Re: [PATCH bpf] xdp: Prevent overflow in devmap_hash cost calculation for 32-bit builds

On Fri, 18 Oct 2019 11:15:39 +0200, Toke Høiland-Jørgensen wrote:

Jakub Kicinski [off-list ref] writes:

quoted

On Thu, 17 Oct 2019 12:57:02 +0200, Toke Høiland-Jørgensen wrote:  

quoted

Tetsuo pointed out that without an explicit cast, the cost calculation for
devmap_hash type maps could overflow on 32-bit builds. This adds the
missing cast.

Fixes: 6f9d451ab1a3 ("xdp: Add devmap_hash map type for looking up devices by hashed index")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 kernel/bpf/devmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index a0a1153da5ae..e34fac6022eb 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c

@@ -128,7 +128,7 @@ static int dev_map_init_map(struct bpf_dtab *dtab, union bpf_attr *attr)
 
 		if (!dtab->n_buckets) /* Overflow check */
 			return -EINVAL;
-		cost += sizeof(struct hlist_head) * dtab->n_buckets;
+		cost += (u64) sizeof(struct hlist_head) * dtab->n_buckets;  

array_size()?  

Well, array_size does this:

	if (check_mul_overflow(a, b, &bytes))
		return SIZE_MAX;

However, we don't to return SIZE_MAX on overflow, we want the
calculation itself to be done in 64 bits so it won't overflow... Or?

Note that array_size calculates on size_t, so it should be fine.
But looking at it, it seems all of this code uses the (u64) cast, 
so I guess that's fine. Clean up for another day :)