Re: [RFC PATCH] make CONFIG_STRICT_DEVMEM a core non-debug feature
From: Kees Cook
Date: Tue Nov 04 2014 - 13:43:06 EST
On Tue, Nov 4, 2014 at 9:23 AM, Leif Lindholm <leif.lindholm@xxxxxxxxxx> wrote:
> Most, but not all, architectures supporting CONFIG_STRICT_DEVMEM treat
> it as a debug feature - although its function is pretty much the
> opposite of debug.
>
> This patch deletes all architecture-specific config options and moves
> the option to core code, as a non-debug option.
>
> Signed-off-by: Leif Lindholm <leif.lindholm@xxxxxxxxxx>
> ---
> Apologies for wide distribution, but this is a bit invasive,
> if trivial.
>
> arch/Kconfig | 3 +++
> arch/arm/Kconfig | 1 +
> arch/arm/Kconfig.debug | 14 --------------
> arch/arm64/Kconfig | 1 +
> arch/arm64/Kconfig.debug | 14 --------------
> arch/powerpc/Kconfig | 1 +
> arch/powerpc/Kconfig.debug | 12 ------------
> arch/s390/Kconfig | 1 +
> arch/s390/Kconfig.debug | 12 ------------
> arch/tile/Kconfig | 4 +---
> arch/unicore32/Kconfig | 1 +
> arch/unicore32/Kconfig.debug | 14 --------------
> arch/x86/Kconfig | 1 +
> arch/x86/Kconfig.debug | 17 -----------------
> drivers/char/Kconfig | 16 ++++++++++++++++
> 15 files changed, 26 insertions(+), 86 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 05d7a8a..e5f4f03 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -484,6 +484,9 @@ config HAVE_IRQ_EXIT_ON_IRQ_STACK
> This spares a stack switch and improves cache usage on softirq
> processing.
>
> +config HAVE_ARCH_RESTRICTED_DEVMEM
> + bool
> +
> #
> # ABI hall of shame
> #
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 89c4b5c..e04910f 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -30,6 +30,7 @@ config ARM
> select HAVE_ARCH_AUDITSYSCALL if (AEABI && !OABI_COMPAT)
> select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
> select HAVE_ARCH_KGDB
> + select HAVE_ARCH_RESTRICTED_DEVMEM if MMU
> select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
> select HAVE_ARCH_TRACEHOOK
> select HAVE_BPF_JIT
> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
> index d8f6a2e..2a02a7d 100644
> --- a/arch/arm/Kconfig.debug
> +++ b/arch/arm/Kconfig.debug
> @@ -14,20 +14,6 @@ config ARM_PTDUMP
> kernel.
> If in doubt, say "N"
>
> -config STRICT_DEVMEM
> - bool "Filter access to /dev/mem"
> - depends on MMU
> - ---help---
> - If this option is disabled, you allow userspace (root) access to all
> - of memory, including kernel and userspace memory. Accidental
> - access to this is obviously disastrous, but specific access can
> - be used by people debugging the kernel.
> -
> - If this option is switched on, the /dev/mem file only allows
> - userspace access to memory mapped peripherals.
> -
> - If in doubt, say Y.
> -
> # RMK wants arm kernels compiled with frame pointers or stack unwinding.
> # If you know what you are doing and are willing to live without stack
> # traces, you can get a slightly smaller kernel by setting this option to
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 9532f8d..90abbb2 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -37,6 +37,7 @@ config ARM64
> select HAVE_ARCH_AUDITSYSCALL
> select HAVE_ARCH_JUMP_LABEL
> select HAVE_ARCH_KGDB
> + select HAVE_ARCH_RESTRICTED_DEVMEM if MMU
> select HAVE_ARCH_TRACEHOOK
> select HAVE_BPF_JIT
> select HAVE_C_RECORDMCOUNT
> diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
> index 0a12933..d0036dd 100644
> --- a/arch/arm64/Kconfig.debug
> +++ b/arch/arm64/Kconfig.debug
> @@ -6,20 +6,6 @@ config FRAME_POINTER
> bool
> default y
>
> -config STRICT_DEVMEM
> - bool "Filter access to /dev/mem"
> - depends on MMU
> - help
> - If this option is disabled, you allow userspace (root) access to all
> - of memory, including kernel and userspace memory. Accidental
> - access to this is obviously disastrous, but specific access can
> - be used by people debugging the kernel.
