On Fri, Oct 7, 2022 at 10:48 AM Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx> wrote: > > 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)) { It bothers me that sec_cnt is initialized in bpf_object__elf_init, but secs are allocated a bit later in bpf_object__elf_collect(). What if we move elf_getshdrnum() call and sec_cnt initialization into bpf_object__elf_collect()? > + 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 >
