[PATCH 12/82] ipv4: Silence intentional wrapping addition

[Kees Cook]

The overflow sanitizer quickly noticed what appears to have been an old
sore spot involving intended wrap around:

[   22.192362] ------------[ cut here ]------------
[   22.193329] UBSAN: signed-integer-overflow in ../arch/x86/include/asm/atomic.h:85:11
[   22.194844] 1469769800 + 1671667352 cannot be represented in type 'int'
[   22.195975] CPU: 2 PID: 2260 Comm: nmbd Not tainted 6.7.0 #1
[   22.196927] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
[   22.198231] Call Trace:
[   22.198641]  <TASK>
[   22.198641]  dump_stack_lvl+0x64/0x80
[   22.199533]  handle_overflow+0x152/0x1a0
[   22.200382]  __ip_select_ident+0xe3/0x100

Explicitly perform a wrapping addition to solve for the needed
-fno-strict-overflow behavior but still allow the sanitizers to operate
correctly.

To see the (unchanged) assembly results more clearly, see:
https://godbolt.org/z/EhYhz6zTT

Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 net/ipv4/route.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 16615d107cf0..c52e85b06fe7 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -473,11 +473,11 @@ static u32 ip_idents_reserve(u32 hash, int segs)
 	if (old != now && cmpxchg(p_tstamp, old, now) == old)
 		delta = get_random_u32_below(now - old);
 
-	/* If UBSAN reports an error there, please make sure your compiler
-	 * supports -fno-strict-overflow before reporting it that was a bug
-	 * in UBSAN, and it has been fixed in GCC-8.
+	/* If UBSAN reports an error there, please make sure your arch's
+	 * atomic_add_return() implementation has been annotated with
+	 * __signed_wrap.
 	 */
-	return atomic_add_return(segs + delta, p_id) - segs;
+	return atomic_add_return(add_wrap(segs, delta), p_id) - segs;
 }
 
 void __ip_select_ident(struct net *net, struct iphdr *iph, int segs)
-- 
2.34.1