Re: [PATCH] UML: add support for KASAN under x86_64

Vincent Whitchurch
2022-05-24T10:34:36Z

On Wed, Mar 11, 2020 at 11:44:37PM +0100, Johannes Berg wrote:
> On Wed, 2020-03-11 at 15:32 -0700, Patricia Alfonso wrote:
> > I'll need some time to investigate these all myself. Having just
> > gotten my first module to run about an hour ago, any more information
> > about how you got these errors would be helpful so I can try to
> > reproduce them on my own.
>
> See the other emails, I was basically just loading random modules. In my
> case cfg80211, mac80211, mac80211-hwsim - those are definitely available
> without any (virtio) hardware requirements, so you could use them.
>
> Note that doing a bunch of vmalloc would likely result in similar
> issues, since the module and vmalloc space is the same on UML.

Old thread, but I had a look at this the other day and I think I got it
working. Since the entire shadow area is mapped at init, we don't need
to do any mappings later.

It works both with and without KASAN_VMALLOC. KASAN_STACK works too
after I disabled sanitization of the stacktrace code. All kasan kunit
tests pass and the test_kasan.ko module works too.

Delta patch against Patricia's is below. The CONFIG_UML checks need to
be replaced with something more appropriate (new config? __weak
functions?) and the free functions should probably be hooked up to
madvise(MADV_DONTNEED) so we discard unused pages in the shadow mapping.

Note that there's a KASAN stack-out-of-bounds splat on startup when just
booting UML. That looks like a real (17-year-old) bug, I've posted a
fix for that:

https://lore.kernel.org/lkml/20220523140403.2361040-1-vincent.whitchurch@axis.com/

8<-----------
diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index a1bd8c07ce14..5f3a4d25d57e 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -12,6 +12,7 @@ config UML
select ARCH_NO_PREEMPT
select HAVE_ARCH_AUDITSYSCALL
select HAVE_ARCH_KASAN if X86_64
+ select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ASM_MODVERSIONS
select HAVE_UID16
@@ -223,7 +224,7 @@ config UML_TIME_TRAVEL_SUPPORT
config KASAN_SHADOW_OFFSET
hex
depends on KASAN
- default 0x7fff8000
+ default 0x100000000000
help
This is the offset at which the ~2.25TB of shadow memory is
mapped and used by KASAN for memory debugging. This can be any
diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
index 1c2d4b29a3d4..a089217e2f0e 100644
--- a/arch/um/kernel/Makefile
+++ b/arch/um/kernel/Makefile
@@ -27,6 +27,9 @@ obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
obj-$(CONFIG_STACKTRACE) += stacktrace.o
obj-$(CONFIG_GENERIC_PCI_IOMAP) += ioport.o

+KASAN_SANITIZE_stacktrace.o := n
+KASAN_SANITIZE_sysrq.o := n
+
USER_OBJS := config.o

include arch/um/scripts/Makefile.rules
diff --git a/arch/um/kernel/mem.c b/arch/um/kernel/mem.c
index 7c3196c297f7..a32cfce53efb 100644
--- a/arch/um/kernel/mem.c
+++ b/arch/um/kernel/mem.c
@@ -33,7 +33,7 @@ void kasan_init(void)
}

static void (*kasan_init_ptr)(void)
-__section(.kasan_init) __used
+__section(".kasan_init") __used
= kasan_init;
#endif

diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index 1113cf5fea25..1f3e620188a2 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -152,7 +152,7 @@ config KASAN_STACK
bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
depends on KASAN_GENERIC || KASAN_SW_TAGS
depends on !ARCH_DISABLE_KASAN_INLINE
- default y if CC_IS_GCC && !UML
+ default y if CC_IS_GCC
help
The LLVM stack address sanitizer has a know problem that
causes excessive stack usage in a lot of functions, see
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index a4f07de21771..d8c518bd0e7d 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -295,8 +295,14 @@ int kasan_populate_vmalloc(unsigned long addr, unsigned long size)
return 0;

shadow_start = (unsigned long)kasan_mem_to_shadow((void *)addr);
- shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
shadow_end = (unsigned long)kasan_mem_to_shadow((void *)addr + size);
+
+ if (IS_ENABLED(CONFIG_UML)) {
+ __memset(kasan_mem_to_shadow((void *)addr), KASAN_VMALLOC_INVALID, shadow_end - shadow_start);
+ return 0;
+ }
+
+ shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
shadow_end = ALIGN(shadow_end, PAGE_SIZE);

ret = apply_to_page_range(&init_mm, shadow_start,
@@ -466,6 +472,10 @@ void kasan_release_vmalloc(unsigned long start, unsigned long end,

if (shadow_end > shadow_start) {
size = shadow_end - shadow_start;
+ if (IS_ENABLED(CONFIG_UML)) {
+ __memset(shadow_start, KASAN_SHADOW_INIT, shadow_end - shadow_start);
+ return;
+ }
apply_to_existing_page_range(&init_mm,
(unsigned long)shadow_start,
size, kasan_depopulate_vmalloc_pte,
@@ -531,6 +541,11 @@ int kasan_alloc_module_shadow(void *addr, size_t size, gfp_t gfp_mask)
if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
return -EINVAL;

+ if (IS_ENABLED(CONFIG_UML)) {
+ __memset((void *)shadow_start, KASAN_SHADOW_INIT, shadow_size);
+ return 0;
+ }
+
ret = __vmalloc_node_range(shadow_size, 1, shadow_start,
shadow_start + shadow_size,
GFP_KERNEL,
@@ -554,6 +569,9 @@ int kasan_alloc_module_shadow(void *addr, size_t size, gfp_t gfp_mask)

void kasan_free_module_shadow(const struct vm_struct *vm)
{
+ if (IS_ENABLED(CONFIG_UML))
+ return;
+
if (vm->flags & VM_KASAN)
vfree(kasan_mem_to_shadow(vm->addr));
}