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Subject[RFC PATCH v2] UML: add support for KASAN under x86_64
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Make KASAN run on User Mode Linux on x86_64.

Depends on Constructor support in UML and is based off of
"[RFC PATCH] um: implement CONFIG_CONSTRUCTORS for modules"
(https://patchwork.ozlabs.org/patch/1234551/) and "[DEMO] um:
demonstrate super early constructors"
(https://patchwork.ozlabs.org/patch/1234553/) by
Johannes.

The location of the KASAN shadow memory, starting at
KASAN_SHADOW_OFFSET, can be configured using the
KASAN_SHADOW_OFFSET option. UML uses roughly 18TB of address
space, and KASAN requires 1/8th of this. The default location of
this offset is 0x100000000000. There is usually enough free space at
this location; however, it is a config option so that it can be
easily changed if needed.

The UML-specific KASAN initializer uses mmap to map
the roughly 2.25TB of shadow memory to the location defined by
KASAN_SHADOW_OFFSET.

Signed-off-by: Patricia Alfonso <trishalfonso@google.com>
---

Changes since v1:
- KASAN has been initialized much earlier.
- With the help of Johannes's RFC patch to implement constructors in
UML and Demo showing how kasan_init could take advantage of these
super early constructors, most of the "KASAN_SANITIZE := n" have
been removed.
- Removed extraneous code
- Fixed typos

arch/um/Kconfig | 10 ++++++++++
arch/um/Makefile | 6 ++++++
arch/um/include/asm/dma.h | 1 +
arch/um/include/asm/kasan.h | 30 ++++++++++++++++++++++++++++++
arch/um/kernel/Makefile | 22 ++++++++++++++++++++++
arch/um/kernel/mem.c | 19 +++++++++----------
arch/um/os-Linux/mem.c | 19 +++++++++++++++++++
arch/um/os-Linux/user_syms.c | 4 ++--
arch/x86/um/Makefile | 3 ++-
arch/x86/um/vdso/Makefile | 3 +++
10 files changed, 104 insertions(+), 13 deletions(-)
create mode 100644 arch/um/include/asm/kasan.h

diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index 0917f8443c28..2b76dc273731 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -8,6 +8,7 @@ config UML
select ARCH_HAS_KCOV
select ARCH_NO_PREEMPT
select HAVE_ARCH_AUDITSYSCALL
+ select HAVE_ARCH_KASAN if X86_64
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ASM_MODVERSIONS
select HAVE_UID16
@@ -200,6 +201,15 @@ config UML_TIME_TRAVEL_SUPPORT

It is safe to say Y, but you probably don't need this.

+config KASAN_SHADOW_OFFSET
+ hex
+ depends on KASAN
+ default 0x100000000000
+ help
+ This is the offset at which the ~2.25TB of shadow memory is
+ initialized and used by KASAN for memory debugging. The default
+ is 0x100000000000.
+
endmenu

source "arch/um/drivers/Kconfig"
diff --git a/arch/um/Makefile b/arch/um/Makefile
index d2daa206872d..28fe7a9a1858 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -75,6 +75,12 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__

+# Kernel config options are not included in USER_CFLAGS, but the option for KASAN
+# should be included if the KASAN config option was set.
+ifdef CONFIG_KASAN
+ USER_CFLAGS+=-DCONFIG_KASAN=y
+endif
+
#This will adjust *FLAGS accordingly to the platform.
include $(ARCH_DIR)/Makefile-os-$(OS)

diff --git a/arch/um/include/asm/dma.h b/arch/um/include/asm/dma.h
index fdc53642c718..8aafd60d62bb 100644
--- a/arch/um/include/asm/dma.h
+++ b/arch/um/include/asm/dma.h
@@ -5,6 +5,7 @@
#include <asm/io.h>

extern unsigned long uml_physmem;
+extern unsigned long long physmem_size;

#define MAX_DMA_ADDRESS (uml_physmem)

diff --git a/arch/um/include/asm/kasan.h b/arch/um/include/asm/kasan.h
new file mode 100644
index 000000000000..ba08061068cf
--- /dev/null
+++ b/arch/um/include/asm/kasan.h
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_UM_KASAN_H
+#define __ASM_UM_KASAN_H
+
+#include <linux/init.h>
+#include <linux/const.h>
+
+#define KASAN_SHADOW_OFFSET _AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
+#ifdef CONFIG_X86_64
+#define KASAN_SHADOW_SIZE 0x100000000000UL
+#else
+#error "KASAN_SHADOW_SIZE is not defined for this sub-architecture"
+#endif /* CONFIG_X86_64 */
+
+// used in kasan_mem_to_shadow to divide by 8
+#define KASAN_SHADOW_SCALE_SHIFT 3
+
+#define KASAN_SHADOW_START (KASAN_SHADOW_OFFSET)
+#define KASAN_SHADOW_END (KASAN_SHADOW_START + KASAN_SHADOW_SIZE)
+
+#ifdef CONFIG_KASAN
+void kasan_init(void);
+#else
+static inline void kasan_init(void) { }
+#endif /* CONFIG_KASAN */
+
+void kasan_map_memory(void *start, unsigned long len);
+void kasan_unpoison_shadow(const void *address, size_t size);
+
+#endif /* __ASM_UM_KASAN_H */
diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
index 5aa882011e04..875e1827588b 100644
--- a/arch/um/kernel/Makefile
+++ b/arch/um/kernel/Makefile
@@ -8,6 +8,28 @@
# kernel.
KCOV_INSTRUMENT := n

