/19/2019 12:36 AM, Daniel Axtens wrote:

KASAN support on Book3S is a bit tricky to get right:

- It would be good to support inline instrumentation so as to be able to
catch stack issues that cannot be caught with outline mode.

- Inline instrumentation requires a fixed offset.

- Book3S runs code in real mode after booting. Most notably a lot of KVM
runs in real mode, and it would be good to be able to instrument it.

- Because code runs in real mode after boot, the offset has to point to
valid memory both in and out of real mode.

[ppc64 mm note: The kernel installs a linear mapping at effective
address c000... onward. This is a one-to-one mapping with physical
memory from 0000... onward. Because of how memory accesses work on
powerpc 64-bit Book3S, a kernel pointer in the linear map accesses the
same memory both with translations on (accessing as an 'effective
address'), and with translations off (accessing as a 'real
address'). This works in both guests and the hypervisor. For more
details, see s5.7 of Book III of version 3 of the ISA, in particular
the Storage Control Overview, s5.7.3, and s5.7.5 - noting that this
KASAN implementation currently only supports Radix.]

One approach is just to give up on inline instrumentation. This way all
checks can be delayed until after everything set is up correctly, and the
address-to-shadow calculations can be overridden. However, the features and
speed boost provided by inline instrumentation are worth trying to do
better.

If _at compile time_ it is known how much contiguous physical memory a
system has, the top 1/8th of the first block of physical memory can be set
aside for the shadow. This is a big hammer and comes with 3 big
consequences:

- there's no nice way to handle physically discontiguous memory, so only
the first physical memory block can be used.

- 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.

Implement and document KASAN this way. The current implementation is Radix
only.

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

At the moment, this physical memory limit must be set _even for outline
mode_. This may be changed in a later series - a different implementation
could be added for outline mode that dynamically allocates shadow at a
fixed offset. For example, see https://patchwork.ozlabs.org/patch/795211/

Suggested-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Balbir Singh <bsingharora@xxxxxxxxx> # ppc64 out-of-line radix version
Cc: Christophe Leroy <christophe.leroy@xxxxxx> # ppc32 version
Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>

---
Changes since v3:
- Address further feedback from Christophe.
- Drop changes to stack walking, it looks like the issue I observed is
related to that particular stack, not stack-walking generally.

Changes since v2:

- Address feedback from Christophe around cleanups and docs.
- Address feedback from Balbir: at this point I don't have a good solution
for the issues you identify around the limitations of the inline implementation
but I think that it's worth trying to get the stack instrumentation support.
I'm happy to have an alternative and more flexible outline mode - I had
envisoned this would be called 'lightweight' mode as it imposes fewer restrictions.
I've linked to your implementation. I think it's best to add it in a follow-up series.
- Made the default PHYS_MEM_SIZE_FOR_KASAN value 1024MB. I think most people have
guests with at least that much memory in the Radix 64s case so it's a much
saner default - it means that if you just turn on KASAN without reading the
docs you're much more likely to have a bootable kernel, which you will never
have if the value is set to zero! I'm happy to bikeshed the value if we want.

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 | 112 ++++++++++++++++++-
arch/powerpc/Kconfig | 2 +
arch/powerpc/Kconfig.debug | 21 ++++
arch/powerpc/Makefile | 11 ++
arch/powerpc/include/asm/book3s/64/hash.h | 4 +
arch/powerpc/include/asm/book3s/64/pgtable.h | 7 ++
arch/powerpc/include/asm/book3s/64/radix.h | 5 +
arch/powerpc/include/asm/kasan.h | 21 +++-
arch/powerpc/kernel/prom.c | 61 +++++++++-
arch/powerpc/mm/kasan/Makefile | 1 +
arch/powerpc/mm/kasan/init_book3s_64.c | 70 ++++++++++++
arch/powerpc/platforms/Kconfig.cputype | 1 +
13 files changed, 316 insertions(+), 8 deletions(-)
create mode 100644 arch/powerpc/mm/kasan/init_book3s_64.c

diff --git a/arch/powerpc/include/asm/kasan.h b/arch/powerpc/include/asm/kasan.h
index 296e51c2f066..f18268cbdc33 100644
--- a/arch/powerpc/include/asm/kasan.h
+++ b/arch/powerpc/include/asm/kasan.h
@@ -2,6 +2,9 @@
#ifndef __ASM_KASAN_H
#define __ASM_KASAN_H
+#include <asm/page.h>
+#include <asm/pgtable.h>

What do you need asm/pgtable.h for ?

