[PATCH v3 bpf] bpf: harden bloom filter sizing and indexing on 32-bit kernels

bloom_map_alloc() has two 32-bit-specific problems when the computed
bitmap reaches the U32_MAX fallback case.

First, BITS_TO_BYTES(U32_MAX) is evaluated with 32-bit arithmetic. The
addition performed by DIV_ROUND_UP wraps, so the map allocates only the
fixed-size bloom filter object while keeping bitset_mask == U32_MAX.
Subsequent updates can then write past the allocated object.

Second, fixing only the allocation size is not sufficient. The bloom hash
is a u32, but set_bit() takes a signed long bit number and x86 test_bit()
eventually feeds the index to variable_test_bit(long, ...). On 32-bit
kernels, hashes in [0x80000000, U32_MAX] therefore become negative bit
offsets. x86 bt/bts with a memory operand interpret those offsets relative
to the supplied base, so a map with bitset_mask == U32_MAX can read or
write before bloom->bitset even after allocating the full 512 MiB bitmap.

Keep the U32_MAX fallback, but split each hash into a word pointer and an
in-word bit number before calling test_bit() or set_bit(). The bitops
argument is then always in [0, BITS_PER_LONG - 1], while BIT_WORD(h) still
selects the intended word in the full bitmap.

Compute the bitset size from (u64)bitset_mask + 1 before passing the final
size to bpf_map_area_alloc(). This fixes the original under-allocation and
keeps the allocated storage consistent with the addressable bitset.

Exploitation note: local privilege escalation is possible on a 32-bit x86
kernel using the under-allocation bug from a binary with CAP_BPF.

Fixes: 9330986c0300 ("bpf: Add bloom filter map implementation")
Cc: [email protected]
Signed-off-by: Jérémy Jean <[email protected]>
Assisted-by: Codex:gpt-5
---
v1: https://lore.kernel.org/bpf/[email protected]/
v2: https://lore.kernel.org/bpf/[email protected]/

Changes in v3:
- Address change request simplification from Andrii

Changes in v2:
- Address AI reviews.
- Split full-width hashes into a word pointer and an in-word bit number
  before calling bitops, so upper-half u32 hashes cannot become negative
  offsets on 32-bit kernels.
- Drop the redundant final u64 add-overflow check; bitset_bytes is bounded
  at 512 MiB once it is computed from bitset_mask.

 kernel/bpf/bloom_filter.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/bloom_filter.c b/kernel/bpf/bloom_filter.c
index b73336c..b63839d 100644
--- a/kernel/bpf/bloom_filter.c
+++ b/kernel/bpf/bloom_filter.c
@@ -41,7 +41,8 @@ static long bloom_map_peek_elem(struct bpf_map *map, void *value)
 
 	for (i = 0; i < bloom->nr_hash_funcs; i++) {
 		h = hash(bloom, value, map->value_size, i);
-		if (!test_bit(h, bloom->bitset))
+		if (!test_bit(h % BITS_PER_LONG,
+			      bloom->bitset + BIT_WORD(h)))
 			return -ENOENT;
 	}
 
@@ -57,9 +58,13 @@ static long bloom_map_push_elem(struct bpf_map *map, void *value, u64 flags)
 	if (flags != BPF_ANY)
 		return -EINVAL;
 
+	/*
+	 * On 32-bit architectures, hashes larger than INT_MAX would be
+	 * treated as negative by set_bit().
+	 */
 	for (i = 0; i < bloom->nr_hash_funcs; i++) {
 		h = hash(bloom, value, map->value_size, i);
-		set_bit(h, bloom->bitset);
+		set_bit(h % BITS_PER_LONG, bloom->bitset + BIT_WORD(h));
 	}
 
 	return 0;
@@ -94,9 +99,10 @@ 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;
 
 	if (attr->key_size != 0 || attr->value_size == 0 ||
 	    attr->max_entries == 0 ||
@@ -127,22 +133,16 @@ 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 = roundup(bitset_bytes, sizeof(unsigned long));
+	bitset_bytes = BITS_TO_LONGS((u64)bitset_mask + 1) * sizeof(unsigned long);
 	bloom = bpf_map_area_alloc(sizeof(*bloom) + bitset_bytes, numa_node);
 
 	if (!bloom)
-- 
2.47.3