From 7cf9cd98cf6f0df3befc167ca6b54c07014d71de Mon Sep 17 00:00:00 2001 From: Leon Hwang Date: Wed, 24 Jun 2026 23:51:14 +0800 Subject: [PATCH 1/2] bpf: Copy per-CPU map value padding in copy_map_value_long() In kernel, per-CPU map elements are stored with round_up(map->value_size, 8) bytes. On UAPI lookup paths, it copies the rounded size for each CPU into a temporary buffer. However, copy_map_value_long() passes 'map->value_size' to bpf_obj_memcpy(). When the map has special fields, bpf_obj_memcpy() copies around those fields with memcpy(), and does not copy the tail padding between 'map->value_size' and round_up(map->value_size, 8). The temporary UAPI lookup buffers are allocated without __GFP_ZERO. As a result, when the per-CPU map's value size is not equal to round_up(map->value_size, 8), UAPI LOOKUP_ELEM and its variants can return stale heap contents from that padding to user space. The same issue applies to bpf_iter for per-CPU maps. Pass round_up(map->value_size, 8) to bpf_obj_memcpy() from copy_map_value_long(), so per-CPU maps both with and without special fields copy the entire per-CPU slot. Remove the now redundant round_up() from bpf_obj_memcpy()'s long_memcpy path. Fixes: 448325199f57 ("bpf: Add copy_map_value_long to copy to remote percpu memory") Signed-off-by: Leon Hwang Signed-off-by: Andrii Nakryiko Link: https://lore.kernel.org/bpf/20260624155115.85196-2-leon.hwang@linux.dev --- include/linux/bpf.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 7719f6528445..ba09795e0bfd 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -570,7 +570,7 @@ static inline void bpf_obj_memcpy(struct btf_record *rec, if (IS_ERR_OR_NULL(rec)) { if (long_memcpy) - bpf_long_memcpy(dst, src, round_up(size, 8)); + bpf_long_memcpy(dst, src, size); else memcpy(dst, src, size); return; @@ -593,7 +593,7 @@ static inline void copy_map_value(struct bpf_map *map, void *dst, void *src) static inline void copy_map_value_long(struct bpf_map *map, void *dst, void *src) { - bpf_obj_memcpy(map->record, dst, src, map->value_size, true); + bpf_obj_memcpy(map->record, dst, src, round_up(map->value_size, 8), true); } static inline void bpf_obj_swap_uptrs(const struct btf_record *rec, void *dst, void *src) From 163944262f8646bf3a1eec557b6aff1f38582a9f Mon Sep 17 00:00:00 2001 From: Leon Hwang Date: Wed, 24 Jun 2026 23:51:15 +0800 Subject: [PATCH 2/2] selftests/bpf: Verify no non-zeroed kernel heap memory exposure When lookup element from those per-CPU maps, which have special field in their values and their value size is not equal to roundup(value_sz, 8), the padding size of temporary non-zeroed kernel heap memory allocated by kvmalloc should not be exposed to user space. Without the fix: test_map_uninit_mem_exposure:FAIL:zeroed tail bytes unexpected memory mismatch actual: 2B 2B 2B 2B expected: 00 00 00 00 Assisted-by: Codex:gpt-5.5 Signed-off-by: Leon Hwang Signed-off-by: Andrii Nakryiko Link: https://lore.kernel.org/bpf/20260624155115.85196-3-leon.hwang@linux.dev --- .../bpf/prog_tests/test_map_uninit.c | 68 +++++++++++++++++++ tools/testing/selftests/bpf/progs/map_kptr.c | 12 ++++ 2 files changed, 80 insertions(+) create mode 100644 tools/testing/selftests/bpf/prog_tests/test_map_uninit.c diff --git a/tools/testing/selftests/bpf/prog_tests/test_map_uninit.c b/tools/testing/selftests/bpf/prog_tests/test_map_uninit.c new file mode 100644 index 000000000000..d0ba2ca587b0 --- /dev/null +++ b/tools/testing/selftests/bpf/prog_tests/test_map_uninit.c @@ -0,0 +1,68 @@ +// SPDX-License-Identifier: GPL-2.0 +#include + +#include "map_kptr.skel.h" + +void test_map_uninit_mem_exposure(void) +{ + size_t value_sz, slot_sz, lookup_sz, tail_sz; + int err, key, nr_cpus, cpu, map_fd; + __u8 *value = NULL, *zero = NULL; + struct bpf_program *prog; + struct map_kptr *skel; + + nr_cpus = libbpf_num_possible_cpus(); + if (!ASSERT_GT(nr_cpus, 0, "libbpf_num_possible_cpus")) + return; + + skel = map_kptr__open(); + if (!ASSERT_OK_PTR(skel, "map_kptr__open")) + return; + + bpf_object__for_each_program(prog, skel->obj) { + err = bpf_program__set_autoload(prog, false); + if (!ASSERT_OK(err, "bpf_program__set_autoload")) + goto out; + } + + err = map_kptr__load(skel); + if (!ASSERT_OK(err, "map_kptr__load")) + goto out; + + value_sz = bpf_map__value_size((skel)->maps.pcpu_array); + slot_sz = roundup(value_sz, 8); + tail_sz = slot_sz - value_sz; + if (!ASSERT_NEQ(tail_sz, 0, "tail_sz")) + goto out; + + lookup_sz = slot_sz * nr_cpus; + map_fd = bpf_map__fd(skel->maps.pcpu_array); + + value = malloc(lookup_sz); + zero = calloc(1, tail_sz); + if (!ASSERT_OK_PTR(value, "malloc value") || !ASSERT_OK_PTR(zero, "calloc zero")) + goto out; + + key = 0; + memset(value, 0x2B, lookup_sz); + err = bpf_map_update_elem(map_fd, &key, value, BPF_ANY); + if (!ASSERT_OK(err, "bpf_map_update_elem")) + goto out; + + memset(value, 0xFF, lookup_sz); + err = bpf_map_lookup_elem(map_fd, &key, value); + if (!ASSERT_OK(err, "bpf_map_lookup_elem")) + goto out; + + for (cpu = 0; cpu < nr_cpus; cpu++) { + __u8 *tail = value + cpu * slot_sz + value_sz; + + if (!ASSERT_MEMEQ(tail, zero, tail_sz, "zeroed tail bytes")) + goto out; + } + +out: + free(zero); + free(value); + map_kptr__destroy(skel); +} diff --git a/tools/testing/selftests/bpf/progs/map_kptr.c b/tools/testing/selftests/bpf/progs/map_kptr.c index 3fbefc568e0a..0d87c97dac99 100644 --- a/tools/testing/selftests/bpf/progs/map_kptr.c +++ b/tools/testing/selftests/bpf/progs/map_kptr.c @@ -4,6 +4,18 @@ #include #include "../test_kmods/bpf_testmod_kfunc.h" +struct map_uninit_value { + struct prog_test_ref_kfunc __kptr_untrusted *unref_ptr; + __u32 data; +} __attribute__((packed)); + +struct { + __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY); + __type(key, int); + __type(value, struct map_uninit_value); + __uint(max_entries, 1); +} pcpu_array SEC(".maps"); + struct map_value { struct prog_test_ref_kfunc __kptr_untrusted *unref_ptr; struct prog_test_ref_kfunc __kptr *ref_ptr;