[RFC PATCH] restrict /dev/mem to idle io memory ranges

From: Dan Williams
Date: Fri Nov 20 2015 - 12:32:24 EST

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This effectively promotes IORESOURCE_BUSY to IORESOURCE_EXCLUSIVE
semantics by default. If userspace really believes it is safe to access
the memory region it can also perform the extra step of disabling an
active driver. This protects device address ranges with read side
effects and otherwise directs userspace to use the driver.

Persistent memory presents a large "mistake surface" to /dev/mem as now
accidental writes can corrupt a filesystem.

Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
arch/arm/Kconfig.debug | 14 --------------
arch/arm64/Kconfig.debug | 14 --------------
arch/powerpc/Kconfig.debug | 12 ------------
arch/s390/Kconfig.debug | 12 ------------
arch/tile/Kconfig | 3 ---
arch/unicore32/Kconfig.debug | 14 --------------
arch/x86/Kconfig.debug | 17 -----------------
kernel/resource.c | 3 +++
lib/Kconfig.debug | 36 ++++++++++++++++++++++++++++++++++++
9 files changed, 39 insertions(+), 86 deletions(-)

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index 259c0ca9c99a..e356357d86bb 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -15,20 +15,6 @@ 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.debug b/arch/arm64/Kconfig.debug
index 04fb73b973f1..e13c4bf84d9e 100644
--- a/arch/arm64/Kconfig.debug
+++ b/arch/arm64/Kconfig.debug
@@ -14,20 +14,6 @@ config ARM64_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.
-
config PID_IN_CONTEXTIDR
bool "Write the current PID to the CONTEXTIDR register"
help
diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index 3a510f4a6b68..a0e44a9c456f 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug
@@ -335,18 +335,6 @@ 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.debug b/arch/s390/Kconfig.debug
index c56878e1245f..26c5d5beb4be 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 106c21bd7f44..7b2d40db11fa 100644
--- a/arch/tile/Kconfig
+++ b/arch/tile/Kconfig
@@ -116,9 +116,6 @@ 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.debug b/arch/unicore32/Kconfig.debug
index 1a3626239843..f075bbe1d46f 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.debug b/arch/x86/Kconfig.debug
index 137dfa96aa14..1116452fcfc2 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/kernel/resource.c b/kernel/resource.c
index f150dbbe6f62..03a8b09f68a8 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -1498,6 +1498,9 @@ int iomem_is_exclusive(u64 addr)
break;
if (p->end < addr)
continue;
+ if (IS_ENABLED(CONFIG_IO_STRICT_DEVMEM)
+ && p->flags & IORESOURCE_BUSY)
+ break;
if (p->flags & IORESOURCE_BUSY &&
p->flags & IORESOURCE_EXCLUSIVE) {
err = 1;
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8c15b29d5adc..a188d7757e26 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1853,3 +1853,39 @@ source "samples/Kconfig"

source "lib/Kconfig.kgdb"

+config STRICT_DEVMEM
+ bool "Filter access to /dev/mem"
+ depends on MMU
+ default y if TILE || PPC || S390
+ ---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 IO_STRICT_DEVMEM
+ bool "Filter I/O access to /dev/mem"
+ depends on STRICT_DEVMEM
+ ---help---
+ If this option is disabled, you allow userspace (root) access
+ to all io memory regardless of whether a driver is actively
+ using that range. 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 *idle* io memory ranges (any non "System
+ RAM" range listed in /proc/iomem). This may break
+ traditional users of /dev/mem if the driver using a given
+ range cannot be disabled.
+
+ If in doubt, say N.

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Next in thread: Arnd Bergmann: "Re: [RFC PATCH] restrict /dev/mem to idle io memory ranges"
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