[PATCH mm] kfence: make reporting sensitive information configurable

From: Marco Elver <elver@google.com>
To: elver@google.com, akpm@linux-foundation.org
Cc: glider@google.com, dvyukov@google.com, andreyknvl@google.com,
	jannh@google.com, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, kasan-dev@googlegroups.com
Subject: [PATCH mm] kfence: make reporting sensitive information configurable
Date: Tue,  9 Feb 2021 16:13:29 +0100	[thread overview]
Message-ID: <20210209151329.3459690-1-elver@google.com> (raw)

We cannot rely on CONFIG_DEBUG_KERNEL to decide if we're running a
"debug kernel" where we can safely show potentially sensitive
information in the kernel log.

Therefore, add the option CONFIG_KFENCE_REPORT_SENSITIVE to decide if we
should add potentially sensitive information to KFENCE reports. The
default behaviour remains unchanged.

Signed-off-by: Marco Elver <elver@google.com>
---
 Documentation/dev-tools/kfence.rst | 6 +++---
 lib/Kconfig.kfence                 | 8 ++++++++
 mm/kfence/core.c                   | 2 +-
 mm/kfence/kfence.h                 | 3 +--
 mm/kfence/report.c                 | 6 +++---
 5 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst
index 58a0a5fa1ddc..5280d644f826 100644
--- a/Documentation/dev-tools/kfence.rst
+++ b/Documentation/dev-tools/kfence.rst
@@ -89,7 +89,7 @@ A typical out-of-bounds access looks like this::
 The header of the report provides a short summary of the function involved in
 the access. It is followed by more detailed information about the access and
 its origin. Note that, real kernel addresses are only shown for
-``CONFIG_DEBUG_KERNEL=y`` builds.
+``CONFIG_KFENCE_REPORT_SENSITIVE=y`` builds.
 
 Use-after-free accesses are reported as::
 
@@ -184,8 +184,8 @@ invalidly written bytes (offset from the address) are shown; in this
 representation, '.' denote untouched bytes. In the example above ``0xac`` is
 the value written to the invalid address at offset 0, and the remaining '.'
 denote that no following bytes have been touched. Note that, real values are
-only shown for ``CONFIG_DEBUG_KERNEL=y`` builds; to avoid information
-disclosure for non-debug builds, '!' is used instead to denote invalidly
+only shown for ``CONFIG_KFENCE_REPORT_SENSITIVE=y`` builds; to avoid
+information disclosure otherwise, '!' is used instead to denote invalidly
 written bytes.
 
 And finally, KFENCE may also report on invalid accesses to any protected page
diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
index 78f50ccb3b45..141494a5f530 100644
--- a/lib/Kconfig.kfence
+++ b/lib/Kconfig.kfence
@@ -55,6 +55,14 @@ config KFENCE_NUM_OBJECTS
 	  pages are required; with one containing the object and two adjacent
 	  ones used as guard pages.
 
+config KFENCE_REPORT_SENSITIVE
+	bool "Show potentially sensitive information in reports"
+	default y if DEBUG_KERNEL
+	help
+	  Show potentially sensitive information such as unhashed pointers,
+	  context bytes on memory corruptions, as well as dump registers in
+	  KFENCE reports.
+
 config KFENCE_STRESS_TEST_FAULTS
 	int "Stress testing of fault handling and error reporting" if EXPERT
 	default 0
diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index cfe3d32ac5b7..5f7e02db5f53 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -648,7 +648,7 @@ void __init kfence_init(void)
 	schedule_delayed_work(&kfence_timer, 0);
 	pr_info("initialized - using %lu bytes for %d objects", KFENCE_POOL_SIZE,
 		CONFIG_KFENCE_NUM_OBJECTS);
-	if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
+	if (IS_ENABLED(CONFIG_KFENCE_REPORT_SENSITIVE))
 		pr_cont(" at 0x%px-0x%px\n", (void *)__kfence_pool,
 			(void *)(__kfence_pool + KFENCE_POOL_SIZE));
 	else
diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
index 1accc840dbbe..48a8196b947b 100644
--- a/mm/kfence/kfence.h
+++ b/mm/kfence/kfence.h
@@ -16,8 +16,7 @@
 
 #include "../slab.h" /* for struct kmem_cache */
 
-/* For non-debug builds, avoid leaking kernel pointers into dmesg. */
-#ifdef CONFIG_DEBUG_KERNEL
+#ifdef CONFIG_KFENCE_REPORT_SENSITIVE
 #define PTR_FMT "%px"
 #else
 #define PTR_FMT "%p"
diff --git a/mm/kfence/report.c b/mm/kfence/report.c
index 901bd7ee83d8..5e2dbabbab1d 100644
--- a/mm/kfence/report.c
+++ b/mm/kfence/report.c
@@ -148,9 +148,9 @@ static void print_diff_canary(unsigned long address, size_t bytes_to_show,
 	for (cur = (const u8 *)address; cur < end; cur++) {
 		if (*cur == KFENCE_CANARY_PATTERN(cur))
 			pr_cont(" .");
-		else if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
+		else if (IS_ENABLED(CONFIG_KFENCE_REPORT_SENSITIVE))
 			pr_cont(" 0x%02x", *cur);
-		else /* Do not leak kernel memory in non-debug builds. */
+		else /* Do not leak kernel memory. */
 			pr_cont(" !");
 	}
 	pr_cont(" ]");
@@ -242,7 +242,7 @@ void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *r
 
 	/* Print report footer. */
 	pr_err("\n");
-	if (IS_ENABLED(CONFIG_DEBUG_KERNEL) && regs)
+	if (IS_ENABLED(CONFIG_KFENCE_REPORT_SENSITIVE) && regs)
 		show_regs(regs);
 	else
 		dump_stack_print_info(KERN_ERR);
-- 
2.30.0.478.g8a0d178c01-goog

