[PATCH net] net: pktgen: fix out-of-bounds write in pktgen_finalize_skb()

[Yuejie Shi <syjcnss@gmail.com>]

pktgen_finalize_skb() spreads `datalen` bytes over `frags` page
fragments, but the loop that fills skb_shinfo(skb)->frags[] is bounded
only by `datalen > 0` -- there is no `i < frags` check.  The
per-fragment length is computed once with a truncating division:

	frag_len = min_t(int, datalen / frags, PAGE_SIZE);

The code just above has already capped datalen at frags * PAGE_SIZE, so
a valid partition does exist, but the truncation means the first
frags - 1 fragments can leave more than PAGE_SIZE behind, and the
last-fragment branch only consumes min(datalen, PAGE_SIZE).  Writing
datalen as q * frags + r with q = datalen / frags, the loop overruns
whenever

	q + r > PAGE_SIZE

because q + r bytes are still outstanding after frags - 1 iterations and
the final fragment can take only PAGE_SIZE of them.  The loop then runs
once more, with i == frags, and writes one element past the array.

frags is clamped to MAX_SKB_FRAGS (17) and frags[] is the last member of
struct skb_shared_info, which sits at the very end of the skb head
allocation, so writing frags[17] writes 16 bytes past that allocation.
With PAGE_SIZE == 4096 every pkt_size in [69675, 69689] hits it
(datalen = pkt_size - 58, giving q = 4095 and r >= 2):

  modprobe pktgen
  echo "add_device dummy0" > /proc/net/pktgen/kpktgend_0
  echo "frags 17"          > /proc/net/pktgen/dummy0
  echo "pkt_size 69675"    > /proc/net/pktgen/dummy0
  echo start               > /proc/net/pktgen/pgctrl

  BUG: KASAN: use-after-free in pktgen_finalize_skb+0x1e8/0x4a4 [pktgen]
  Write of size 8 at addr ffff0000c6b40000 by task kpktgend_0/97
   __asan_store8+0xe0/0xe4
   pktgen_finalize_skb+0x1e8/0x4a4 [pktgen]
   pktgen_thread_worker+0x1f3c/0x2d50 [pktgen]
   kthread+0x1c4/0x1e0
  The buggy address belongs to the physical page:
  page: refcount:0 mapcount:0 mapping:0000000000000000 pfn:0x106b40

nr_frags ends up at 18 as well, so skb_release_data() later walks one
entry past the array too.

Compute the fragment length with DIV_ROUND_UP() instead.  datalen is
still <= frags * PAGE_SIZE so the result cannot exceed PAGE_SIZE, but
the fragments now cover datalen exactly and the loop always terminates
on the last one.  As a side effect pktgen stops emitting zero-length
fragments when datalen < frags.  Bound the loop with `i < frags` as
well, so that a future arithmetic mistake cannot walk off the array
again.

Reaching this needs real root: pktgen creates its /proc/net/pktgen
entries with proc_mkdir()/proc_create_data() at mode 0600 owned by
global root, so unlike /proc/net itself they are not remapped to the
owner of a user namespace, and the module has to be loaded first.  It is
still a plain heap overflow behind a documented interface.

Fixes: 7d36a991e8d3 ("pktgen: create num frags requested")
Cc: stable@vger.kernel.org
Signed-off-by: Yuejie Shi <syjcnss@gmail.com>
---
 net/core/pktgen.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index 8e185b318288..2dbb7caacd0d 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -2843,8 +2843,8 @@ static void pktgen_finalize_skb(struct pktgen_dev *pkt_dev, struct sk_buff *skb,
 		}
 
 		i = 0;
-		frag_len = min_t(int, datalen / frags, PAGE_SIZE);
-		while (datalen > 0) {
+		frag_len = min_t(int, DIV_ROUND_UP(datalen, frags), PAGE_SIZE);
+		while (datalen > 0 && i < frags) {
 			if (unlikely(!pkt_dev->page)) {
 				int node = numa_node_id();
 
--
2.51.0