Re: [PATCH] libbpf: Fix NULL pointer dereference in find_extern_btf_id

[Stanislav Fomichev <sdf@google.com>]

On 03/18, zhangmingyi wrote:
> From: Mingyi Zhang <zhangmingyi5@huawei.com>
> 
> During our fuzz testing, we encountered the following error:
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x00000000005915bb in __interceptor_strcmp.part.0 ()
> (gdb) bt
>     #0  0x00000000005915bb in __interceptor_strcmp.part.0 ()
>     #1  0x000000000087dc65 in __wrap_strcmp ()
>     #2  0x0000000000951ded in find_extern_btf_id () at libbpf.c:3508
>     #3  0x000000000094d7a1 in bpf_object.collect_externs () at libbpf.c:3712
>     #4  0x000000000092be3b in bpf_object_open () at libbpf.c:7433
>     #5  0x000000000092c046 in bpf_object.open_mem () at libbpf.c:7497
>     #6  0x0000000000924afa in LLVMFuzzerTestOneInput () at fuzz/bpf-object-fuzzer.c:16
>     #7  0x000000000060be11 in testblitz_engine::fuzzer::Fuzzer::run_one ()
>     #8  0x000000000087ad92 in tracing::span::Span::in_scope ()
>     #9  0x00000000006078aa in testblitz_engine::fuzzer::util::walkdir ()
>     #10 0x00000000005f3217 in testblitz_engine::entrypoint::main::{{closure}} ()
>     #11 0x00000000005f2601 in main ()
> (gdb)
> 
> tname = btf__name_by_offset(btf, t->name_off);
> if (strcmp(tname, ext_name))
>         continue;
> 
> tname is passed directly into strcmp without a null pointer check.
> When t(btf_type)->name_off >= btf->hdr->str_len, tname is NULL. normally,
> that's not likely to happen.
> Considering that the bpf_object__open_mem interface is a direct API
> provided to users, which reads directly from memory. There may be an
> input similar to this fuzzing, leading to a Segmentation fault.

Are you trying to parse completely bogus elf obj files?
I don't think we have been hardening against those cases. I see
a bunch of other places where we assume the return of btf__name_by_offset
is non-null. Do we need to audit all those places as well?