[PATCH] hpet: allow user controlled mmap for user processes

From: Clemens Ladisch
Date: Sun Sep 29 2013 - 16:34:20 EST

From: Prarit Bhargava <prarit@xxxxxxxxxx>

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.

[v2]: Clemens suggested modifying the Kconfig help text and making the
default setting configurable.
[v3]: Fixed up Documentation and Kconfig entries, default now "Y"
[v4]: After testing, found that I need to modify CONFIG_HPET_MMAP_DEFAULT usage
[v5]: Fixed up Documentation, Kconfig entry, and log message [CL]

Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
Acked-by: Matt Wilson <msw@xxxxxxxxxx>
Signed-off-by: Clemens Ladisch <clemens@xxxxxxxxxx>
Documentation/kernel-parameters.txt | 3 +++
drivers/char/Kconfig | 10 ++++++++--
drivers/char/hpet.c | 24 ++++++++++++++++++++++--
3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 539a236..6a7b656 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1064,6 +1064,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
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
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 1421997..fa3243d 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -522,10 +522,16 @@ config HPET_MMAP
If you say Y here, user applications will be able to mmap
the HPET registers.

+ 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.

tristate "Hangcheck timer"
diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index d6568a6..964d002 100644
--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -367,12 +367,29 @@ static unsigned int hpet_poll(struct file *file, poll_table * wait)
return 0;

+static int hpet_mmap_enabled = 1;
+static int hpet_mmap_enabled = 0;
+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)
struct hpet_dev *devp;
unsigned long addr;

+ if (!hpet_mmap_enabled)
+ return -EACCES;
devp = file->private_data;
addr = devp->hd_hpets->hp_hpet_phys;

@@ -381,10 +398,13 @@ static int hpet_mmap(struct file *file, struct vm_area_struct *vma)

vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
return vm_iomap_memory(vma, addr, PAGE_SIZE);
+static int hpet_mmap(struct file *file, struct vm_area_struct *vma)
return -ENOSYS;

static int hpet_fasync(int fd, struct file *file, int on)
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