Re: [PATCH] proc: Disable /proc/$pid/wchan

[Kees Cook]

On Thu, Sep 23, 2021 at 04:31:05PM -0700, Kees Cook wrote:
> The /proc/$pid/wchan file has been broken by default on x86_64 for 4
> years now[1]. As this remains a potential leak of either kernel
> addresses (when symbolization fails) or limited observation of kernel
> function progress, just remove the contents for good.
> 
> Unconditionally set the contents to "0" and also mark the wchan
> field in /proc/$pid/stat with 0.

I forgot to CC Qi Zheng on this patch. Now corrected. :)

> This leaves kernel/sched/fair.c as the only user of get_wchan(). But
> again, since this was broken for 4 years, was this profiling logic
> actually doing anything useful?

If the fair scheduler would actually benefit from still using get_wchan,
I think this patch:
https://lore.kernel.org/all/20210831083625.59554-1-zhengqi.arch@xxxxxxxxxxxxx/
should still be applied too.

If not, we can rip get_wchan() out completely (across all
architectures).

-Kees

> [1] https://lore.kernel.org/lkml/20210922001537.4ktg3r2ky3b3r6yp@treble/
> 
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Vito Caputo <vcaputo@xxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/x86/kernel/process.c |  2 +-
>  fs/proc/array.c           | 16 +++++-----------
>  fs/proc/base.c            | 16 +---------------
>  3 files changed, 7 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 1d9463e3096b..84a4f9f3f0c2 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -937,7 +937,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
>  }
>  
>  /*
> - * Called from fs/proc with a reference on @p to find the function
> + * Called from scheduler with a reference on @p to find the function
>   * which called into schedule(). This needs to be done carefully
>   * because the task might wake up and we might look at a stack
>   * changing under us.
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 49be8c8ef555..8a4ecfd901b8 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -452,7 +452,7 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
>  static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>  			struct pid *pid, struct task_struct *task, int whole)
>  {
> -	unsigned long vsize, eip, esp, wchan = 0;
> +	unsigned long vsize, eip, esp;
>  	int priority, nice;
>  	int tty_pgrp = -1, tty_nr = 0;
>  	sigset_t sigign, sigcatch;
> @@ -540,8 +540,6 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>  		unlock_task_sighand(task, &flags);
>  	}
>  
> -	if (permitted && (!whole || num_threads < 2))
> -		wchan = get_wchan(task);
>  	if (!whole) {
>  		min_flt = task->min_flt;
>  		maj_flt = task->maj_flt;
> @@ -600,16 +598,12 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>  	seq_put_decimal_ull(m, " ", sigcatch.sig[0] & 0x7fffffffUL);
>  
>  	/*
> -	 * We used to output the absolute kernel address, but that's an
> -	 * information leak - so instead we show a 0/1 flag here, to signal
> -	 * to user-space whether there's a wchan field in /proc/PID/wchan.
> -	 *
> +	 * We used to output the absolute kernel address, and then just
> +	 * a symbol. But both are information leaks, so just report 0
> +	 * to indicate there is no wchan field in /proc/$PID/wchan.
>  	 * This works with older implementations of procps as well.
>  	 */
> -	if (wchan)
> -		seq_puts(m, " 1");
> -	else
> -		seq_puts(m, " 0");
> +	seq_puts(m, " 0");
>  
>  	seq_put_decimal_ull(m, " ", 0);
>  	seq_put_decimal_ull(m, " ", 0);
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 533d5836eb9a..52484cd77f99 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -378,24 +378,10 @@ static const struct file_operations proc_pid_cmdline_ops = {
>  };
>  
>  #ifdef CONFIG_KALLSYMS
> -/*
> - * Provides a wchan file via kallsyms in a proper one-value-per-file format.
> - * Returns the resolved symbol.  If that fails, simply return the address.
> - */
>  static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
>  			  struct pid *pid, struct task_struct *task)
>  {
> -	unsigned long wchan;
> -
> -	if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
> -		wchan = get_wchan(task);
> -	else
> -		wchan = 0;
> -
> -	if (wchan)
> -		seq_printf(m, "%ps", (void *) wchan);
> -	else
> -		seq_putc(m, '0');
> +	seq_putc(m, '0');
>  
>  	return 0;
>  }
> -- 
> 2.30.2
> 

-- 
Kees Cook