[3.19.y-ckt stable] Patch "bpf, array: fix heap out-of-bounds access when updating elements" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    bpf, array: fix heap out-of-bounds access when updating elements

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt12.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From 77f70114917d4c16ffa96a5f232fcf3f20d6c16c Mon Sep 17 00:00:00 2001
From: Daniel Borkmann <daniel at iogearbox.net>
Date: Mon, 30 Nov 2015 13:02:56 +0100
Subject: bpf, array: fix heap out-of-bounds access when updating elements

commit fbca9d2d35c6ef1b323fae75cc9545005ba25097 upstream.

During own review but also reported by Dmitry's syzkaller [1] it has been
noticed that we trigger a heap out-of-bounds access on eBPF array maps
when updating elements. This happens with each map whose map->value_size
(specified during map creation time) is not multiple of 8 bytes.

In array_map_alloc(), elem_size is round_up(attr->value_size, 8) and
used to align array map slots for faster access. However, in function
array_map_update_elem(), we update the element as ...

memcpy(array->value + array->elem_size * index, value, array->elem_size);

... where we access 'value' out-of-bounds, since it was allocated from
map_update_elem() from syscall side as kmalloc(map->value_size, GFP_USER)
and later on copied through copy_from_user(value, uvalue, map->value_size).
Thus, up to 7 bytes, we can access out-of-bounds.

Same could happen from within an eBPF program, where in worst case we
access beyond an eBPF program's designated stack.

Since 1be7f75d1668 ("bpf: enable non-root eBPF programs") didn't hit an
official release yet, it only affects priviledged users.

In case of array_map_lookup_elem(), the verifier prevents eBPF programs
from accessing beyond map->value_size through check_map_access(). Also
from syscall side map_lookup_elem() only copies map->value_size back to
user, so nothing could leak.

  [1] http://github.com/google/syzkaller

Fixes: 28fbcfa08d8e ("bpf: add array type of eBPF maps")
Reported-by: Dmitry Vyukov <dvyukov at google.com>
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Acked-by: Alexei Starovoitov <ast at kernel.org>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/bpf/arraymap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 9eb4d8a..264073e 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -109,7 +109,7 @@ static int array_map_update_elem(struct bpf_map *map, void *key, void *value,
 		/* all elements already exist */
 		return -EEXIST;

-	memcpy(array->value + array->elem_size * index, value, array->elem_size);
+	memcpy(array->value + array->elem_size * index, value, map->value_size);
 	return 0;
 }

--
1.9.1