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Subject[patch 3/4 -mm] flex_array: poison free elements
Newly initialized flex_array's and/or flex_array_part's are now poisoned
with a new poison value, FLEX_ARRAY_FREE. It's value is similar to
POISON_FREE used in the various slab allocators, but is different to
distinguish between flex array's poisoned kmem and slab allocator
poisoned kmem.

This will allow us to identify flex_array_part's that only contain free
elements (and free them with an addition to the flex_array API). This
could also be extended in the future to identify `get' uses on elements
that have not been `put'.

If __GFP_ZERO is passed for a part's gfp mask, the poisoning is avoided.
These elements are considered to be in-use since they have been
initialized.

Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: David Rientjes <rientjes@google.com>
---
include/linux/poison.h | 3 +++
lib/flex_array.c | 15 +++++++--------
2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/include/linux/poison.h b/include/linux/poison.h
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -65,6 +65,9 @@
#define MUTEX_DEBUG_INIT 0x11
#define MUTEX_DEBUG_FREE 0x22

+/********** lib/flex_array.c **********/
+#define FLEX_ARRAY_FREE 0x6c /* for use-after-free poisoning */
+
/********** security/ **********/
#define KEY_DESTROY 0xbd

diff --git a/lib/flex_array.c b/lib/flex_array.c
--- a/lib/flex_array.c
+++ b/lib/flex_array.c
@@ -113,6 +113,8 @@ struct flex_array *flex_array_alloc(int element_size, unsigned int total,
return NULL;
ret->element_size = element_size;
ret->total_nr_elements = total;
+ if (elements_fit_in_base(ret) && !(flags & __GFP_ZERO))
+ memset(ret->parts[0], FLEX_ARRAY_FREE, bytes_left_in_base());
return ret;
}

@@ -159,15 +161,12 @@ __fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
{
struct flex_array_part *part = fa->parts[part_nr];
if (!part) {
- /*
- * This leaves the part pages uninitialized
- * and with potentially random data, just
- * as if the user had kmalloc()'d the whole.
- * __GFP_ZERO can be used to zero it.
- */
- part = kmalloc(FLEX_ARRAY_PART_SIZE, flags);
+ part = kmalloc(sizeof(struct flex_array_part), flags);
if (!part)
return NULL;
+ if (!(flags & __GFP_ZERO))
+ memset(part, FLEX_ARRAY_FREE,
+ sizeof(struct flex_array_part));
fa->parts[part_nr] = part;
}
return part;
@@ -228,7 +227,7 @@ int flex_array_clear(struct flex_array *fa, unsigned int element_nr)
return -EINVAL;
}
dst = &part->elements[index_inside_part(fa, element_nr)];
- memset(dst, 0, fa->element_size);
+ memset(dst, FLEX_ARRAY_FREE, fa->element_size);
return 0;
}


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