net-gro: fix use-after-free read in napi_gro_frags()

[ Upstream commit a4270d6795b0580287453ea55974d948393e66ef ]

If a network driver provides to napi_gro_frags() an
skb with a page fragment of exactly 14 bytes, the call
to gro_pull_from_frag0() will 'consume' the fragment
by calling skb_frag_unref(skb, 0), and the page might
be freed and reused.

Reading eth->h_proto at the end of napi_frags_skb() might
read mangled data, or crash under specific debugging features.

BUG: KASAN: use-after-free in napi_frags_skb net/core/dev.c:5833 [inline]
BUG: KASAN: use-after-free in napi_gro_frags+0xc6f/0xd10 net/core/dev.c:5841
Read of size 2 at addr ffff88809366840c by task syz-executor599/8957

CPU: 1 PID: 8957 Comm: syz-executor599 Not tainted 5.2.0-rc1+ #32
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:188
 __kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 kasan_report+0x12/0x20 mm/kasan/common.c:614
 __asan_report_load_n_noabort+0xf/0x20 mm/kasan/generic_report.c:142
 napi_frags_skb net/core/dev.c:5833 [inline]
 napi_gro_frags+0xc6f/0xd10 net/core/dev.c:5841
 tun_get_user+0x2f3c/0x3ff0 drivers/net/tun.c:1991
 tun_chr_write_iter+0xbd/0x156 drivers/net/tun.c:2037
 call_write_iter include/linux/fs.h:1872 [inline]
 do_iter_readv_writev+0x5f8/0x8f0 fs/read_write.c:693
 do_iter_write fs/read_write.c:970 [inline]
 do_iter_write+0x184/0x610 fs/read_write.c:951
 vfs_writev+0x1b3/0x2f0 fs/read_write.c:1015
 do_writev+0x15b/0x330 fs/read_write.c:1058

Fixes: a50e233c50db ("net-gro: restore frag0 optimization")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/core/dev.c b/net/core/dev.c
index 13a82744a00a5916efe0d1f878db5f1dd60d7052..985058c45f7b1622aeef480cfad704eca37d0327 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5725,7 +5725,6 @@ static struct sk_buff *napi_frags_skb(struct napi_struct *napi)
        skb_reset_mac_header(skb);
        skb_gro_reset_offset(skb);
 
-       eth = skb_gro_header_fast(skb, 0);
        if (unlikely(skb_gro_header_hard(skb, hlen))) {
                eth = skb_gro_header_slow(skb, hlen, 0);
                if (unlikely(!eth)) {
@@ -5735,6 +5734,7 @@ static struct sk_buff *napi_frags_skb(struct napi_struct *napi)
                        return NULL;
                }
        } else {
+               eth = (const struct ethhdr *)skb->data;
                gro_pull_from_frag0(skb, hlen);
                NAPI_GRO_CB(skb)->frag0 += hlen;
                NAPI_GRO_CB(skb)->frag0_len -= hlen;