On Mon, Oct 10, 2022 at 05:44:20PM -0700, Andrii Nakryiko wrote: > 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()? Ack. My rational for placing it there was that it's closer to other elf_*() helper calls, but having it close to the allocation where it's used seems like a better option. Will change accordingly and send a v2 based on top of bpf-next. > > + 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 > >