> -
> - If this option is switched on, the /dev/mem file only allows
> - userspace access to memory mapped peripherals.
> -
> - If in doubt, say Y.
> -
> config PID_IN_CONTEXTIDR
> bool "Write the current PID to the CONTEXTIDR register"
> help
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 88eace4..cd35068 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -148,6 +148,7 @@ config PPC
> select HAVE_ARCH_AUDITSYSCALL
> select ARCH_SUPPORTS_ATOMIC_RMW
> select DCACHE_WORD_ACCESS if PPC64 && CPU_LITTLE_ENDIAN
> + select HAVE_ARCH_RESTRICTED_DEVMEM
>
> config GENERIC_CSUM
> def_bool CPU_LITTLE_ENDIAN
> diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
> index ec2e40f..6f11382 100644
> --- a/arch/powerpc/Kconfig.debug
> +++ b/arch/powerpc/Kconfig.debug
> @@ -334,18 +334,6 @@ config PPC_EARLY_DEBUG_CPM_ADDR
> platform probing is done, all platforms selected must
> share the same address.
>
> -config STRICT_DEVMEM
> - def_bool y
> - prompt "Filter access to /dev/mem"
> - help
> - This option restricts access to /dev/mem. If this option is
> - disabled, you allow userspace access to all memory, including
> - kernel and userspace memory. Accidental memory access is likely
> - to be disastrous.
> - Memory access is required for experts who want to debug the kernel.
> -
> - If you are unsure, say Y.
> -
> config FAIL_IOMMU
> bool "Fault-injection capability for IOMMU"
> depends on FAULT_INJECTION
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index f2cf1f9..fe92272 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -109,6 +109,7 @@ config S390
> select HAVE_ALIGNED_STRUCT_PAGE if SLUB
> select HAVE_ARCH_AUDITSYSCALL
> select HAVE_ARCH_JUMP_LABEL if !MARCH_G5
> + select HAVE_ARCH_RESTRICTED_DEVMEM
> select HAVE_ARCH_SECCOMP_FILTER
> select HAVE_ARCH_TRACEHOOK
> select HAVE_ARCH_TRANSPARENT_HUGEPAGE if 64BIT
> diff --git a/arch/s390/Kconfig.debug b/arch/s390/Kconfig.debug
> index c56878e..26c5d5be 100644
> --- a/arch/s390/Kconfig.debug
> +++ b/arch/s390/Kconfig.debug
> @@ -5,18 +5,6 @@ config TRACE_IRQFLAGS_SUPPORT
>
> source "lib/Kconfig.debug"
>
> -config STRICT_DEVMEM
> - def_bool y
> - prompt "Filter access to /dev/mem"
> - ---help---
> - This option restricts access to /dev/mem. If this option is
> - disabled, you allow userspace access to all memory, including
> - kernel and userspace memory. Accidental memory access is likely
> - to be disastrous.
> - Memory access is required for experts who want to debug the kernel.
> -
> - If you are unsure, say Y.
> -
> config S390_PTDUMP
> bool "Export kernel pagetable layout to userspace via debugfs"
> depends on DEBUG_KERNEL
> diff --git a/arch/tile/Kconfig b/arch/tile/Kconfig
> index 7cca418..f881168 100644
> --- a/arch/tile/Kconfig
> +++ b/arch/tile/Kconfig
> @@ -27,6 +27,7 @@ config TILE
> select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
> select HAVE_DEBUG_STACKOVERFLOW
> select ARCH_WANT_FRAME_POINTERS
> + select HAVE_ARCH_RESTRICTED_DEVMEM
>
> # FIXME: investigate whether we need/want these options.
> # select HAVE_IOREMAP_PROT
> @@ -110,9 +111,6 @@ config ARCH_DISCONTIGMEM_DEFAULT
> config TRACE_IRQFLAGS_SUPPORT
> def_bool y
>
> -config STRICT_DEVMEM
> - def_bool y
> -
> # SMP is required for Tilera Linux.
