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libbpf: Use elf_getshdrnum() instead of e_shnum

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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
This commit is contained in:
Shung-Hsi Yu 2022-10-12 10:23:51 +08:00 committed by Andrii Nakryiko
parent 6e73e683b6
commit 51deedc9b8
1 changed files with 9 additions and 4 deletions
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13
tools/lib/bpf/libbpf.c
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@ -597,7 +597,7 @@ 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;
@ -3312,10 +3312,15 @@ 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;