syzbot reported an array-index-out-of-bounds when printing out bpf
insns. Further investigation shows the insn is illegal but
is printed out due to log level 1 or 2 before actual insn verification
in do_check().
This particular illegal insn is a MOVSX insn with offset value 2.
The legal offset value for MOVSX should be 8, 16 and 32.
The disasm sign-extension-size array index is calculated as
(insn->off / 8) - 1
and offset value 2 gives an out-of-bound index -1.
Tighten the checking for MOVSX insn in disasm.c to avoid
array-index-out-of-bounds issue.
Reported-by: syzbot+3758842a6c01012aa73b@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: f835bb622299 ("bpf: Add kernel/bpftool asm support for new instructions")
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
kernel/bpf/disasm.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c
index d7bff608f299..ef7c107c7b8f 100644
--- a/kernel/bpf/disasm.c
+++ b/kernel/bpf/disasm.c
@@ -162,7 +162,8 @@ static bool is_sdiv_smod(const struct bpf_insn *insn)
static bool is_movsx(const struct bpf_insn *insn)
{
- return BPF_OP(insn->code) == BPF_MOV && insn->off != 0;
+ return BPF_OP(insn->code) == BPF_MOV &&
+ (insn->off == 8 || insn->off == 16 || insn->off == 32);
}
void print_bpf_insn(const struct bpf_insn_cbs *cbs,
--
2.34.1
