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, on the other hand,
implemented an extra logic that, when e_shnum is zero and e_shoff is not
zero, 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.
Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40868
Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40957
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@xxxxxxxx>
---
To be honest I'm not sure if any of the BPF toolchain will produce such
ELF binary. Tools like readelf simply refuse to dump section header
table when e_shnum==0 && e_shoff !=0 case is encountered.
While we can use same approach as readelf, opting for a coherent view
with libelf for now since that should be less confusing.
---
tools/lib/bpf/libbpf.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 184ce1684dcd..a64e13c654f3 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -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;
@@ -1369,6 +1369,13 @@ static int bpf_object__elf_init(struct bpf_object *obj)
goto errout;
}
+ 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));
+ err = -LIBBPF_ERRNO__FORMAT;
+ goto errout;
+ }
+
/* Elf is corrupted/truncated, avoid calling elf_strptr. */
if (!elf_rawdata(elf_getscn(elf, obj->efile.shstrndx), NULL)) {
pr_warn("elf: failed to get section names strings from %s: %s\n",
@@ -3315,7 +3322,6 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
* section. e_shnum does include sec #0, so e_shnum is the necessary
* size of an array to keep all the sections.
*/
- obj->efile.sec_cnt = obj->efile.ehdr->e_shnum;
obj->efile.secs = calloc(obj->efile.sec_cnt, sizeof(*obj->efile.secs));
if (!obj->efile.secs)
return -ENOMEM;
--
2.37.3
