templates: Disable the os-prober by default

The os-prober is enabled by default what may lead to potentially
dangerous use cases and borderline opening attack vectors. This
patch disables the os-prober, adds warning messages and updates
GRUB_DISABLE_OS_PROBER configuration option documentation. This
way we make it clear that the os-prober usage is not recommended.

Simplistic nature of this change allows downstream vendors, who
really want os-prober to be enabled out of the box in their
relevant products, easily revert to it's old behavior.

Reported-by: NyankoSec (<nyanko@10x.moe>, https://twitter.com/NyankoSec),
             working with SSD Secure Disclosure
Signed-off-by: Alex Burmashev <alexander.burmashev@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -1519,10 +1519,13 @@ boot sequence.  If you have problems, set this option to @samp{text} and
 GRUB will tell Linux to boot in normal text mode.
 
 @item GRUB_DISABLE_OS_PROBER
-Normally, @command{grub-mkconfig} will try to use the external
-@command{os-prober} program, if installed, to discover other operating
-systems installed on the same system and generate appropriate menu entries
-for them.  Set this option to @samp{true} to disable this.
+The @command{grub-mkconfig} has a feature to use the external
+@command{os-prober} program to discover other operating systems installed on
+the same machine and generate appropriate menu entries for them. It is disabled
+by default since automatic and silent execution of @command{os-prober}, and
+creating boot entries based on that data, is a potential attack vector. Set
+this option to @samp{false} to enable this feature in the
+@command{grub-mkconfig} command.
 
 @item GRUB_OS_PROBER_SKIP_LIST
 List of space-separated FS UUIDs of filesystems to be ignored from os-prober
@@ -1850,10 +1853,9 @@ than zero; otherwise 0.
 @section Multi-boot manual config
 
 Currently autogenerating config files for multi-boot environments depends on
-os-prober and has several shortcomings. While fixing it is scheduled for the
-next release, meanwhile you can make use of the power of GRUB syntax and do it
-yourself. A possible configuration is detailed here, feel free to adjust to your
-needs.
+os-prober and has several shortcomings. Due to that it is disabled by default.
+It is advised to use the power of GRUB syntax and do it yourself. A possible
+configuration is detailed here, feel free to adjust to your needs.
 
 First create a separate GRUB partition, big enough to hold GRUB. Some of the
 following entries show how to load OS installer images from this same partition,

util/grub.d/30_os-prober.in

@@ -26,7 +26,8 @@ export TEXTDOMAINDIR="@localedir@"
 
 . "$pkgdatadir/grub-mkconfig_lib"
 
-if [ "x${GRUB_DISABLE_OS_PROBER}" = "xtrue" ]; then
+if [ "x${GRUB_DISABLE_OS_PROBER}" = "xfalse" ]; then
+  gettext_printf "os-prober will not be executed to detect other bootable partitions.\nSystems on them will not be added to the GRUB boot configuration.\nCheck GRUB_DISABLE_OS_PROBER documentation entry.\n"
   exit 0
 fi
 
@@ -39,6 +40,8 @@ OSPROBED="`os-prober | tr ' ' '^' | paste -s -d ' '`"
 if [ -z "${OSPROBED}" ] ; then
   # empty os-prober output, nothing doing
   exit 0
+else
+  grub_warn "$(gettext_printf "os-prober was executed to detect other bootable partitions.\nIt's output will be used to detect bootable binaries on them and create new boot entries.")"
 fi
 
 osx_entry() {