[PATCH 6.8 320/715] bpf: Fix hashtab overflow check on 32-bit arches

Sasha Levin
2024-03-24T22:40:12Z

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

[ Upstream commit 6787d916c2cf9850c97a0a3f73e08c43e7d973b1 ]

The hashtab code relies on roundup_pow_of_two() to compute the number of
hash buckets, and contains an overflow check by checking if the
resulting value is 0. However, on 32-bit arches, the roundup code itself
can overflow by doing a 32-bit left-shift of an unsigned long value,
which is undefined behaviour, so it is not guaranteed to truncate
neatly. This was triggered by syzbot on the DEVMAP_HASH type, which
contains the same check, copied from the hashtab code. So apply the same
fix to hashtab, by moving the overflow check to before the roundup.

Fixes: daaf427c6ab3 ("bpf: fix arraymap NULL deref and missing overflow and zero size checks")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Message-ID: <20240307120340.99577-3-toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/hashtab.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 03a6a2500b6ab..3a088a5349bc0 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -499,7 +499,13 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
num_possible_cpus());
}

- /* hash table size must be power of 2 */
+ /* hash table size must be power of 2; roundup_pow_of_two() can overflow
+ * into UB on 32-bit arches, so check that first
+ */
+ err = -E2BIG;
+ if (htab->map.max_entries > 1UL << 31)
+ goto free_htab;
+
htab->n_buckets = roundup_pow_of_two(htab->map.max_entries);

htab->elem_size = sizeof(struct htab_elem) +
@@ -509,10 +515,8 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
else
htab->elem_size += round_up(htab->map.value_size, 8);

- err = -E2BIG;
- /* prevent zero size kmalloc and check for u32 overflow */
- if (htab->n_buckets == 0 ||
- htab->n_buckets > U32_MAX / sizeof(struct bucket))
+ /* check for u32 overflow */
+ if (htab->n_buckets > U32_MAX / sizeof(struct bucket))
goto free_htab;

err = bpf_map_init_elem_count(&htab->map);
--
2.43.0