6.1-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jérémy Jean <redacted>
commit 11c1e836710dcba03e50454a4eedfdbaf8d3050e upstream.
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")
Signed-off-by: Jérémy Jean <redacted>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Cc: <redacted>
Link: https://lore.kernel.org/bpf/20260805060228.2703051-1-Jeremy.Jean@oss.cyber.gouv.fr (local)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Assisted-by: Codex:gpt-5
---
kernel/bpf/bloom_filter.c | 19 +++++++++----------
1 file changed, 9 insertions(+), 10 deletions(-)
--- a/kernel/bpf/bloom_filter.c
+++ b/kernel/bpf/bloom_filter.c
@@ -49,7 +49,7 @@ static int bloom_map_peek_elem(struct bp
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;
}
@@ -65,9 +65,13 @@ static int bloom_map_push_elem(struct bp
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;@@ -102,9 +106,10 @@ static int bloom_map_alloc_check(union b
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 (!bpf_capable())
return ERR_PTR(-EPERM);@@ -138,22 +143,16 @@ static struct bpf_map *bloom_map_alloc(u
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)