xfrm: validate template mode

XFRM mode parameters passed as part of the user templates
in the IP_XFRM_POLICY are never properly validated. Passing
values other than valid XFRM modes can cause stack-out-of-bounds
reads to occur later in the XFRM processing:

[  140.535608] ================================================================
[  140.543058] BUG: KASAN: stack-out-of-bounds in xfrm_state_find+0x17e4/0x1cc4
[  140.550306] Read of size 4 at addr ffffffc0238a7a58 by task repro/5148
[  140.557369]
[  140.558927] Call trace:
[  140.558936] dump_backtrace+0x0/0x388
[  140.558940] show_stack+0x24/0x30
[  140.558946] __dump_stack+0x24/0x2c
[  140.558949] dump_stack+0x8c/0xd0
[  140.558956] print_address_description+0x74/0x234
[  140.558960] kasan_report+0x240/0x264
[  140.558963] __asan_report_load4_noabort+0x2c/0x38
[  140.558967] xfrm_state_find+0x17e4/0x1cc4
[  140.558971] xfrm_resolve_and_create_bundle+0x40c/0x1fb8
[  140.558975] xfrm_lookup+0x238/0x1444
[  140.558977] xfrm_lookup_route+0x48/0x11c
[  140.558984] ip_route_output_flow+0x88/0xc4
[  140.558991] raw_sendmsg+0xa74/0x266c
[  140.558996] inet_sendmsg+0x258/0x3b0
[  140.559002] sock_sendmsg+0xbc/0xec
[  140.559005] SyS_sendto+0x3a8/0x5a8
[  140.559008] el0_svc_naked+0x34/0x38
[  140.559009]
[  140.592245] page dumped because: kasan: bad access detected
[  140.597981] page_owner info is not active (free page?)
[  140.603267]
[  140.653503] ================================================================
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_user.c

@@ -1455,6 +1455,9 @@ static int validate_tmpl(int nr, struct xfrm_user_tmpl *ut, u16 family)
 		    (ut[i].family != prev_family))
 			return -EINVAL;
 
+		if (ut[i].mode >= XFRM_MODE_MAX)
+			return -EINVAL;
+
 		prev_family = ut[i].family;
 
 		switch (ut[i].family) {