x86: introduce /dev/mem restrictions with a config option

This patch introduces a restriction on /dev/mem: Only non-memory can be
read or written unless the newly introduced config option is set.

The X server needs access to /dev/mem for the PCI space, but it doesn't need
access to memory; both the file permissions and SELinux permissions of /dev/mem
just make X effectively super-super powerful. With the exception of the
BIOS area, there's just no valid app that uses /dev/mem on actual memory.
Other popular users of /dev/mem are rootkits and the like.
(note: mmap access of memory via /dev/mem was already not allowed since
a really long time)

People who want to use /dev/mem for kernel debugging can enable the config
option.

The restrictions of this patch have been in the Fedora and RHEL kernels for
at least 4 years without any problems.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

5 files changed, 80 insertions, 0 deletions

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 610aaecc19f8..0c1890c41279 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -5,6 +5,18 @@ config TRACE_IRQFLAGS_SUPPORT
 
 source "lib/Kconfig.debug"
 
+config NONPROMISC_DEVMEM
+        bool "Disable promiscuous /dev/mem"
+        default y
+        help
+          The /dev/mem file by default 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. With this config
+          option, you allow userspace access to all of memory, including
+          kernel and userspace memory. Accidental access to this is
+          obviously disasterous, but specific access can be used by people
+          debugging the kernel.
+
 config EARLY_PRINTK
         bool "Early printk" if EMBEDDED
         default y
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index 9ec62da85fd7..39852d539018 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -227,6 +227,25 @@ static inline int page_kills_ppro(unsigned long pagenr)
         return 0;
 }
 
+/*
+ * devmem_is_allowed() checks to see if /dev/mem access to a certain address
+ * is valid. The argument is a physical page number.
+ *
+ *
+ * On x86, access has to be given to the first megabyte of ram because that area
+ * contains bios code and data regions used by X and dosemu and similar apps.
+ * Access has to be given to non-kernel-ram areas as well, these contain the PCI
+ * mmio resources as well as potential bios/acpi data regions.
+ */
+int devmem_is_allowed(unsigned long pagenr)
+{
+        if (pagenr <= 256)
+                return 1;
+        if (!page_is_ram(pagenr))
+                return 1;
+        return 0;
+}
+
 #ifdef CONFIG_HIGHMEM
 pte_t *kmap_pte;
 pgprot_t kmap_prot;
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 1ff7906a9a4d..49c274ee2fba 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -664,6 +664,26 @@ EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
 
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
+/*
+ * devmem_is_allowed() checks to see if /dev/mem access to a certain address
+ * is valid. The argument is a physical page number.
+ *
+ *
+ * On x86, access has to be given to the first megabyte of ram because that area
+ * contains bios code and data regions used by X and dosemu and similar apps.
+ * Access has to be given to non-kernel-ram areas as well, these contain the PCI
+ * mmio resources as well as potential bios/acpi data regions.
+ */
+int devmem_is_allowed(unsigned long pagenr)
+{
+        if (pagenr <= 256)
+                return 1;
+        if (!page_is_ram(pagenr))
+                return 1;
+        return 0;
+}
+
+
 static struct kcore_list kcore_mem, kcore_vmalloc, kcore_kernel,
                          kcore_modules, kcore_vsyscall;
 
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 20070b7c573d..dcf6e31970a1 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -108,6 +108,30 @@ static inline int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
 }
 #endif
 
+#ifdef CONFIG_NONPROMISC_DEVMEM
+static inline int range_is_allowed(unsigned long from, unsigned long to)
+{
+        unsigned long cursor;
+
+        cursor = from >> PAGE_SHIFT;
+        while ((cursor << PAGE_SHIFT) < to) {
+                if (!devmem_is_allowed(cursor)) {
+                        printk(KERN_INFO "Program %s tried to read /dev/mem "
+                                "between %lx->%lx.\n",
+                                current->comm, from, to);
+                        return 0;
+                }
+                cursor++;
+        }
+        return 1;
+}
+#else
+static inline int range_is_allowed(unsigned long from, unsigned long to)
+{
+        return 1;
+}
+#endif
+
 /*
  * This funcion reads the *physical* memory. The f_pos points directly to the 
  * memory location. 
@@ -157,6 +181,8 @@ static ssize_t read_mem(struct file * file, char __user * buf,
                  */
                 ptr = xlate_dev_mem_ptr(p);
 
+                if (!range_is_allowed(p, p+count))
+                        return -EPERM;
                 if (copy_to_user(buf, ptr, sz))
                         return -EFAULT;
                 buf += sz;
@@ -214,6 +240,8 @@ static ssize_t write_mem(struct file * file, const char __user * buf,
                  */
                 ptr = xlate_dev_mem_ptr(p);
 
+                if (!range_is_allowed(p, p+sz))
+                        return -EPERM;
                 copied = copy_from_user(ptr, buf, sz);
                 if (copied) {
                         written += sz - copied;
diff --git a/include/asm-x86/page.h b/include/asm-x86/page.h
index 6724a4bc6b7a..b381f4a5a0bd 100644
--- a/include/asm-x86/page.h
+++ b/include/asm-x86/page.h
@@ -47,6 +47,7 @@
 #ifndef __ASSEMBLY__
 
 extern int page_is_ram(unsigned long pagenr);
+extern int devmem_is_allowed(unsigned long pagenr);
 
 extern unsigned long max_pfn_mapped;