drivers/char/hpet.c: allow user controlled mmap for user processes

The CONFIG_HPET_MMAP Kconfig option exposes the memory map of the HPET
registers to userspace. The Kconfig help points out that in some cases
this can be a security risk as some systems may erroneously configure the
map such that additional data is exposed to userspace.

This is a problem for distributions -- some users want the MMAP
functionality but it comes with a significant security risk. In an effort
to mitigate this risk, and due to the low number of users of the MMAP
functionality, I've introduced a kernel parameter, hpet_mmap_enable, that
is required in order to actually have the HPET MMAP exposed.

Signed-off-by: Prarit Bhargava <[e-mail]>
Acked-by: Matt Wilson <[e-mail]>
Signed-off-by: Clemens Ladisch <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Tomas Winkler <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/kernel-parameters.txt

···
 				VIA, nVidia)
 			verbose: show contents of HPET registers during setup
 
+	hpet_mmap=	[X86, HPET_MMAP] Allow userspace to mmap HPET
+			registers.  Default set by CONFIG_HPET_MMAP_DEFAULT.
+
 	hugepages=	[HW,X86-32,IA-64] HugeTLB pages to allocate at boot.
 	hugepagesz=	[HW,IA-64,PPC,X86-64] The size of the HugeTLB pages.
 			On x86-64 and powerpc, this option can be specified

drivers/char/Kconfig

···
 	  If you say Y here, user applications will be able to mmap
 	  the HPET registers.
 
+config HPET_MMAP_DEFAULT
+	bool "Enable HPET MMAP access by default"
+	default y
+	depends on HPET_MMAP
+	help
 	  In some hardware implementations, the page containing HPET
 	  registers may also contain other things that shouldn't be
-	  exposed to the user.  If this applies to your hardware,
-	  say N here.
+	  exposed to the user.  This option selects the default (if
+	  kernel parameter hpet_mmap is not set) user access to the
+	  registers for applications that require it.
 
 config HANGCHECK_TIMER
 	tristate "Hangcheck timer"

drivers/char/hpet.c

···
 	return 0;
 }
 
+#ifdef CONFIG_HPET_MMAP
+#ifdef CONFIG_HPET_MMAP_DEFAULT
+static int hpet_mmap_enabled = 1;
+#else
+static int hpet_mmap_enabled = 0;
+#endif
+
+static __init int hpet_mmap_enable(char *str)
+{
+	get_option(&str, &hpet_mmap_enabled);
+	pr_info("HPET mmap %s\n", hpet_mmap_enabled ? "enabled" : "disabled");
+	return 1;
+}
+__setup("hpet_mmap", hpet_mmap_enable);
+
 static int hpet_mmap(struct file *file, struct vm_area_struct *vma)
 {
-#ifdef	CONFIG_HPET_MMAP
 	struct hpet_dev *devp;
 	unsigned long addr;
+
+	if (!hpet_mmap_enabled)
+		return -EACCES;
 
 	devp = file->private_data;
 	addr = devp->hd_hpets->hp_hpet_phys;
···
 
 	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
 	return vm_iomap_memory(vma, addr, PAGE_SIZE);
-#else
-	return -ENOSYS;
-#endif
 }
+#else
+static int hpet_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	return -ENOSYS;
+}
+#endif
 
 static int hpet_fasync(int fd, struct file *file, int on)
 {