linux/Documentation/arm/memory.rst
Linus Walleij c12366ba44 ARM: 9015/2: Define the virtual space of KASan's shadow region
Define KASAN_SHADOW_OFFSET,KASAN_SHADOW_START and KASAN_SHADOW_END for
the Arm kernel address sanitizer. We are "stealing" lowmem (the 4GB
addressable by a 32bit architecture) out of the virtual address
space to use as shadow memory for KASan as follows:

 +----+ 0xffffffff
 |    |
 |    | |-> Static kernel image (vmlinux) BSS and page table
 |    |/
 +----+ PAGE_OFFSET
 |    |
 |    | |->  Loadable kernel modules virtual address space area
 |    |/
 +----+ MODULES_VADDR = KASAN_SHADOW_END
 |    |
 |    | |-> The shadow area of kernel virtual address.
 |    |/
 +----+->  TASK_SIZE (start of kernel space) = KASAN_SHADOW_START the
 |    |   shadow address of MODULES_VADDR
 |    | |
 |    | |
 |    | |-> The user space area in lowmem. The kernel address
 |    | |   sanitizer do not use this space, nor does it map it.
 |    | |
 |    | |
 |    | |
 |    | |
 |    |/
 ------ 0

0 .. TASK_SIZE is the memory that can be used by shared
userspace/kernelspace. It us used for userspace processes and for
passing parameters and memory buffers in system calls etc. We do not
need to shadow this area.

KASAN_SHADOW_START:
 This value begins with the MODULE_VADDR's shadow address. It is the
 start of kernel virtual space. Since we have modules to load, we need
 to cover also that area with shadow memory so we can find memory
 bugs in modules.

KASAN_SHADOW_END
 This value is the 0x100000000's shadow address: the mapping that would
 be after the end of the kernel memory at 0xffffffff. It is the end of
 kernel address sanitizer shadow area. It is also the start of the
 module area.

KASAN_SHADOW_OFFSET:
 This value is used to map an address to the corresponding shadow
 address by the following formula:

   shadow_addr = (address >> 3) + KASAN_SHADOW_OFFSET;

 As you would expect, >> 3 is equal to dividing by 8, meaning each
 byte in the shadow memory covers 8 bytes of kernel memory, so one
 bit shadow memory per byte of kernel memory is used.

 The KASAN_SHADOW_OFFSET is provided in a Kconfig option depending
 on the VMSPLIT layout of the system: the kernel and userspace can
 split up lowmem in different ways according to needs, so we calculate
 the shadow offset depending on this.

When kasan is enabled, the definition of TASK_SIZE is not an 8-bit
rotated constant, so we need to modify the TASK_SIZE access code in the
*.s file.

The kernel and modules may use different amounts of memory,
according to the VMSPLIT configuration, which in turn
determines the PAGE_OFFSET.

We use the following KASAN_SHADOW_OFFSETs depending on how the
virtual memory is split up:

- 0x1f000000 if we have 1G userspace / 3G kernelspace split:
  - The kernel address space is 3G (0xc0000000)
  - PAGE_OFFSET is then set to 0x40000000 so the kernel static
    image (vmlinux) uses addresses 0x40000000 .. 0xffffffff
  - On top of that we have the MODULES_VADDR which under
    the worst case (using ARM instructions) is
    PAGE_OFFSET - 16M (0x01000000) = 0x3f000000
    so the modules use addresses 0x3f000000 .. 0x3fffffff
  - So the addresses 0x3f000000 .. 0xffffffff need to be
    covered with shadow memory. That is 0xc1000000 bytes
    of memory.
  - 1/8 of that is needed for its shadow memory, so
    0x18200000 bytes of shadow memory is needed. We
    "steal" that from the remaining lowmem.
  - The KASAN_SHADOW_START becomes 0x26e00000, to
    KASAN_SHADOW_END at 0x3effffff.
  - Now we can calculate the KASAN_SHADOW_OFFSET for any
    kernel address as 0x3f000000 needs to map to the first
    byte of shadow memory and 0xffffffff needs to map to
    the last byte of shadow memory. Since:
    SHADOW_ADDR = (address >> 3) + KASAN_SHADOW_OFFSET
    0x26e00000 = (0x3f000000 >> 3) + KASAN_SHADOW_OFFSET
    KASAN_SHADOW_OFFSET = 0x26e00000 - (0x3f000000 >> 3)
    KASAN_SHADOW_OFFSET = 0x26e00000 - 0x07e00000
    KASAN_SHADOW_OFFSET = 0x1f000000

