From: Shung-Hsi Yu shung-hsi.yu@suse.com
[ Upstream commit 51deedc9b8680953437dfe359e5268120de10e30 ]
This commit replace e_shnum with the elf_getshdrnum() helper to fix two oss-fuzz-reported heap-buffer overflow in __bpf_object__open. Both reports are incorrectly marked as fixed and while still being reproducible in the latest libbpf.
# clusterfuzz-testcase-minimized-bpf-object-fuzzer-5747922482888704 libbpf: loading object 'fuzz-object' from buffer libbpf: sec_cnt is 0 libbpf: elf: section(1) .data, size 0, link 538976288, flags 2020202020202020, type=2 libbpf: elf: section(2) .data, size 32, link 538976288, flags 202020202020ff20, type=1 ================================================================= ==13==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6020000000c0 at pc 0x0000005a7b46 bp 0x7ffd12214af0 sp 0x7ffd12214ae8 WRITE of size 4 at 0x6020000000c0 thread T0 SCARINESS: 46 (4-byte-write-heap-buffer-overflow-far-from-bounds) #0 0x5a7b45 in bpf_object__elf_collect /src/libbpf/src/libbpf.c:3414:24 #1 0x5733c0 in bpf_object_open /src/libbpf/src/libbpf.c:7223:16 #2 0x5739fd in bpf_object__open_mem /src/libbpf/src/libbpf.c:7263:20 ...
The issue lie in libbpf's direct use of e_shnum field in ELF header as the section header count. Where as libelf implemented an extra logic that, when e_shnum == 0 && e_shoff != 0, will use sh_size member of the initial section header as the real section header count (part of ELF spec to accommodate situation where section header counter is larger than SHN_LORESERVE).
The above inconsistency lead to libbpf writing into a zero-entry calloc area. So intead of using e_shnum directly, use the elf_getshdrnum() helper provided by libelf to retrieve the section header counter into sec_cnt.
Fixes: 0d6988e16a12 ("libbpf: Fix section counting logic") Fixes: 25bbbd7a444b ("libbpf: Remove assumptions about uniqueness of .rodata/.data/.bss maps") Signed-off-by: Shung-Hsi Yu shung-hsi.yu@suse.com Signed-off-by: Andrii Nakryiko andrii@kernel.org Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40868 Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40957 Link: https://lore.kernel.org/bpf/20221012022353.7350-2-shung-hsi.yu@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- tools/lib/bpf/libbpf.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index 264790796001..7f3cec7e7349 100644 --- a/tools/lib/bpf/libbpf.c +++ b/tools/lib/bpf/libbpf.c @@ -590,7 +590,7 @@ struct elf_state { size_t shstrndx; /* section index for section name strings */ size_t strtabidx; struct elf_sec_desc *secs; - int sec_cnt; + size_t sec_cnt; int btf_maps_shndx; __u32 btf_maps_sec_btf_id; int text_shndx; @@ -3282,10 +3282,15 @@ static int bpf_object__elf_collect(struct bpf_object *obj) Elf64_Shdr *sh;
/* ELF section indices are 0-based, but sec #0 is special "invalid" - * section. e_shnum does include sec #0, so e_shnum is the necessary - * size of an array to keep all the sections. + * section. Since section count retrieved by elf_getshdrnum() does + * include sec #0, it is already the necessary size of an array to keep + * all the sections. */ - obj->efile.sec_cnt = obj->efile.ehdr->e_shnum; + if (elf_getshdrnum(obj->efile.elf, &obj->efile.sec_cnt)) { + pr_warn("elf: failed to get the number of sections for %s: %s\n", + obj->path, elf_errmsg(-1)); + return -LIBBPF_ERRNO__FORMAT; + } obj->efile.secs = calloc(obj->efile.sec_cnt, sizeof(*obj->efile.secs)); if (!obj->efile.secs) return -ENOMEM;