[PATCH bpf] bpf: harden bloom filter allocation sizing on 32-bit kernels

bloom_map_alloc() can under-allocate bloom filter storage on 32-bit
kernels when the bitset size reaches the U32_MAX fallback case.

BITS_TO_BYTES(U32_MAX) is evaluated with 32-bit arithmetic and wraps to 0,
while bitset_mask remains U32_MAX. As a result, later bloom filter updates
can write beyond the allocated object.

I reached local privilege escalation using only this bug from a binary with
CAP_BPF.

Compute the bitset size from bitset_mask in u64 and check the final
allocation size before calling bpf_map_area_alloc().

Fixes: 9330986c0300 ("bpf: Add bloom filter map implementation")
Cc: [email protected]
Signed-off-by: Jérémy Jean <[email protected]>
---
 kernel/bpf/bloom_filter.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/bloom_filter.c b/kernel/bpf/bloom_filter.c
index b73336c976b..8e6532fa418 100644
--- a/kernel/bpf/bloom_filter.c
+++ b/kernel/bpf/bloom_filter.c
@@ -94,9 +94,10 @@ static int bloom_map_alloc_check(union bpf_attr *attr)
 
 static struct bpf_map *bloom_map_alloc(union bpf_attr *attr)
 {
-	u32 bitset_bytes, bitset_mask, nr_hash_funcs, nr_bits;
+	u32 bitset_mask, nr_hash_funcs, nr_bits;
 	int numa_node = bpf_map_attr_numa_node(attr);
 	struct bpf_bloom_filter *bloom;
+	u64 bitset_bytes, alloc_size;
 
 	if (attr->key_size != 0 || attr->value_size == 0 ||
 	    attr->max_entries == 0 ||
@@ -127,23 +128,21 @@ static struct bpf_map *bloom_map_alloc(union bpf_attr *attr)
 	if (check_mul_overflow(attr->max_entries, nr_hash_funcs, &nr_bits) ||
 	    check_mul_overflow(nr_bits / 5, (u32)7, &nr_bits) ||
 	    nr_bits > (1UL << 31)) {
-		/* The bit array size is 2^32 bits but to avoid overflowing the
-		 * u32, we use U32_MAX, which will round up to the equivalent
-		 * number of bytes
-		 */
-		bitset_bytes = BITS_TO_BYTES(U32_MAX);
 		bitset_mask = U32_MAX;
 	} else {
 		if (nr_bits <= BITS_PER_LONG)
 			nr_bits = BITS_PER_LONG;
 		else
 			nr_bits = roundup_pow_of_two(nr_bits);
-		bitset_bytes = BITS_TO_BYTES(nr_bits);
 		bitset_mask = nr_bits - 1;
 	}
 
+	bitset_bytes = BITS_TO_BYTES((u64)bitset_mask + 1);
 	bitset_bytes = roundup(bitset_bytes, sizeof(unsigned long));
-	bloom = bpf_map_area_alloc(sizeof(*bloom) + bitset_bytes, numa_node);
+	if (check_add_overflow((u64)sizeof(*bloom), bitset_bytes, &alloc_size))
+		return ERR_PTR(-E2BIG);
+
+	bloom = bpf_map_area_alloc(alloc_size, numa_node);
 
 	if (!bloom)
 		return ERR_PTR(-ENOMEM);
-- 
2.47.3