Re: [PATCH v7 02/10] vsprintf: Consistent %pK handling for kptr_restrict == 0

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 17 Apr 2019 13:53:42 +0200
Petr Mladek <pmladek@suse.com> wrote:

> restricted_pointer() pretends that it prints the address when kptr_restrict
> is set to zero. But it is never called in this situation. Instead,
> pointer() falls back to ptr_to_id() and hashes the pointer.
> 
> This patch removes the potential confusion. klp_restrict is checked only
> in restricted_pointer().
> 
> It actually fixes a small race when the address might get printed unhashed:
> 
> CPU0                            CPU1
> 
> pointer()
>   if (!kptr_restrict)
>      /* for example set to 2 */
>   restricted_pointer()
> 				/* echo 0 >/proc/sys/kernel/kptr_restrict */
> 				proc_dointvec_minmax_sysadmin()
> 				  klpr_restrict = 0;
>     switch(kptr_restrict)
>       case 0:
> 	break:
> 
>     number()
> 
> Fixes: commit ef0010a30935de4e0211 ("vsprintf: don't use 'restricted_pointer()' when not restricting")
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Tobin Harding <me@tobin.cc>
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve

> ---
>  lib/vsprintf.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index eb7b4a06e1f0..2af48948a973 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -725,8 +725,8 @@ char *restricted_pointer(char *buf, char *end, const void *ptr,
>  {
>  	switch (kptr_restrict) {
>  	case 0:
> -		/* Always print %pK values */
> -		break;
> +		/* Handle as %p, hash and do _not_ leak addresses. */
> +		return ptr_to_id(buf, end, ptr, spec);
>  	case 1: {
>  		const struct cred *cred;
>  
> @@ -2041,8 +2041,6 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  			return buf;
>  		}
>  	case 'K':
> -		if (!kptr_restrict)
> -			break;
>  		return restricted_pointer(buf, end, ptr, spec);
>  	case 'N':
>  		return netdev_bits(buf, end, ptr, spec, fmt);