[SRU][F][PATCH 1/1] bpf: Fix DEVMAP_HASH overflow check on 32-bit arches

[Koichiro Den]

From: Toke Høiland-Jørgensen <toke at redhat.com>

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/000000000000ed666a0611af6818@google.com
Reported-and-tested-by: syzbot+8cd36f6b65f3cafd400a at syzkaller.appspotmail.com
Signed-off-by: Toke Høiland-Jørgensen <toke at redhat.com>
Message-ID: <20240307120340.99577-2-toke at redhat.com>
Signed-off-by: Alexei Starovoitov <ast at kernel.org>
(backported from commit 281d464a34f540de166cee74b723e97ac2515ec3)
[koichiroden: Adjusted context due to missing commits:
 commit 844f157f6c0a ("bpf: Eliminate rlimit-based memory accounting for devmap maps")
 commit 96360004b862 ("xdp: Make devmap flush_list common for all map instances")]
CVE-2024-26885
Signed-off-by: Koichiro Den <koichiro.den at canonical.com>
---
 kernel/bpf/devmap.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 4b2819b0a05a..b3b5c8f71f58 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -130,10 +130,12 @@ 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 *);
-- 
2.43.0