- 0x5f000000 if we have 2G userspace / 2G kernelspace split:
  - The kernel space is 2G (0x80000000)
  - PAGE_OFFSET is set to 0x80000000 so the kernel static
    image uses 0x80000000 .. 0xffffffff.
  - On top of that we have the MODULES_VADDR which under
    the worst case (using ARM instructions) is
    PAGE_OFFSET - 16M (0x01000000) = 0x7f000000
    so the modules use addresses 0x7f000000 .. 0x7fffffff
  - So the addresses 0x7f000000 .. 0xffffffff need to be
    covered with shadow memory. That is 0x81000000 bytes
    of memory.
  - 1/8 of that is needed for its shadow memory, so
    0x10200000 bytes of shadow memory is needed. We
    "steal" that from the remaining lowmem.
  - The KASAN_SHADOW_START becomes 0x6ee00000, to
    KASAN_SHADOW_END at 0x7effffff.
  - Now we can calculate the KASAN_SHADOW_OFFSET for any
    kernel address as 0x7f000000 needs to map to the first
    byte of shadow memory and 0xffffffff needs to map to
    the last byte of shadow memory. Since:
    SHADOW_ADDR = (address >> 3) + KASAN_SHADOW_OFFSET
    0x6ee00000 = (0x7f000000 >> 3) + KASAN_SHADOW_OFFSET
    KASAN_SHADOW_OFFSET = 0x6ee00000 - (0x7f000000 >> 3)
    KASAN_SHADOW_OFFSET = 0x6ee00000 - 0x0fe00000
    KASAN_SHADOW_OFFSET = 0x5f000000

- 0x9f000000 if we have 3G userspace / 1G kernelspace split,
  and this is the default split for ARM:
  - The kernel address space is 1GB (0x40000000)
  - PAGE_OFFSET is set to 0xc0000000 so the kernel static
    image uses 0xc0000000 .. 0xffffffff.
  - On top of that we have the MODULES_VADDR which under
    the worst case (using ARM instructions) is
    PAGE_OFFSET - 16M (0x01000000) = 0xbf000000
    so the modules use addresses 0xbf000000 .. 0xbfffffff
  - So the addresses 0xbf000000 .. 0xffffffff need to be
    covered with shadow memory. That is 0x41000000 bytes
    of memory.
  - 1/8 of that is needed for its shadow memory, so
    0x08200000 bytes of shadow memory is needed. We
    "steal" that from the remaining lowmem.
  - The KASAN_SHADOW_START becomes 0xb6e00000, to
    KASAN_SHADOW_END at 0xbfffffff.
  - Now we can calculate the KASAN_SHADOW_OFFSET for any
    kernel address as 0xbf000000 needs to map to the first
    byte of shadow memory and 0xffffffff needs to map to
    the last byte of shadow memory. Since:
    SHADOW_ADDR = (address >> 3) + KASAN_SHADOW_OFFSET
    0xb6e00000 = (0xbf000000 >> 3) + KASAN_SHADOW_OFFSET
    KASAN_SHADOW_OFFSET = 0xb6e00000 - (0xbf000000 >> 3)
    KASAN_SHADOW_OFFSET = 0xb6e00000 - 0x17e00000
    KASAN_SHADOW_OFFSET = 0x9f000000

- 0x8f000000 if we have 3G userspace / 1G kernelspace with
  full 1 GB low memory (VMSPLIT_3G_OPT):
  - The kernel address space is 1GB (0x40000000)
  - PAGE_OFFSET is set to 0xb0000000 so the kernel static
    image uses 0xb0000000 .. 0xffffffff.
  - On top of that we have the MODULES_VADDR which under
    the worst case (using ARM instructions) is
    PAGE_OFFSET - 16M (0x01000000) = 0xaf000000
    so the modules use addresses 0xaf000000 .. 0xaffffff
  - So the addresses 0xaf000000 .. 0xffffffff need to be
    covered with shadow memory. That is 0x51000000 bytes
    of memory.
  - 1/8 of that is needed for its shadow memory, so
    0x0a200000 bytes of shadow memory is needed. We
    "steal" that from the remaining lowmem.
  - The KASAN_SHADOW_START becomes 0xa4e00000, to
    KASAN_SHADOW_END at 0xaeffffff.
  - Now we can calculate the KASAN_SHADOW_OFFSET for any
    kernel address as 0xaf000000 needs to map to the first
    byte of shadow memory and 0xffffffff needs to map to
    the last byte of shadow memory. Since:
    SHADOW_ADDR = (address >> 3) + KASAN_SHADOW_OFFSET
    0xa4e00000 = (0xaf000000 >> 3) + KASAN_SHADOW_OFFSET
    KASAN_SHADOW_OFFSET = 0xa4e00000 - (0xaf000000 >> 3)
    KASAN_SHADOW_OFFSET = 0xa4e00000 - 0x15e00000
    KASAN_SHADOW_OFFSET = 0x8f000000

- The default value of 0xffffffff for KASAN_SHADOW_OFFSET
  is an error value. We should always match one of the
  above shadow offsets.