> config SMP
> def_bool y
> diff --git a/arch/unicore32/Kconfig b/arch/unicore32/Kconfig
> index 928237a..574b07f 100644
> --- a/arch/unicore32/Kconfig
> +++ b/arch/unicore32/Kconfig
> @@ -18,6 +18,7 @@ config UNICORE32
> select ARCH_WANT_FRAME_POINTERS
> select GENERIC_IOMAP
> select MODULES_USE_ELF_REL
> + select HAVE_ARCH_RESTRICTED_DEVMEM if MMU
> help
> UniCore-32 is 32-bit Instruction Set Architecture,
> including a series of low-power-consumption RISC chip
> diff --git a/arch/unicore32/Kconfig.debug b/arch/unicore32/Kconfig.debug
> index 1a36262..f075bbe 100644
> --- a/arch/unicore32/Kconfig.debug
> +++ b/arch/unicore32/Kconfig.debug
> @@ -2,20 +2,6 @@ menu "Kernel hacking"
>
> source "lib/Kconfig.debug"
>
> -config STRICT_DEVMEM
> - bool "Filter access to /dev/mem"
> - depends on MMU
> - ---help---
> - If this option is disabled, you allow userspace (root) access to all
> - of memory, including kernel and userspace memory. Accidental
> - access to this is obviously disastrous, but specific access can
> - be used by people debugging the kernel.
> -
> - If this option is switched on, the /dev/mem file only allows
> - userspace access to memory mapped peripherals.
> -
> - If in doubt, say Y.
> -
> config EARLY_PRINTK
> def_bool DEBUG_OCD
> help
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index ded8a67..91392bf 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -137,6 +137,7 @@ config X86
> select HAVE_ACPI_APEI_NMI if ACPI
> select ACPI_LEGACY_TABLES_LOOKUP if ACPI
> select X86_FEATURE_NAMES if PROC_FS
> + select HAVE_ARCH_RESTRICTED_DEVMEM
>
> config INSTRUCTION_DECODER
> def_bool y
> diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
> index 61bd2ad..e35d4a1 100644
> --- a/arch/x86/Kconfig.debug
> +++ b/arch/x86/Kconfig.debug
> @@ -5,23 +5,6 @@ config TRACE_IRQFLAGS_SUPPORT
>
> source "lib/Kconfig.debug"
>
> -config STRICT_DEVMEM
> - bool "Filter access to /dev/mem"
> - ---help---
> - If this option is disabled, you allow userspace (root) access to all
> - of memory, including kernel and userspace memory. Accidental
> - access to this is obviously disastrous, but specific access can
> - be used by people debugging the kernel. Note that with PAT support
> - enabled, even in this case there are restrictions on /dev/mem
> - use due to the cache aliasing requirements.
> -
> - If this option is switched on, the /dev/mem file only allows
> - userspace access to PCI space and the BIOS code and data regions.
> - This is sufficient for dosemu and X and all common users of
> - /dev/mem.
> -
> - If in doubt, say Y.
> -
> config X86_VERBOSE_BOOTUP
> bool "Enable verbose x86 bootup info messages"
> default y
> diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
> index efefd12..39f7817 100644
> --- a/drivers/char/Kconfig
> +++ b/drivers/char/Kconfig
> @@ -6,6 +6,22 @@ menu "Character devices"
>
> source "drivers/tty/Kconfig"
>
> +config STRICT_DEVMEM
> + bool "Reduced access to /dev/mem"
> + depends on HAVE_ARCH_RESTRICTED_DEVMEM
> + default y
> + help
> + If this option is disabled, you allow userspace (root) access to all
> + of memory, including kernel and userspace memory. Accidental
> + access to this is obviously disastrous, but specific access can
> + be used by people debugging the kernel.
> +
> + If this option is switched on, the /dev/mem file restricts userspace
> + access to an architecture-specific subset of the physical address
> + space.
Great consolidation, thanks! I would probably expand this help text a
bit to include some of details mentioned in the x86 portion of the
option. For example:
If this option is switched on, the /dev/mem file restricts userspace
access to an architecture-specific subset of the physical address
space. For example on x86, PCI space and BIOS code and data
regions. This is sufficient for things like dosemu and non-KMS
Xorg and all common users of /dev/mem.
Thanks!
-Kees
> +
> + If in doubt, say Y.
> +
> config DEVKMEM
> bool "/dev/kmem virtual device support"
> default y
> --
> 1.7.10.4
>
--
Kees Cook
Chrome OS Security
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