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

[Helge Deller]

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@xxxxxx>

Helge

> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Link: https://lore.kernel.org/all/20210103142726.GC30643@xsang-OptiPlex-9020/
> Reported-by: Vito Caputo <vcaputo@xxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/20210921193249.el476vlhg5k6lfcq@xxxxxxxxxxxxxxxxxxxxxxxx/
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/CAG48ez2zC=+PuNgezH53HBPZ8CXU5H=vkWx7nJs60G8RXt3w0Q@xxxxxxxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>   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 */