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Subject[PATCH 4.4 66/74] pppoe: take ->needed_headroom of lower device into account on xmit
4.4-stable review patch.  If anyone has any objections, please let me know.


From: Guillaume Nault <>

[ Upstream commit 02612bb05e51df8489db5e94d0cf8d1c81f87b0c ]

In pppoe_sendmsg(), reserving dev->hard_header_len bytes of headroom
was probably fine before the introduction of ->needed_headroom in
commit f5184d267c1a ("net: Allow netdevices to specify needed head/tailroom").

But now, virtual devices typically advertise the size of their overhead
in dev->needed_headroom, so we must also take it into account in
Allocation size of skb is also updated to take dev->needed_tailroom
into account and replace the arbitrary 32 bytes with the real size of
a PPPoE header.

This issue was discovered by syzbot, who connected a pppoe socket to a
gre device which had dev->header_ops->create == ipgre_header and
dev->hard_header_len == 0. Therefore, PPPoE didn't reserve any
headroom, and dev_hard_header() crashed when ipgre_header() tried to
prepend its header to skb->data.

skbuff: skb_under_panic: text:000000001d390b3a len:31 put:24
head:00000000d8ed776f data:000000008150e823 tail:0x7 end:0xc0 dev:gre0
------------[ cut here ]------------
kernel BUG at net/core/skbuff.c:104!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
(ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 3670 Comm: syzkaller801466 Not tainted
4.15.0-rc7-next-20180115+ #97
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:skb_panic+0x162/0x1f0 net/core/skbuff.c:100
RSP: 0018:ffff8801d9bd7840 EFLAGS: 00010282
RAX: 0000000000000083 RBX: ffff8801d4f083c0 RCX: 0000000000000000
RDX: 0000000000000083 RSI: 1ffff1003b37ae92 RDI: ffffed003b37aefc
RBP: ffff8801d9bd78a8 R08: 1ffff1003b37ae8a R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffffffff86200de0
R13: ffffffff84a981ad R14: 0000000000000018 R15: ffff8801d2d34180
FS: 00000000019c4880(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000208bc000 CR3: 00000001d9111001 CR4: 00000000001606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
skb_under_panic net/core/skbuff.c:114 [inline]
skb_push+0xce/0xf0 net/core/skbuff.c:1714
ipgre_header+0x6d/0x4e0 net/ipv4/ip_gre.c:879
dev_hard_header include/linux/netdevice.h:2723 [inline]
pppoe_sendmsg+0x58e/0x8b0 drivers/net/ppp/pppoe.c:890
sock_sendmsg_nosec net/socket.c:630 [inline]
sock_sendmsg+0xca/0x110 net/socket.c:640
sock_write_iter+0x31a/0x5d0 net/socket.c:909
call_write_iter include/linux/fs.h:1775 [inline]
do_iter_readv_writev+0x525/0x7f0 fs/read_write.c:653
do_iter_write+0x154/0x540 fs/read_write.c:932
vfs_writev+0x18a/0x340 fs/read_write.c:977
do_writev+0xfc/0x2a0 fs/read_write.c:1012
SYSC_writev fs/read_write.c:1085 [inline]
SyS_writev+0x27/0x30 fs/read_write.c:1082

Admittedly PPPoE shouldn't be allowed to run on non Ethernet-like
interfaces, but reserving space for ->needed_headroom is a more
fundamental issue that needs to be addressed first.

Same problem exists for __pppoe_xmit(), which also needs to take
dev->needed_headroom into account in skb_cow_head().

Fixes: f5184d267c1a ("net: Allow netdevices to specify needed head/tailroom")
Signed-off-by: Guillaume Nault <>
Reviewed-by: Xin Long <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
drivers/net/ppp/pppoe.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -860,6 +860,7 @@ static int pppoe_sendmsg(struct socket *
struct pppoe_hdr *ph;
struct net_device *dev;
char *start;
+ int hlen;

if (sock_flag(sk, SOCK_DEAD) || !(sk->sk_state & PPPOX_CONNECTED)) {
@@ -878,16 +879,16 @@ static int pppoe_sendmsg(struct socket *
if (total_len > (dev->mtu + dev->hard_header_len))
goto end;

- skb = sock_wmalloc(sk, total_len + dev->hard_header_len + 32,
+ hlen = LL_RESERVED_SPACE(dev);
+ skb = sock_wmalloc(sk, hlen + sizeof(*ph) + total_len +
+ dev->needed_tailroom, 0, GFP_KERNEL);
if (!skb) {
error = -ENOMEM;
goto end;

/* Reserve space for headers. */
- skb_reserve(skb, dev->hard_header_len);
+ skb_reserve(skb, hlen);

skb->dev = dev;
@@ -948,7 +949,7 @@ static int __pppoe_xmit(struct sock *sk,
/* Copy the data if there is no space for the header or if it's
* read-only.
- if (skb_cow_head(skb, sizeof(*ph) + dev->hard_header_len))
+ if (skb_cow_head(skb, LL_RESERVED_SPACE(dev) + sizeof(*ph)))
goto abort;

__skb_push(skb, sizeof(*ph));

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