[PATCH v1] arch/x86/kernel: check the return value of insn_decode_kernel()

[PATCH v1] arch/x86/kernel: check the return value of insn_decode_kernel()

[Li Zhong]

insn_decode() could fail and the insn.length could be invalid. So we
need to check the return value first.

Signed-off-by: Li Zhong <floridsleeves@gmail.com>
---
 arch/x86/kernel/jump_label.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
index f5b8ef02d172..3ecaf0cc71d0 100644
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -20,9 +20,10 @@
 int arch_jump_entry_size(struct jump_entry *entry)
 {
 	struct insn insn = {};
+	int ret;
 
-	insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
-	BUG_ON(insn.length != 2 && insn.length != 5);
+	ret = insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
+	BUG_ON(ret < 0 || insn.length != 2 && insn.length != 5);
 
 	return insn.length;
 }
-- 
2.25.1

Re: [PATCH v1] arch/x86/kernel: check the return value of insn_decode_kernel()

[Steven Rostedt]

On Fri,  2 Sep 2022 00:47:06 -0700
Li Zhong <floridsleeves@gmail.com> wrote:

> @@ -20,9 +20,10 @@
>  int arch_jump_entry_size(struct jump_entry *entry)
>  {
>  	struct insn insn = {};
> +	int ret;
>  
> -	insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
> -	BUG_ON(insn.length != 2 && insn.length != 5);
> +	ret = insn_decode_kernel(&insn, (void *)jump_entry_code(entry));

It's highly unlikely that length will be 2 or 5 if ret is not zero (as it
is initialized to zero going into this function).

> +	BUG_ON(ret < 0 || insn.length != 2 && insn.length != 5);

In any case, you need parenthesis around the && condition.

-- Steve


>  
>  	return insn.length;

Re: [PATCH v1] arch/x86/kernel: check the return value of insn_decode_kernel()

[Li Zhong]

On Tue, Sep 6, 2022 at 6:58 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri,  2 Sep 2022 00:47:06 -0700
> Li Zhong <floridsleeves@gmail.com> wrote:
>
> > @@ -20,9 +20,10 @@
> >  int arch_jump_entry_size(struct jump_entry *entry)
> >  {
> >       struct insn insn = {};
> > +     int ret;
> >
> > -     insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
> > -     BUG_ON(insn.length != 2 && insn.length != 5);
> > +     ret = insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
>
> It's highly unlikely that length will be 2 or 5 if ret is not zero (as it
> is initialized to zero going into this function).
>

In this case, I think maybe just BUG_ON(ret<0) is enough. However, the code
that decides the value of insn->length could be modified in the future. And
there are other variables insn->next_byte and insn->kaddr that are related to
insn->length, which could also be modified. So I guess we can preserve all the
assertion conditions to guarantee all assumptions are satisfied.

> > +     BUG_ON(ret < 0 || insn.length != 2 && insn.length != 5);
>
> In any case, you need parenthesis around the && condition.
>

Thanks. Will add this in v2 patch.

> -- Steve
>
>
> >
> >       return insn.length;