[PATCH 4.4 29/43] net: ipv6: RTF_PCPU should not be settable from userspace

From: Greg Kroah-Hartman
Date: Mon May 01 2017 - 17:29:09 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: David Ahern <dsa@xxxxxxxxxxxxxxxxxxx>

[ Upstream commit 557c44be917c322860665be3d28376afa84aa936 ]

Andrey reported a fault in the IPv6 route code:

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Modules linked in:
CPU: 1 PID: 4035 Comm: a.out Not tainted 4.11.0-rc7+ #250
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff880069809600 task.stack: ffff880062dc8000
RIP: 0010:ip6_rt_cache_alloc+0xa6/0x560 net/ipv6/route.c:975
RSP: 0018:ffff880062dced30 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: ffff8800670561c0 RCX: 0000000000000006
RDX: 0000000000000003 RSI: ffff880062dcfb28 RDI: 0000000000000018
RBP: ffff880062dced68 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffff880062dcfb28 R14: dffffc0000000000 R15: 0000000000000000
FS: 00007feebe37e7c0(0000) GS:ffff88006cb00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000205a0fe4 CR3: 000000006b5c9000 CR4: 00000000000006e0
Call Trace:
ip6_pol_route+0x1512/0x1f20 net/ipv6/route.c:1128
ip6_pol_route_output+0x4c/0x60 net/ipv6/route.c:1212

Andrey's syzkaller program passes rtmsg.rtmsg_flags with the RTF_PCPU bit
set. Flags passed to the kernel are blindly copied to the allocated
rt6_info by ip6_route_info_create making a newly inserted route appear
as though it is a per-cpu route. ip6_rt_cache_alloc sees the flag set
and expects rt->dst.from to be set - which it is not since it is not
really a per-cpu copy. The subsequent call to __ip6_dst_alloc then
generates the fault.

Fix by checking for the flag and failing with EINVAL.

Fixes: d52d3997f843f ("ipv6: Create percpu rt6_info")
Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: David Ahern <dsa@xxxxxxxxxxxxxxxxxxx>
Acked-by: Martin KaFai Lau <kafai@xxxxxx>
Tested-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
include/uapi/linux/ipv6_route.h | 2 +-
net/ipv6/route.c | 4 ++++
2 files changed, 5 insertions(+), 1 deletion(-)

--- a/include/uapi/linux/ipv6_route.h
+++ b/include/uapi/linux/ipv6_route.h
@@ -34,7 +34,7 @@
#define RTF_PREF(pref) ((pref) << 27)
#define RTF_PREF_MASK 0x18000000

-#define RTF_PCPU 0x40000000
+#define RTF_PCPU 0x40000000 /* read-only: can not be set by user */
#define RTF_LOCAL 0x80000000

--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1758,6 +1758,10 @@ static struct rt6_info *ip6_route_info_c
int addr_type;
int err = -EINVAL;

+ /* RTF_PCPU is an internal flag; can not be set by userspace */
+ if (cfg->fc_flags & RTF_PCPU)
+ goto out;
if (cfg->fc_dst_len > 128 || cfg->fc_src_len > 128)
goto out;