[5.4] bpf: Fix DEVMAP_HASH overflow check on 32-bit arches

Commit Message

Pu Lehui Sept. 20, 2024, 10:49 a.m. UTC

From: Toke Høiland-Jørgensen <[e-mail]>

[ Upstream commit 281d464a34f540de166cee74b723e97ac2515ec3 ]

The devmap code allocates a number hash buckets equal to the next power
of two of the max_entries value provided when creating the map. When
rounding up to the next power of two, the 32-bit variable storing the
number of buckets can overflow, and the code checks for overflow by
checking if the truncated 32-bit value is equal to 0. However, on 32-bit
arches the rounding up itself can overflow mid-way through, because it
ends up doing a left-shift of 32 bits on an unsigned long value. If the
size of an unsigned long is four bytes, this is undefined behaviour, so
there is no guarantee that we'll end up with a nice and tidy 0-value at
the end.

Syzbot managed to turn this into a crash on arm32 by creating a
DEVMAP_HASH with max_entries > 0x80000000 and then trying to update it.
Fix this by moving the overflow check to before the rounding up
operation.

Fixes: 6f9d451ab1a3 ("xdp: Add devmap_hash map type for looking up devices by hashed index")
Link: https://lore.kernel.org/r/[e-mail]
Reported-and-tested-by: [e-mail]
Signed-off-by: Toke Høiland-Jørgensen <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Pu Lehui <[e-mail]>
---
 kernel/bpf/devmap.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Patch

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 4b2819b0a05a..2370fc31169f 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -130,10 +130,13 @@  static int dev_map_init_map(struct bpf_dtab *dtab, union bpf_attr *attr)
 	cost = (u64) sizeof(struct list_head) * num_possible_cpus();
 
 	if (attr->map_type == BPF_MAP_TYPE_DEVMAP_HASH) {
-		dtab->n_buckets = roundup_pow_of_two(dtab->map.max_entries);
-
-		if (!dtab->n_buckets) /* Overflow check */
+		/* hash table size must be power of 2; roundup_pow_of_two() can
+		 * overflow into UB on 32-bit arches, so check that first
+		 */
+		if (dtab->map.max_entries > 1UL << 31)
 			return -EINVAL;
+
+		dtab->n_buckets = roundup_pow_of_two(dtab->map.max_entries);
 		cost += (u64) sizeof(struct hlist_head) * dtab->n_buckets;
 	} else {
 		cost += (u64) dtab->map.max_entries * sizeof(struct bpf_dtab_netdev *);