linux-hardening - Re: [PATCH 1/3] Revert "proc/wchan: use printk format instead of lookup_symbol

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Date:   Fri, 24 Sep 2021 10:17:53 +0200
From:   Helge Deller <deller@....de>
To:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Vito Caputo <vcaputo@...garu.com>,
        Jann Horn <jannh@...gle.com>, stable@...r.kernel.org,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Tobin C. Harding" <me@...in.cc>,
        Tycho Andersen <tycho@...ho.pizza>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Mark Rutland <mark.rutland@....com>,
        Jens Axboe <axboe@...nel.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Stefan Metzmacher <metze@...ba.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Michal Hocko <mhocko@...e.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ohhoon Kwon <ohoono.kwon@...sung.com>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        x86@...nel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/3] Revert "proc/wchan: use printk format instead of
 lookup_symbol_name()"

On 9/24/21 8:20 AM, Kees Cook wrote:
> This reverts commit 152c432b128cb043fc107e8f211195fe94b2159c.
>
> When a kernel address couldn't be symbolized for /proc/$pid/wchan, it
> would leak the raw value, a potential information exposure. This is a
> regression compared to the safer pre-v5.12 behavior.

Instead of reverting, another possibility might be to depend on
CONFIG_KALLSYMS before using the %ps format specifier and print "0" otherwise.
If it can't be symbolized it's most likely not a valid kernel address
and as such wouldn't leak anything....
But well,
Acked-by: Helge Deller <deller@....de>

Helge

> Reported-by: kernel test robot <oliver.sang@...el.com>
> Link: https://lore.kernel.org/all/20210103142726.GC30643@xsang-OptiPlex-9020/
> Reported-by: Vito Caputo <vcaputo@...garu.com>
> Link: https://lore.kernel.org/lkml/20210921193249.el476vlhg5k6lfcq@shells.gnugeneration.com/
> Reported-by: Jann Horn <jannh@...gle.com>
> Link: https://lore.kernel.org/lkml/CAG48ez2zC=+PuNgezH53HBPZ8CXU5H=vkWx7nJs60G8RXt3w0Q@mail.gmail.com/
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>   fs/proc/base.c | 19 +++++++++++--------
>   1 file changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 533d5836eb9a..1f394095eb88 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -67,6 +67,7 @@
>   #include <linux/mm.h>
>   #include <linux/swap.h>
>   #include <linux/rcupdate.h>
> +#include <linux/kallsyms.h>
>   #include <linux/stacktrace.h>
>   #include <linux/resource.h>
>   #include <linux/module.h>
> @@ -386,17 +387,19 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
>   			  struct pid *pid, struct task_struct *task)
>   {
>   	unsigned long wchan;
> +	char symname[KSYM_NAME_LEN];
>
> -	if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
> -		wchan = get_wchan(task);
> -	else
> -		wchan = 0;
> +	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
> +		goto print0;
>
> -	if (wchan)
> -		seq_printf(m, "%ps", (void *) wchan);
> -	else
> -		seq_putc(m, '0');
> +	wchan = get_wchan(task);
> +	if (wchan && !lookup_symbol_name(wchan, symname)) {
> +		seq_puts(m, symname);
> +		return 0;
> +	}
>
> +print0:
> +	seq_putc(m, '0');
>   	return 0;
>   }
>   #endif /* CONFIG_KALLSYMS */
>