kfence: allow re-enabling KFENCE after system startup

Patch series "provide the flexibility to enable KFENCE", v3.

If CONFIG_CONTIG_ALLOC is not supported, we fallback to try
alloc_pages_exact().  Allocating pages in this way has limits about
MAX_ORDER (default 11).  So we will not support allocating kfence pool
after system startup with a large KFENCE_NUM_OBJECTS.

When handling failures in kfence_init_pool_late(), we pair
free_pages_exact() to alloc_pages_exact() for compatibility consideration,
though it actually does the same as free_contig_range().

This patch (of 2):

If once KFENCE is disabled by:
echo 0 > /sys/module/kfence/parameters/sample_interval
KFENCE could never be re-enabled until next rebooting.

Allow re-enabling it by writing a positive num to sample_interval.

Link: https://lkml.kernel.org/r/20220307074516.6920-1-dtcccc@linux.alibaba.com
Link: https://lkml.kernel.org/r/20220307074516.6920-2-dtcccc@linux.alibaba.com
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 13128fa..caa4e84 100644 (file)
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -38,14 +38,17 @@
 #define KFENCE_WARN_ON(cond)                                                   \
        ({                                                                     \
                const bool __cond = WARN_ON(cond);                             \
-               if (unlikely(__cond))                                          \
+               if (unlikely(__cond)) {                                        \
                        WRITE_ONCE(kfence_enabled, false);                     \
+                       disabled_by_warn = true;                               \
+               }                                                              \
                __cond;                                                        \
        })
 
 /* === Data ================================================================= */
 
 static bool kfence_enabled __read_mostly;
+static bool disabled_by_warn __read_mostly;
 
 unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
 EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
@@ -55,6 +58,7 @@ EXPORT_SYMBOL_GPL(kfence_sample_interval); /* Export for test modules. */
 #endif
 #define MODULE_PARAM_PREFIX "kfence."
 
+static int kfence_enable_late(void);
 static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
 {
        unsigned long num;
@@ -65,10 +69,11 @@ static int param_set_sample_interval(const char *val, const struct kernel_param
 
        if (!num) /* Using 0 to indicate KFENCE is disabled. */
                WRITE_ONCE(kfence_enabled, false);
-       else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
-               return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */
 
        *((unsigned long *)kp->arg) = num;
+
+       if (num && !READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
+               return disabled_by_warn ? -EINVAL : kfence_enable_late();
        return 0;
 }
 
@@ -787,6 +792,16 @@ void __init kfence_init(void)
                (void *)(__kfence_pool + KFENCE_POOL_SIZE));
 }
 
+static int kfence_enable_late(void)
+{
+       if (!__kfence_pool)
+               return -EINVAL;
+
+       WRITE_ONCE(kfence_enabled, true);
+       queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
+       return 0;
+}
+
 void kfence_shutdown_cache(struct kmem_cache *s)
 {
        unsigned long flags;