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Subject[PATCH mm] kfence: report sensitive information based on no_hash_pointers
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.

Instead, simply rely on the newly introduced "no_hash_pointers" to print
unhashed kernel pointers, as well as decide if our reports can include
other potentially sensitive information such as registers and corrupted

Cc: Timur Tabi <>
Signed-off-by: Marco Elver <>

Depends on "lib/vsprintf: no_hash_pointers prints all addresses as
unhashed", which was merged into mainline yesterday:

Documentation/dev-tools/kfence.rst | 8 ++++----
mm/kfence/core.c | 10 +++-------
mm/kfence/kfence.h | 7 -------
mm/kfence/kfence_test.c | 2 +-
mm/kfence/report.c | 18 ++++++++++--------
5 files changed, 18 insertions(+), 27 deletions(-)

diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst
index 58a0a5fa1ddc..fdf04e741ea5 100644
--- a/Documentation/dev-tools/kfence.rst
+++ b/Documentation/dev-tools/kfence.rst
@@ -88,8 +88,8 @@ 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.
+its origin. Note that, real kernel addresses are only shown when using the
+kernel command line option ``no_hash_pointers``.

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 if the kernel was booted with ``no_hash_pointers``; 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/mm/kfence/core.c b/mm/kfence/core.c
index cfe3d32ac5b7..3b8ec938470a 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -646,13 +646,9 @@ void __init kfence_init(void)

WRITE_ONCE(kfence_enabled, true);
schedule_delayed_work(&kfence_timer, 0);
- pr_info("initialized - using %lu bytes for %d objects", KFENCE_POOL_SIZE,
- pr_cont(" at 0x%px-0x%px\n", (void *)__kfence_pool,
- (void *)(__kfence_pool + KFENCE_POOL_SIZE));
- else
- pr_cont("\n");
+ pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
+ CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
+ (void *)(__kfence_pool + KFENCE_POOL_SIZE));

void kfence_shutdown_cache(struct kmem_cache *s)
diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
index 1accc840dbbe..24065321ff8a 100644
--- a/mm/kfence/kfence.h
+++ b/mm/kfence/kfence.h
@@ -16,13 +16,6 @@

#include "../slab.h" /* for struct kmem_cache */

-/* For non-debug builds, avoid leaking kernel pointers into dmesg. */
-#define PTR_FMT "%px"
-#define PTR_FMT "%p"
* Get the canary byte pattern for @addr. Use a pattern that varies based on the
* lower 3 bits of the address, to detect memory corruptions with higher
diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
index db1bb596acaf..4acf4251ee04 100644
--- a/mm/kfence/kfence_test.c
+++ b/mm/kfence/kfence_test.c
@@ -146,7 +146,7 @@ static bool report_matches(const struct expect_report *r)

- cur += scnprintf(cur, end - cur, " 0x" PTR_FMT, (void *)r->addr);
+ cur += scnprintf(cur, end - cur, " 0x%p", (void *)r->addr);

spin_lock_irqsave(&observed.lock, flags);
if (!report_available())
diff --git a/mm/kfence/report.c b/mm/kfence/report.c
index 901bd7ee83d8..4a424de44e2d 100644
--- a/mm/kfence/report.c
+++ b/mm/kfence/report.c
@@ -19,6 +19,8 @@

#include "kfence.h"

+extern bool no_hash_pointers;
/* Helper function to either print to a seq_file or to console. */
__printf(2, 3)
static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)
@@ -118,7 +120,7 @@ void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *met

- "kfence-#%zd [0x" PTR_FMT "-0x" PTR_FMT
+ "kfence-#%zd [0x%p-0x%p"
", size=%d, cache=%s] allocated by task %d:\n",
meta - kfence_metadata, (void *)start, (void *)(start + size - 1), size,
(cache && cache->name) ? cache->name : "<destroyed>", meta->;
@@ -148,7 +150,7 @@ 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 (no_hash_pointers)
pr_cont(" 0x%02x", *cur);
else /* Do not leak kernel memory in non-debug builds. */
pr_cont(" !");
@@ -201,7 +203,7 @@ void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *r

pr_err("BUG: KFENCE: out-of-bounds %s in %pS\n\n", get_access_type(is_write),
(void *)stack_entries[skipnr]);
- pr_err("Out-of-bounds %s at 0x" PTR_FMT " (%luB %s of kfence-#%zd):\n",
+ pr_err("Out-of-bounds %s at 0x%p (%luB %s of kfence-#%zd):\n",
get_access_type(is_write), (void *)address,
left_of_object ? meta->addr - address : address - meta->addr,
left_of_object ? "left" : "right", object_index);
@@ -210,24 +212,24 @@ void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *r
pr_err("BUG: KFENCE: use-after-free %s in %pS\n\n", get_access_type(is_write),
(void *)stack_entries[skipnr]);
- pr_err("Use-after-free %s at 0x" PTR_FMT " (in kfence-#%zd):\n",
+ pr_err("Use-after-free %s at 0x%p (in kfence-#%zd):\n",
get_access_type(is_write), (void *)address, object_index);
pr_err("BUG: KFENCE: memory corruption in %pS\n\n", (void *)stack_entries[skipnr]);
- pr_err("Corrupted memory at 0x" PTR_FMT " ", (void *)address);
+ pr_err("Corrupted memory at 0x%p ", (void *)address);
print_diff_canary(address, 16, meta);
pr_cont(" (in kfence-#%zd):\n", object_index);
pr_err("BUG: KFENCE: invalid %s in %pS\n\n", get_access_type(is_write),
(void *)stack_entries[skipnr]);
- pr_err("Invalid %s at 0x" PTR_FMT ":\n", get_access_type(is_write),
+ pr_err("Invalid %s at 0x%p:\n", get_access_type(is_write),
(void *)address);
pr_err("BUG: KFENCE: invalid free in %pS\n\n", (void *)stack_entries[skipnr]);
- pr_err("Invalid free of 0x" PTR_FMT " (in kfence-#%zd):\n", (void *)address,
+ pr_err("Invalid free of 0x%p (in kfence-#%zd):\n", (void *)address,
@@ -242,7 +244,7 @@ void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *r

/* Print report footer. */
+ if (no_hash_pointers && regs)
 \ /
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