When we do this, TASK_SIZE will sometimes get a bit odd values
that will not fit into immediate mov assembly instructions.
To account for this, we need to rewrite some assembly using
TASK_SIZE like this:

-       mov     r1, #TASK_SIZE
+       ldr     r1, =TASK_SIZE

or

-       cmp     r4, #TASK_SIZE
+       ldr     r0, =TASK_SIZE
+       cmp     r4, r0

this is done to avoid the immediate #TASK_SIZE that need to
fit into a limited number of bits.

Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: kasan-dev@googlegroups.com
Cc: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Ard Biesheuvel <ardb@kernel.org> # QEMU/KVM/mach-virt/LPAE/8G
Tested-by: Florian Fainelli <f.fainelli@gmail.com> # Brahma SoCs
Tested-by: Ahmad Fatoum <a.fatoum@pengutronix.de> # i.MX6Q
Reported-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Abbott Liu <liuwenliang@huawei.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
2020-10-27 12:11:08 +00:00

104 lines
3.7 KiB
ReStructuredText

=================================
Kernel Memory Layout on ARM Linux
=================================
Russell King <rmk@arm.linux.org.uk>
November 17, 2005 (2.6.15)
This document describes the virtual memory layout which the Linux
kernel uses for ARM processors. It indicates which regions are
free for platforms to use, and which are used by generic code.
The ARM CPU is capable of addressing a maximum of 4GB virtual memory
space, and this must be shared between user space processes, the
kernel, and hardware devices.
As the ARM architecture matures, it becomes necessary to reserve
certain regions of VM space for use for new facilities; therefore
this document may reserve more VM space over time.
=============== =============== ===============================================
Start End Use
=============== =============== ===============================================
ffff8000 ffffffff copy_user_page / clear_user_page use.
For SA11xx and Xscale, this is used to
setup a minicache mapping.
ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
ffff1000 ffff7fff Reserved.
Platforms must not use this address range.
ffff0000 ffff0fff CPU vector page.
The CPU vectors are mapped here if the
CPU supports vector relocation (control
register V bit.)
fffe0000 fffeffff XScale cache flush area. This is used
in proc-xscale.S to flush the whole data
cache. (XScale does not have TCM.)
fffe8000 fffeffff DTCM mapping area for platforms with
DTCM mounted inside the CPU.
fffe0000 fffe7fff ITCM mapping area for platforms with
ITCM mounted inside the CPU.
ffc80000 ffefffff Fixmap mapping region. Addresses provided
by fix_to_virt() will be located here.
ffc00000 ffc7ffff Guard region
ff800000 ffbfffff Permanent, fixed read-only mapping of the
firmware provided DT blob
fee00000 feffffff Mapping of PCI I/O space. This is a static
mapping within the vmalloc space.
VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
Memory returned by vmalloc/ioremap will
be dynamically placed in this region.
Machine specific static mappings are also
located here through iotable_init().
VMALLOC_START is based upon the value
of the high_memory variable, and VMALLOC_END
is equal to 0xff800000.
PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
This maps the platforms RAM, and typically
maps all platform RAM in a 1:1 relationship.
PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
One way of mapping HIGHMEM pages into kernel
space.
MODULES_VADDR MODULES_END-1 Kernel module space
Kernel modules inserted via insmod are
placed here using dynamic mappings.
TASK_SIZE MODULES_VADDR-1 KASAn shadow memory when KASan is in use.
The range from MODULES_VADDR to the top
of the memory is shadowed here with 1 bit
per byte of memory.
00001000 TASK_SIZE-1 User space mappings
Per-thread mappings are placed here via
the mmap() system call.
00000000 00000fff CPU vector page / null pointer trap
CPUs which do not support vector remapping
place their vector page here. NULL pointer
dereferences by both the kernel and user
space are also caught via this mapping.
=============== =============== ===============================================
Please note that mappings which collide with the above areas may result
in a non-bootable kernel, or may cause the kernel to (eventually) panic
at run time.
Since future CPUs may impact the kernel mapping layout, user programs
must not access any memory which is not mapped inside their 0x0001000
to TASK_SIZE address range. If they wish to access these areas, they
must set up their own mappings using open() and mmap().