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 <prarit@redhat.com>
Acked-by: Matt Wilson <msw@amazon.com>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 882a40d405c8..9ca3e74a10e1 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1070,6 +1070,9 @@ 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