Build failure due to circular inclusion of asm/pgtable.h:

CC arch/powerpc/kernel/asm-offsets.s
In file included from ./arch/powerpc/include/asm/nohash/32/pgtable.h:77:0,
from ./arch/powerpc/include/asm/nohash/pgtable.h:8,
from ./arch/powerpc/include/asm/pgtable.h:20,
from ./arch/powerpc/include/asm/kasan.h:6,
from ./include/linux/kasan.h:14,
from ./include/linux/slab.h:136,
from ./include/linux/crypto.h:19,
from ./include/crypto/hash.h:11,
from ./include/linux/uio.h:10,
from ./include/linux/socket.h:8,
from ./include/linux/compat.h:15,
from arch/powerpc/kernel/asm-offsets.c:14:
./include/asm-generic/fixmap.h: In function 'fix_to_virt':
./arch/powerpc/include/asm/fixmap.h:28:22: error: 'KASAN_SHADOW_START' undeclared (first use in this function)
#define FIXADDR_TOP (KASAN_SHADOW_START - PAGE_SIZE)
^
./include/asm-generic/fixmap.h:21:27: note: in expansion of macro 'FIXADDR_TOP'
#define __fix_to_virt(x) (FIXADDR_TOP - ((x) << PAGE_SHIFT))
^
./include/asm-generic/fixmap.h:33:9: note: in expansion of macro '__fix_to_virt'
return __fix_to_virt(idx);
^
./arch/powerpc/include/asm/fixmap.h:28:22: note: each undeclared identifier is reported only once for each function it appears in
#define FIXADDR_TOP (KASAN_SHADOW_START - PAGE_SIZE)
^
./include/asm-generic/fixmap.h:21:27: note: in expansion of macro 'FIXADDR_TOP'
#define __fix_to_virt(x) (FIXADDR_TOP - ((x) << PAGE_SHIFT))
^
./include/asm-generic/fixmap.h:33:9: note: in expansion of macro '__fix_to_virt'
return __fix_to_virt(idx);
^
In file included from ./include/linux/bug.h:5:0,
from ./include/linux/thread_info.h:12,
from ./include/asm-generic/preempt.h:5,
from ./arch/powerpc/include/generated/asm/preempt.h:1,
from ./include/linux/preempt.h:78,
from ./include/linux/spinlock.h:51,
from ./include/linux/seqlock.h:36,
from ./include/linux/time.h:6,
from ./include/linux/compat.h:10,
from arch/powerpc/kernel/asm-offsets.c:14:
./include/asm-generic/fixmap.h: In function 'virt_to_fix':
./arch/powerpc/include/asm/fixmap.h:28:22: error: 'KASAN_SHADOW_START' undeclared (first use in this function)
#define FIXADDR_TOP (KASAN_SHADOW_START - PAGE_SIZE)
^
./arch/powerpc/include/asm/bug.h:73:27: note: in definition of macro 'BUG_ON'
if (__builtin_constant_p(x)) { \
^
./include/asm-generic/fixmap.h:38:18: note: in expansion of macro 'FIXADDR_TOP'
BUG_ON(vaddr >= FIXADDR_TOP || vaddr < FIXADDR_START);
^
In file included from ./arch/powerpc/include/asm/nohash/32/pgtable.h:77:0,
from ./arch/powerpc/include/asm/nohash/pgtable.h:8,
from ./arch/powerpc/include/asm/pgtable.h:20,
from ./arch/powerpc/include/asm/kasan.h:6,
from ./include/linux/kasan.h:14,
from ./include/linux/slab.h:136,
from ./include/linux/crypto.h:19,
from ./include/crypto/hash.h:11,
from ./include/linux/uio.h:10,
from ./include/linux/socket.h:8,
from ./include/linux/compat.h:15,
from arch/powerpc/kernel/asm-offsets.c:14:
./arch/powerpc/include/asm/fixmap.h: In function '__set_fixmap':
./arch/powerpc/include/asm/fixmap.h:28:22: error: 'KASAN_SHADOW_START' undeclared (first use in this function)
#define FIXADDR_TOP (KASAN_SHADOW_START - PAGE_SIZE)
^
./include/asm-generic/fixmap.h:21:27: note: in expansion of macro 'FIXADDR_TOP'
#define __fix_to_virt(x) (FIXADDR_TOP - ((x) << PAGE_SHIFT))
^
./arch/powerpc/include/asm/fixmap.h:102:18: note: in expansion of macro '__fix_to_virt'
map_kernel_page(__fix_to_virt(idx), phys, flags);
^
make[2]: *** [arch/powerpc/kernel/asm-offsets.s] Error 1
make[1]: *** [prepare0] Error 2
make: *** [sub-make] Error 2

+
#ifdef CONFIG_KASAN
#define _GLOBAL_KASAN(fn) _GLOBAL(__##fn)
#define _GLOBAL_TOC_KASAN(fn) _GLOBAL_TOC(__##fn)
@@ -14,13 +17,19 @@
#ifndef __ASSEMBLY__
-#include <asm/page.h>
+#ifdef CONFIG_KASAN
+void kasan_init(void);
+#else
+static inline void kasan_init(void) { }
+#endif

I don't think it is worth moving this. Just keep everything out of the #ifdef CONFIG_PPC32. Having undefined/unused functions there shouldn't matter.