+# The way UMl deals with the stack causes seemingly false positive KASAN
+# reports such as:
+# BUG: KASAN: stack-out-of-bounds in show_stack+0x15e/0x1fb
+# Read of size 8 at addr 000000006184bbb0 by task swapper/1
+# ==================================================================
+# BUG: KASAN: stack-out-of-bounds in dump_trace+0x141/0x1c5
+# Read of size 8 at addr 0000000071057eb8 by task swapper/1
+# ==================================================================
+# BUG: KASAN: stack-out-of-bounds in get_wchan+0xd7/0x138
+# Read of size 8 at addr 0000000070e8fc80 by task systemd/1
+#
+# With these files removed from instrumentation, those reports are
+# eliminated, but KASAN still repeatedly reports a bug on syscall_stub_data:
+# ==================================================================
+# BUG: KASAN: stack-out-of-bounds in syscall_stub_data+0x299/0x2bf
+# Read of size 128 at addr 0000000071457c50 by task swapper/1
+
+KASAN_SANITIZE_stacktrace.o := n
+KASAN_SANITIZE_sysrq.o := n
+KASAN_SANITIZE_process.o := n
+
+
CPPFLAGS_vmlinux.lds := -DSTART=$(LDS_START) \
-DELF_ARCH=$(LDS_ELF_ARCH) \
-DELF_FORMAT=$(LDS_ELF_FORMAT) \
diff --git a/arch/um/kernel/mem.c b/arch/um/kernel/mem.c
index 32fc941c80f7..7b7b8a0ee724 100644
--- a/arch/um/kernel/mem.c
+++ b/arch/um/kernel/mem.c
@@ -18,21 +18,20 @@
#include <kern_util.h>
#include <mem_user.h>
#include <os.h>
+#include <linux/sched/task.h>

-extern int printf(const char *msg, ...);
-static void early_print(void)
+#ifdef CONFIG_KASAN
+void kasan_init(void)
{
- printf("I'm super early, before constructors\n");
+ kasan_map_memory((void *)KASAN_SHADOW_START, KASAN_SHADOW_SIZE);
+ init_task.kasan_depth = 0;
+ os_info("KernelAddressSanitizer initialized\n");
}

-static void __attribute__((constructor)) constructor_test(void)
-{
- printf("yes, you can see it\n");
-}
-
-static void (*early_print_ptr)(void)
+static void (*kasan_init_ptr)(void)
__attribute__((section(".kasan_init"), used))
- = early_print;
+= kasan_init;
+#endif

/* allocated in paging_init, zeroed in mem_init, and unchanged thereafter */
unsigned long *empty_zero_page = NULL;
diff --git a/arch/um/os-Linux/mem.c b/arch/um/os-Linux/mem.c
index 3c1b77474d2d..da7039721d35 100644
--- a/arch/um/os-Linux/mem.c
+++ b/arch/um/os-Linux/mem.c
@@ -17,6 +17,25 @@
#include <init.h>
#include <os.h>

+/**
+ * kasan_map_memory() - maps memory from @start with a size of @len.
+ * The allocated memory is filled with zeroes upon success.
+ * @start: the start address of the memory to be mapped
+ * @len: the length of the memory to be mapped
+ *
+ * This function is used to map shadow memory for KASAN in uml
+ */
+void kasan_map_memory(void *start, size_t len)
+{
+ if (mmap(start,
+ len,
+ PROT_READ|PROT_WRITE,
+ MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE|MAP_NORESERVE,
+ -1,
+ 0) == MAP_FAILED)
+ os_info("Couldn't allocate shadow memory %s", strerror(errno));
+}
+
/* Set by make_tempfile() during early boot. */
static char *tempdir = NULL;

diff --git a/arch/um/os-Linux/user_syms.c b/arch/um/os-Linux/user_syms.c
index 715594fe5719..cb667c9225ab 100644
--- a/arch/um/os-Linux/user_syms.c
+++ b/arch/um/os-Linux/user_syms.c
@@ -27,10 +27,10 @@ EXPORT_SYMBOL(strstr);
#ifndef __x86_64__
extern void *memcpy(void *, const void *, size_t);
EXPORT_SYMBOL(memcpy);
-#endif
-
EXPORT_SYMBOL(memmove);
EXPORT_SYMBOL(memset);
+#endif
+
EXPORT_SYMBOL(printf);

/* Here, instead, I can provide a fake prototype. Yes, someone cares: genksyms.
diff --git a/arch/x86/um/Makefile b/arch/x86/um/Makefile
index 33c51c064c77..7dbd76c546fe 100644
--- a/arch/x86/um/Makefile
+++ b/arch/x86/um/Makefile
@@ -26,7 +26,8 @@ else

obj-y += syscalls_64.o vdso/

-subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o ../entry/thunk_64.o
+subarch-y = ../lib/csum-partial_64.o ../lib/memcpy_64.o ../entry/thunk_64.o \
+ ../lib/memmove_64.o ../lib/memset_64.o

endif

diff --git a/arch/x86/um/vdso/Makefile b/arch/x86/um/vdso/Makefile
index 0caddd6acb22..450efa0fb694 100644
--- a/arch/x86/um/vdso/Makefile
+++ b/arch/x86/um/vdso/Makefile
@@ -3,6 +3,9 @@
# Building vDSO images for x86.
#

+# do not instrument on vdso because KASAN is not compatible with user mode
+KASAN_SANITIZE := n
+
# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
KCOV_INSTRUMENT := n

--
2.25.0.341.g760bfbb309-goog
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