From 90a545e981267e917b9d698ce07affd69787db87 Mon Sep 17 00:00:00 2001 From: Dan Williams Date: Mon, 23 Nov 2015 15:49:03 -0800 Subject: [PATCH] restrict /dev/mem to idle io memory ranges 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. In general if a device driver is busily using a memory region it already informs other parts of the kernel to not touch it via request_mem_region(). /dev/mem should honor the same safety restriction by default. Debugging a device driver from userspace becomes more difficult with this enabled. Any application using /dev/mem or mmap of sysfs pci resources will now need to perform the extra step of either: 1/ Disabling the driver, for example: echo > /dev/bus//drivers//unbind 2/ Rebooting with "iomem=relaxed" on the command line 3/ Recompiling with CONFIG_IO_STRICT_DEVMEM=n Traditional users of /dev/mem like dosemu are unaffected because the first 1MB of memory is not subject to the IO_STRICT_DEVMEM restriction. Legacy X configurations use /dev/mem to talk to graphics hardware, but that functionality has since moved to kernel graphics drivers. Cc: Arnd Bergmann Cc: Russell King Cc: Andrew Morton Cc: Greg Kroah-Hartman Acked-by: Kees Cook Acked-by: Ingo Molnar Signed-off-by: Dan Williams --- kernel/resource.c | 11 +++++++++-- lib/Kconfig.debug | 25 +++++++++++++++++++++---- 2 files changed, 30 insertions(+), 6 deletions(-) diff --git a/kernel/resource.c b/kernel/resource.c index f150dbbe6f62..09c0597840b0 100644 --- a/kernel/resource.c +++ b/kernel/resource.c @@ -1498,8 +1498,15 @@ int iomem_is_exclusive(u64 addr) break; if (p->end < addr) continue; - if (p->flags & IORESOURCE_BUSY && - p->flags & IORESOURCE_EXCLUSIVE) { + /* + * A resource is exclusive if IORESOURCE_EXCLUSIVE is set + * or CONFIG_IO_STRICT_DEVMEM is enabled and the + * resource is busy. + */ + if ((p->flags & IORESOURCE_BUSY) == 0) + continue; + if (IS_ENABLED(CONFIG_IO_STRICT_DEVMEM) + || p->flags & IORESOURCE_EXCLUSIVE) { err = 1; break; } diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 289dfcbc14eb..073496dea848 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -1869,9 +1869,26 @@ config STRICT_DEVMEM 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 this option is switched on, and IO_STRICT_DEVMEM=n, 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 + default 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 kernel drivers. + + If this option is switched on, the /dev/mem file only allows + userspace access to *idle* io-memory ranges (see /proc/iomem) This + may break traditional users of /dev/mem (dosemu, legacy X, etc...) + if the driver using a given range cannot be disabled. If in doubt, say Y.