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Subject[kernel-hardening] [RFC V2 1/6] lib: vsprintf: additional kernel pointer filtering options
Date
Add the kptr_restrict setting of 3 which results in both
%p and %pK values being replaced by zeros.

Add an additional %pP value inspired by the Grsecurity
option which explicitly whitelists pointers for output.

Amend scripts/checkpatch.pl to handle %pP.

This patch is based on work by William Roberts
<william.c.roberts@intel.com>

Signed-off-by: Tobin C. Harding <me@tobin.cc>
---
Documentation/printk-formats.txt | 8 +++++
Documentation/sysctl/kernel.txt | 4 +++
kernel/sysctl.c | 3 +-
lib/vsprintf.c | 78 ++++++++++++++++++++++++++--------------
scripts/checkpatch.pl | 2 +-
5 files changed, 66 insertions(+), 29 deletions(-)

diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 361789d..16c9abc 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -97,6 +97,14 @@ For printing kernel pointers which should be hidden from unprivileged
users. The behaviour of ``%pK`` depends on the ``kptr_restrict sysctl`` - see
Documentation/sysctl/kernel.txt for more details.

+::
+
+ %pP 0x01234567 or 0x0123456789abcdef
+
+For printing kernel pointers which should always be shown, even to
+unprivileged users.
+
+
Struct Resources
================

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 694968c..7ee183af 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -394,6 +394,10 @@ values to unprivileged users is a concern.
When kptr_restrict is set to (2), kernel pointers printed using
%pK will be replaced with 0's regardless of privileges.

+When kptr_restrict is set to (3), kernel pointers printed using
+%p and %pK will be replaced with 0's regardless of privileges,
+however kernel pointers printed using %pP will continue to be printed.
+
==============================================================

l2cr: (PPC only)
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 6648fbb..37ba637 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -129,6 +129,7 @@ static unsigned long one_ul = 1;
static int one_hundred = 100;
static int one_thousand = 1000;
#ifdef CONFIG_PRINTK
+static int three = 3;
static int ten_thousand = 10000;
#endif
#ifdef CONFIG_PERF_EVENTS
@@ -851,7 +852,7 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec_minmax_sysadmin,
.extra1 = &zero,
- .extra2 = &two,
+ .extra2 = &three,
},
#endif
{
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 86c3385..e6eace0 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -396,6 +396,16 @@ struct printf_spec {
#define FIELD_WIDTH_MAX ((1 << 23) - 1)
#define PRECISION_MAX ((1 << 15) - 1)

+int kptr_restrict __read_mostly;
+
+/*
+ * return non-zero if we should cleanse pointers for %p and %pK specifiers.
+ */
+static inline int kptr_restrict_cleanse_kernel_pointers(void)
+{
+ return kptr_restrict >= 3;
+}
+
static noinline_for_stack
char *number(char *buf, char *end, unsigned long long num,
struct printf_spec spec)
@@ -1591,8 +1601,6 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
return widen_string(buf, buf - buf_start, end, spec);
}

-int kptr_restrict __read_mostly;
-
/*
* Show a '%p' thing. A kernel extension is that the '%p' is followed
* by an extra set of alphanumeric characters that are extended format
@@ -1664,6 +1672,7 @@ int kptr_restrict __read_mostly;
* Do not use this feature without some mechanism to verify the
* correctness of the format string and va_list arguments.
* - 'K' For a kernel pointer that should be hidden from unprivileged users
+ * - 'P' For a kernel pointer that should be shown to all users
* - 'NF' For a netdev_features_t
* - 'h[CDN]' For a variable-length buffer, it prints it as a hex string with
* a certain separator (' ' by default):
@@ -1703,6 +1712,8 @@ int kptr_restrict __read_mostly;
* Note: The difference between 'S' and 'F' is that on ia64 and ppc64
* function pointers are really function descriptors, which contain a
* pointer to the real address.
+ *
+ * Note: That for kptr_restrict set to 3, %p and %pK have the same meaning.
*/
static noinline_for_stack
char *pointer(const char *fmt, char *buf, char *end, void *ptr,
@@ -1710,7 +1721,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
{
const int default_width = 2 * sizeof(void *);

- if (!ptr && *fmt != 'K') {
+ if (!ptr && *fmt != 'K' && !kptr_restrict_cleanse_kernel_pointers()) {
/*
* Print (null) with the same width as a pointer so it makes
* tabular output look nice.
@@ -1791,6 +1802,31 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
va_end(va);
return buf;
}
+ case 'N':
+ return netdev_bits(buf, end, ptr, fmt);
+ case 'a':
+ return address_val(buf, end, ptr, fmt);
+ case 'd':
+ return dentry_name(buf, end, ptr, spec, fmt);
+ case 'C':
+ return clock(buf, end, ptr, spec, fmt);
+ case 'D':
+ return dentry_name(buf, end,
+ ((const struct file *)ptr)->f_path.dentry,
+ spec, fmt);
+#ifdef CONFIG_BLOCK
+ case 'g':
+ return bdev_name(buf, end, ptr, spec, fmt);
+#endif
+
+ case 'G':
+ return flags_string(buf, end, ptr, fmt);
+ case 'P':
+ /*
+ * An explicitly whitelisted kernel pointer should never be
+ * cleansed
+ */
+ break;
case 'K':
switch (kptr_restrict) {
case 0:
@@ -1825,39 +1861,27 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
ptr = NULL;
break;
}
- case 2:
+ case 2: /* cleanse %pK for kptr_restrict >= 2 */
default:
- /* Always print 0's for %pK */
ptr = NULL;
break;
}
- break;
-
- case 'N':
- return netdev_bits(buf, end, ptr, fmt);
- case 'a':
- return address_val(buf, end, ptr, fmt);
- case 'd':
- return dentry_name(buf, end, ptr, spec, fmt);
- case 'C':
- return clock(buf, end, ptr, spec, fmt);
- case 'D':
- return dentry_name(buf, end,
- ((const struct file *)ptr)->f_path.dentry,
- spec, fmt);
-#ifdef CONFIG_BLOCK
- case 'g':
- return bdev_name(buf, end, ptr, spec, fmt);
-#endif
-
- case 'G':
- return flags_string(buf, end, ptr, fmt);
+ break; /* case 'K' */
case 'O':
switch (fmt[1]) {
case 'F':
return device_node_string(buf, end, ptr, spec, fmt + 1);
}
+ /* fall through */
+ default:
+ /*
+ * Plain pointers (%p) are always cleansed in higher restriction
+ * levels.
+ */
+ if (kptr_restrict >= 3)
+ ptr = NULL;
}
+
spec.flags |= SMALL;
if (spec.field_width == -1) {
spec.field_width = default_width;
@@ -1865,7 +1889,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
}
spec.base = 16;

- return number(buf, end, (unsigned long) ptr, spec);
+ return number(buf, end, (unsigned long long) ptr, spec);
}

/*
diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index dd2c262..eabc56c 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -5762,7 +5762,7 @@ sub process {
for (my $count = $linenr; $count <= $lc; $count++) {
my $fmt = get_quoted_string($lines[$count - 1], raw_line($count, 0));
$fmt =~ s/%%//g;
- if ($fmt =~ /(\%[\*\d\.]*p(?![\WFfSsBKRraEhMmIiUDdgVCbGNO]).)/) {
+ if ($fmt =~ /(\%[\*\d\.]*p(?![\WFfSsBKRraEhMmIiUDdgVCbGNOP]).)/) {
$bad_extension = $1;
last;
}
--
2.7.4
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