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bpf: Fix hashtab overflow check on 32-bit arches 

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: daaf427c6a ("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>
This commit is contained in:
Toke Høiland-Jørgensen 2024-03-07 13:03:36 +01:00 committed by Alexei Starovoitov
parent 281d464a34
commit 6787d916c2
1 changed files with 9 additions and 5 deletions
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14
kernel/bpf/hashtab.c
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@ -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);