Re: Issue 60887 in oss-fuzz: elfutils:fuzz-libelf: Direct-leak in __libelf_decompress_zlib

[Mark Wielaard <mark@klomp.org>]

Hi,

On Sat, Jul 29, 2023 at 03:00:49PM -0700, evv… via monorail via Elfutils-devel wrote:
> 
> Comment #1 on issue 60887 by evv...@gmail.com: elfutils:fuzz-libelf: Direct-leak in __libelf_decompress_zlib
> https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=60887#c1
> 
> The full backtrace is
> ```
> ==178009==ERROR: LeakSanitizer: detected memory leaks
> Direct leak of 1 byte(s) in 1 object(s) allocated from:
>     #0 0x52efd6 in __interceptor_malloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:69:3
>     #1 0x57a228 in __libelf_decompress_zlib /src/elfutils/libelf/elf_compress.c:370:19
>     #2 0x57a987 in __libelf_decompress /src/elfutils/libelf/elf_compress.c:440:12
>     #3 0x57a987 in __libelf_decompress_elf /src/elfutils/libelf/elf_compress.c:500:7
>     #4 0x57629f in get_zdata /src/elfutils/libelf/elf_strptr.c:45:17
>     #5 0x575c5e in elf_strptr /src/elfutils/libelf/elf_strptr.c:135:38
>     #6 0x56c5b3 in fuzz_logic_one /src/fuzz-libelf.c:40:26
>     #7 0x56cc7f in LLVMFuzzerTestOneInput /src/fuzz-libelf.c:88:3
> ```
> 
> I haven't figured out how to trigger that memory leak without the fuzz target
> but as far as I can tell `fuzz_logic_one` was inspired by the elfgetzdata test in
> the sense that it calls elf_nextscn/elf_strptr/elf_compress.
> 
> The code triggering the memory leak is
> https://github.com/google/oss-fuzz/blob/24328c88fd610decaf311020ffc7073aec1db252/projects/elfutils/fuzz-libelf.c#L27C6-L27C20

Thanks, I can replicate it with that and valgrind.  The issue is when
elf_strptr has (partially) uncompressed the section data (to read the
uncompressed string), the program never requests the (uncompressed)
section data, but does (re)compress it.

Working on a fix.

Cheers,

Mark