On 2/9/22 1:11 AM, Hou Tao wrote:
Now kfunc call uses s32 to represent the offset between the address
of kfunc and __bpf_call_base, but it doesn't check whether or not
s32 will be overflowed, so add an extra checking to reject these
invalid kfunc calls.
Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
The patch itself looks good. But the commit message
itself doesn't specify whether this is a theoretical case or
could really happen in practice. I look at the patch history,
and find the become commit message in v1 of the patch ([1]):
> Since commit b2eed9b58811 ("arm64/kernel: kaslr: reduce module
> randomization range to 2 GB"), for arm64 whether KASLR is enabled
> or not, the module is placed within 2GB of the kernel region, so
> s32 in bpf_kfunc_desc is sufficient to represente the offset of
> module function relative to __bpf_call_base. The only thing needed
> is to override bpf_jit_supports_kfunc_call().
So it does look like the overflow is possible.
So I suggest you add more description on *when* the overflow
may happen in this patch.
And you can also retain your previous selftest patch to test
this verifier change.
[1]
https://lore.kernel.org/bpf/20220119144942.305568-1-houtao1@xxxxxxxxxx/
---
v3:
* call BPF_CALL_IMM() once (suggested by Yonghong)
v2: https://lore.kernel.org/bpf/20220208123348.40360-1-houtao1@xxxxxxxxxx
* instead of checking the overflow in selftests, just reject
these kfunc calls directly in verifier
v1: https://lore.kernel.org/bpf/20220206043107.18549-1-houtao1@xxxxxxxxxx
---
kernel/bpf/verifier.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1ae41d0cf96c..eb72e6139e2b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1842,6 +1842,7 @@ static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, s16 offset)
struct bpf_kfunc_desc *desc;
const char *func_name;
struct btf *desc_btf;
+ unsigned long call_imm;
unsigned long addr;
int err;
@@ -1926,9 +1927,17 @@ static int add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, s16 offset)
return -EINVAL;
}
+ call_imm = BPF_CALL_IMM(addr);
+ /* Check whether or not the relative offset overflows desc->imm */
+ if ((unsigned long)(s32)call_imm != call_imm) {
+ verbose(env, "address of kernel function %s is out of range\n",
+ func_name);
+ return -EINVAL;
+ }
+
desc = &tab->descs[tab->nr_descs++];
desc->func_id = func_id;
- desc->imm = BPF_CALL_IMM(addr);
+ desc->imm = call_imm;
desc->offset = offset;
err = btf_distill_func_proto(&env->log, desc_btf,
func_proto, func_name,
