Re: [RFC PATCH v2] UML: add support for KASAN under x86_64

From: Dmitry Vyukov
Date: Tue Feb 11 2020 - 08:03:12 EST

On Mon, Feb 10, 2020 at 11:58 PM 'Patricia Alfonso' via kasan-dev
<kasan-dev@xxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> ---
>
> 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


I started reviewing this, but I am spotting things that I already
commented on, like shadow start and about shadow size const. Please
either address them, or answer why they are not addressed, or add some
kind of TODOs so that I don't write the same comment again.


> 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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