On 5/20/22 4:50 PM, Yonghong Song wrote:
On 5/20/22 4:37 AM, Shung-Hsi Yu wrote:
The BPF_SIZE check in the beginning of check_ld_imm() actually guard
against program with JMP instructions that goes to the second
instruction of BPF_LD_IMM64, but may be easily dismissed as an simple
opcode check that's duplicating the effort of bpf_opcode_in_insntable().
Add comment to better reflect the importance of the check.
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx>
---
kernel/bpf/verifier.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 79a2695ee2e2..133929751f80 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9921,6 +9921,10 @@ static int check_ld_imm(struct bpf_verifier_env
*env, struct bpf_insn *insn)
struct bpf_map *map;
int err;
+ /* checks that this is not the second part of BPF_LD_IMM64, which is
+ * skipped over during opcode check, but a JMP with invalid
offset may
+ * cause check_ld_imm() to be called upon it.
+ */
The check_ld_imm() call context is:
} else if (class == BPF_LD) {
u8 mode = BPF_MODE(insn->code);
if (mode == BPF_ABS || mode == BPF_IND) {
err = check_ld_abs(env, insn);
if (err)
return err;
} else if (mode == BPF_IMM) {
err = check_ld_imm(env, insn);
if (err)
return err;
env->insn_idx++;
sanitize_mark_insn_seen(env);
} else {
verbose(env, "invalid BPF_LD mode\n");
return -EINVAL;
}
}
which is a normal checking of LD_imm64 insn.
I think the to-be-added comment is incorrect and unnecessary.
Okay, double check again and now I understand what happens
when hitting the second insn of ldimm64 with a branch target.
Here we have BPF_LD = 0 and BPF_IMM = 0, so for a branch
target to the 2nd part of ldimm64, it will come to
check_ld_imm() and have error "invalid BPF_LD_IMM insn"
So check_ld_imm() is to check whether the insn is a
*legal* insn for the first part of ldimm64.
So the comment may be rewritten as below.
This is to verify whether an insn is a BPF_LD_IMM64
or not. But since BPF_LD = 0 and BPF_IMM = 0, if the branch
target comes to the second part of BPF_LD_IMM64,
the control may come here as well.
if (BPF_SIZE(insn->code) != BPF_DW) {
verbose(env, "invalid BPF_LD_IMM insn\n");
return -EINVAL;
