kfence: add sysfs interface to disable kfence for selected slabs.

By default kfence allocation can happen for any slab object, whose size is
up to PAGE_SIZE, as long as that allocation is the first allocation after
expiration of kfence sample interval.  But in certain debugging scenarios
we may be interested in debugging corruptions involving some specific slub
objects like dentry or ext4_* etc.  In such cases limiting kfence for
allocations involving only specific slub objects will increase the
probablity of catching the issue since kfence pool will not be consumed by
other slab objects.

This patch introduces a sysfs interface
'/sys/kernel/slab/<name>/skip_kfence' to disable kfence for specific
slabs.  Having the interface work in this way does not impact
current/default behavior of kfence and allows us to use kfence for
specific slabs (when needed) as well.  The decision to skip/use kfence is
taken depending on whether kmem_cache.flags has (newly introduced)
SLAB_SKIP_KFENCE flag set or not.

Link: https://lkml.kernel.org/r/20220814195353.2540848-1-imran.f.khan@oracle.com
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 33 insertions, 0 deletions

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index c252081b11df..8c08ae2101d7 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -1003,6 +1003,13 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
 		return NULL;
 	}
 
+	/*
+	 * Skip allocations for this slab, if KFENCE has been disabled for
+	 * this slab.
+	 */
+	if (s->flags & SLAB_SKIP_KFENCE)
+		return NULL;
+
 	if (atomic_inc_return(&kfence_allocation_gate) > 1)
 		return NULL;
 #ifdef CONFIG_KFENCE_STATIC_KEYS
diff --git a/mm/slub.c b/mm/slub.c
index 862dbd9af4f5..6953c3367bc2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5745,6 +5745,29 @@ STAT_ATTR(CPU_PARTIAL_NODE, cpu_partial_node);
 STAT_ATTR(CPU_PARTIAL_DRAIN, cpu_partial_drain);
 #endif	/* CONFIG_SLUB_STATS */
 
+#ifdef CONFIG_KFENCE
+static ssize_t skip_kfence_show(struct kmem_cache *s, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", !!(s->flags & SLAB_SKIP_KFENCE));
+}
+
+static ssize_t skip_kfence_store(struct kmem_cache *s,
+			const char *buf, size_t length)
+{
+	int ret = length;
+
+	if (buf[0] == '0')
+		s->flags &= ~SLAB_SKIP_KFENCE;
+	else if (buf[0] == '1')
+		s->flags |= SLAB_SKIP_KFENCE;
+	else
+		ret = -EINVAL;
+
+	return ret;
+}
+SLAB_ATTR(skip_kfence);
+#endif
+
 static struct attribute *slab_attrs[] = {
 	&slab_size_attr.attr,
 	&object_size_attr.attr,
@@ -5812,6 +5835,9 @@ static struct attribute *slab_attrs[] = {
 	&failslab_attr.attr,
 #endif
 	&usersize_attr.attr,
+#ifdef CONFIG_KFENCE
+	&skip_kfence_attr.attr,
+#endif
 
 	NULL
 };