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From: Shung-Hsi Yu <shung-hsi.yu@suse.com>
To: bpf@vger.kernel.org, Andrii Nakryiko <andrii@kernel.org>
Cc: Shung-Hsi Yu <shung-hsi.yu@suse.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>
Subject: [PATCH bpf 1/3] libbpf: use elf_getshdrnum() instead of e_shnum
Date: Sat,  8 Oct 2022 01:48:14 +0800	[thread overview]
Message-ID: <20221007174816.17536-2-shung-hsi.yu@suse.com> (raw)
In-Reply-To: <20221007174816.17536-1-shung-hsi.yu@suse.com>

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

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@suse.com>

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));
+		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;

  reply	other threads:[~2022-10-07 17:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-07 17:48 [PATCH bpf 0/3] libbpf: fix fuzzer-reported issues Shung-Hsi Yu
2022-10-07 17:48 ` Shung-Hsi Yu [this message]
2022-10-11  0:44   ` [PATCH bpf 1/3] libbpf: use elf_getshdrnum() instead of e_shnum Andrii Nakryiko
2022-10-11  3:55     ` Shung-Hsi Yu
2022-10-11 14:53       ` Shung-Hsi Yu
2022-10-11 16:06         ` Andrii Nakryiko
2022-10-12  1:50           ` Shung-Hsi Yu
2022-10-07 17:48 ` [PATCH bpf 2/3] libbpf: fix null-pointer dereference in find_prog_by_sec_insn() Shung-Hsi Yu
2022-10-07 17:48 ` [PATCH bpf 3/3] libbpf: deal with section with no data gracefully Shung-Hsi Yu
2022-10-11  0:47 ` [PATCH bpf 0/3] libbpf: fix fuzzer-reported issues Andrii Nakryiko

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