#define KASAN_SHADOW_SCALE_SHIFT 3
#define KASAN_SHADOW_START (KASAN_SHADOW_OFFSET + \
(PAGE_OFFSET >> KASAN_SHADOW_SCALE_SHIFT))
+#ifdef CONFIG_PPC32
+
#define KASAN_SHADOW_OFFSET ASM_CONST(CONFIG_KASAN_SHADOW_OFFSET)
#define KASAN_SHADOW_END 0UL
@@ -30,11 +39,17 @@

Keep the block below out of the CONFIG_PPC32 ifdef, don't need to move kasan_init()

#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
+
+#define KASAN_SHADOW_SIZE ((u64)CONFIG_PHYS_MEM_SIZE_FOR_KASAN * \
+ 1024 * 1024 * 1 / 8)

What about:

(ASM_CONST(CONFIG_PHYS_MEM_SIZE_FOR_KASAN) * SZ_1G) >> KASAN_SHADOW_SCALE_SHIFT

+
+#endif /* CONFIG_PPC_BOOK3S_64 */
#endif /* __ASSEMBLY */
#endif
diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 6620f37abe73..f8ef0074b320 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 +450,26 @@ static bool validate_mem_limit(u64 base, u64 *size)
{
u64 max_mem = 1UL << (MAX_PHYSMEM_BITS);
+ /*
+ * To handle the NUMA/discontiguous memory case, don't allow a block
+ * to be added if it falls completely beyond the configured physical
+ * memory. Print an informational message.
+ *
+ * Frustratingly we also see this with qemu - it seems to split the
+ * specified memory into a number of smaller blocks. If this happens
+ * under qemu, it probably represents misconfiguration. So we want
+ * the message to be noticeable, but not shouty.
+ *
+ * See Documentation/powerpc/kasan.txt
+ */
+ if (IS_ENABLED(CONFIG_KASAN) &&
+ (base >= ((u64)CONFIG_PHYS_MEM_SIZE_FOR_KASAN << 20))) {
+ pr_warn("KASAN: not adding memory block at %llx (size %llx)\n"
+ "This could be due to discontiguous memory or kernel misconfiguration.",
+ base, *size);
+ return false;
+ }
+
if (base >= max_mem)
return false;
if ((base + *size) > max_mem)
@@ -572,8 +593,10 @@ 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);
+ top_phys_addr = max(top_phys_addr, base + size);

Build failure, you have to cast (base + size) to (phys_addr_t) as phys_addr_t is not always u64.

CC arch/powerpc/kernel/asm-offsets.s
CALL scripts/checksyscalls.sh
CALL scripts/atomic/check-atomics.sh
CC arch/powerpc/kernel/prom.o
In file included from arch/powerpc/kernel/prom.c:15:0:
arch/powerpc/kernel/prom.c: In function 'early_init_dt_add_memory_arch':
./include/linux/kernel.h:844:29: error: comparison of distinct pointer types lacks a cast [-Werror]
(!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
^
./include/linux/kernel.h:858:4: note: in expansion of macro '__typecheck'
(__typecheck(x, y) && __no_side_effects(x, y))
^
./include/linux/kernel.h:868:24: note: in expansion of macro '__safe_cmp'
__builtin_choose_expr(__safe_cmp(x, y), \
^
./include/linux/kernel.h:884:19: note: in expansion of macro '__careful_cmp'
#define max(x, y) __careful_cmp(x, y, >)
^
arch/powerpc/kernel/prom.c:598:20: note: in expansion of macro 'max'
top_phys_addr = max(top_phys_addr, base + size);
^
cc1: all warnings being treated as errors
make[3]: *** [arch/powerpc/kernel/prom.o] Error 1
make[2]: *** [arch/powerpc/kernel] Error 2
make[1]: *** [arch/powerpc] Error 2
make: *** [sub-make] Error 2

+ }
}
}
@@ -613,6 +636,8 @@ 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 +676,40 @@ 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. We shouldn't even be able to get this
+ * far, but we do in qemu. If we continue and turn
+ * relocations on, we'll take fatal page faults for
+ * memory that's not physically present. Instead,
+ * panic() here: it will be saved to __log_buf even if
+ * it doesn't get printed to the console.
+ */
+ panic("Tried to boot a KASAN kernel configured for %u MB with only %llu MB! Aborting.",
+ CONFIG_PHYS_MEM_SIZE_FOR_KASAN,
+ (u64)(top_phys_addr >> 20));
+ } 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 physical memory detected is at least %llu MB.\n"
+ "Memory above the compiled limit will not be used!\n"
+ "===========================================\n",
+ CONFIG_PHYS_MEM_SIZE_FOR_KASAN,
+ (u64)(top_phys_addr >> 20));
+ }
+
+ 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

Christophe

Next message: David Howells: "Re: [PATCH 0/2] pipe: Fixes [ver #2]"
Previous message: Chuhong Yuan: "[PATCH] media: omap3isp: fix the incorrect check in omap3isp_csiphy_acquire"
In reply to: Daniel Axtens: "[PATCH v4 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support"
Next in thread: Daniel Axtens: "Re: [PATCH v4 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support"
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