Re: [PATCH] tracing: Correct the length check which causes memory corruption

[Steven Rostedt]

On Wed, 9 Jun 2021 14:43:51 -0700
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Wed, Jun 9, 2021 at 1:52 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > >
> > > That "sizeof(*entry)" is clearly wrong, because it doesn't take the
> > > unsized array into account.  
> >
> > Correct. That's because I forgot that the structure has that empty array :-(  
> 
> Note that 'sparse' does have the option to warn about odd flexible
> array uses. Including 'sizeof()'.
> 
> You can do something like
> 
>     CF='-Wflexible-array-sizeof' make C=2 kernel/trace/trace.o
> 
> and you'll see
> 
>   kernel/trace/trace.c:1022:17: warning: using sizeof on a flexible structure

	alloc = sizeof(*entry) + size + 2; /* possible \n added */

Entry is created via a macro. But I guess that too could use a struct_size().

>   kernel/trace/trace.c:2645:17: warning: using sizeof on a flexible structure

		memset(event, 0, sizeof(*event));

Hah, this is the part that is clearing the first part of the
ring_buffer_event structure of that temp buffer. Where it's setting
"type_len" to zero. It doesn't care about the extended portion here.

>   kernel/trace/trace.c:2739:41: warning: using sizeof on a flexible structure

This is the code we are talking about now.

>   kernel/trace/trace.c:3290:16: warning: using sizeof on a flexible structure

This is like the first one. A macro created the structure, but probably
could be switched over.

>   kernel/trace/trace.c:3350:16: warning: using sizeof on a flexible structure

Same as the first occurrence.

>   kernel/trace/trace.c:6989:16: warning: using sizeof on a flexible structure

Also the same as the first.

>   kernel/trace/trace.c:7070:16: warning: using sizeof on a flexible structure

And same as the first.

> 
> and I suspect every single one of those should be using
> 'struct_size()' instead for a sizeof() on the base structure plus some
> manual arithmetic (or, as in the case of this bug, _without_ the extra
> arithmetic).

Most of the above are for structures that hold dynamic size strings.
But I have no problem converting them to struct_size. But since they
are created by FTRACE_ENTRY*() macros in kernel/trace/trace_entries.h,
we need to be careful in picking the field to use.

> 
> And yeah, it isn't just the tracing code that does this. We have it
> all over, so that sparse check isn't on by default. Sparse is pretty
> darn noisy even without it, but it can be worth using that
> CF='-Wflexible-array-sizeof' on individual files that you want to
> check.

Could be a task for an intern to do?

-- Steve