Re: [PATCH 1/3] Revert "proc/wchan: use printk format instead of lookup_symbol_name()"

From: Helge Deller <deller@gmx.de>
To: Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: kernel test robot <oliver.sang@intel.com>,
	Vito Caputo <vcaputo@pengaru.com>, Jann Horn <jannh@google.com>,
	stable@vger.kernel.org, Qi Zheng <zhengqi.arch@bytedance.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	"Tobin C. Harding" <me@tobin.cc>,
	Tycho Andersen <tycho@tycho.pizza>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Mark Rutland <mark.rutland@arm.com>, Jens Axboe <axboe@kernel.dk>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Lai Jiangshan <laijs@linux.alibaba.com>,
	Stefan Metzmacher <metze@samba.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Michal Hocko <mhocko@suse.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Ohhoon Kwon <ohoono.kwon@samsung.com>,
	YiFei Zhu <yifeifz2@illinois.edu>,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org,
	x86@kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 1/3] Revert "proc/wchan: use printk format instead of lookup_symbol_name()"
Date: Fri, 24 Sep 2021 10:17:53 +0200	[thread overview]
Message-ID: <16ebd28d-9d99-d217-c62f-03d7c158ac84@gmx.de> (raw)
In-Reply-To: <20210924062006.231699-2-keescook@chromium.org>

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@gmx.de>

Helge

> Reported-by: kernel test robot <oliver.sang@intel.com>
> Link: https://lore.kernel.org/all/20210103142726.GC30643@xsang-OptiPlex-9020/
> Reported-by: Vito Caputo <vcaputo@pengaru.com>
> Link: https://lore.kernel.org/lkml/20210921193249.el476vlhg5k6lfcq@shells.gnugeneration.com/
> Reported-by: Jann Horn <jannh@google.com>
> Link: https://lore.kernel.org/lkml/CAG48ez2zC=+PuNgezH53HBPZ8CXU5H=vkWx7nJs60G8RXt3w0Q@mail.gmail.com/
> Cc: stable@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.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 */
>