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Date:   Thu, 8 Jun 2023 09:47:04 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, gregkh@linuxfoundation.org,
        pbonzini@redhat.com, linux-kernel@vger.kernel.org,
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        will@kernel.org, longman@redhat.com, boqun.feng@gmail.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        paulmck@kernel.org, frederic@kernel.org, quic_neeraju@quicinc.com,
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        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        rcu@vger.kernel.org, tj@kernel.org, tglx@linutronix.de,
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Subject: Re: [PATCH v2 0/2] Lock and Pointer guards
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On Thu, Jun 08, 2023 at 08:45:53AM -0700, Linus Torvalds wrote:
> So for convenient automatic pointer freeing, you want an interface
> much more akin to
> 
>         struct whatever *ptr __automatic_kfree = kmalloc(...);
> 
> which is much more legible, doesn't have any type mis-use issues, and
> is also just trivially dealt with by a
> 
>   static inline void automatic_kfree_wrapper(void *pp)
>   { void *p = *(void **)pp; if (p) kfree(p); }
>   #define __automatic_kfree \
>         __attribute__((__cleanup__(automatic_kfree_wrapper)))
>   #define no_free_ptr(p) \
>         ({ __auto_type __ptr = (p); (p) = NULL; __ptr; })
> 
> which I just tested generates the sane code even for the "set the ptr
> to NULL and return success" case.
> 
> The above allows you to trivially do things like
> 
>         struct whatever *p __automatic_kfree = kmalloc(..);
> 
>         if (!do_something(p))
>                 return -ENOENT;
> 
>         return no_free_ptr(p);

I am a little worried about how (any version so far of) this API could go
wrong, e.g. if someone uses this and does "return p" instead of "return
no_free_ptr(p)", it'll return a freed pointer. I was hoping we could do
something like this to the end of automatic_kfree_wrapper():

	*(void **)pp = NULL;

i.e. if no_free_ptr() goes missing, "return p" will return NULL, which
is much easier to track down that dealing with later use-after-free bugs,
etc. Unfortunately, the __cleanup ordering is _after_ the compiler stores
the return value...

static inline void cleanup_info(struct info **p)
{
	free(*p);
	*p = NULL; /* this is effectively ignored */
}

struct info *do_something(int f)
{
	struct info *var __attribute__((__cleanup__(cleanup_info))) =
		malloc(1024);

	process(var);

	return var; /* oops, forgot to disable cleanup */
}

compile down to:

do_something:
        pushq   %rbx
        movl    $1024, %edi
        call    malloc
        movq    %rax, %rbx
        movq    %rax, %rdi
        call    process
        movq    %rbx, %rdi
        call    free
        movq    %rbx, %rax	; uses saved copy of malloc return
        popq    %rbx
        ret

The point being, if we can proactively make this hard to shoot ourselves in
the foot, that would be nice. :)

-- 
Kees Cook