Re: [PATCH] vsprintf: don't obfuscate NULL and error pointers

[Kees Cook <keescook@chromium.org>]

On Mon, Feb 17, 2020 at 11:28:03PM +0100, Ilya Dryomov wrote:
> I don't see what security concern is addressed by obfuscating NULL
> and IS_ERR() error pointers, printed with %p/%pK.  Given the number
> of sites where %p is used (over 10000) and the fact that NULL pointers
> aren't uncommon, it probably wouldn't take long for an attacker to
> find the hash that corresponds to 0.  Although harder, the same goes
> for most common error values, such as -1, -2, -11, -14, etc.
> 
> The NULL part actually fixes a regression: NULL pointers weren't
> obfuscated until commit 3e5903eb9cff ("vsprintf: Prevent crash when
> dereferencing invalid pointers") which went into 5.2.  I'm tacking
> the IS_ERR() part on here because error pointers won't leak kernel
> addresses and printing them as pointers shouldn't be any different
> from e.g. %d with PTR_ERR_OR_ZERO().  Obfuscating them just makes
> debugging based on existing pr_debug and friends excruciating.
> 
> Note that the "always print 0's for %pK when kptr_restrict == 2"
> behaviour which goes way back is left as is.
> 
> Example output with the patch applied:
> 
>                             ptr         error-ptr              NULL
> %p:            0000000001f8cc5b  fffffffffffffff2  0000000000000000
> %pK, kptr = 0: 0000000001f8cc5b  fffffffffffffff2  0000000000000000
> %px:           ffff888048c04020  fffffffffffffff2  0000000000000000
> %pK, kptr = 1: ffff888048c04020  fffffffffffffff2  0000000000000000
> %pK, kptr = 2: 0000000000000000  0000000000000000  0000000000000000

This seems reasonable. Though I wonder -- since the efault string is
exposed now -- should this instead print all the error-ptr strings
instead of the unsigned negative pointer value?

-Kees

> 
> Fixes: 3e5903eb9cff ("vsprintf: Prevent crash when dereferencing invalid pointers")
> Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
> ---
>  lib/vsprintf.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 7c488a1ce318..f0f0522cd5a7 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -794,6 +794,13 @@ static char *ptr_to_id(char *buf, char *end, const void *ptr,
>  	unsigned long hashval;
>  	int ret;
>  
> +	/*
> +	 * Print the real pointer value for NULL and error pointers,
> +	 * as they are not actual addresses.
> +	 */
> +	if (IS_ERR_OR_NULL(ptr))
> +		return pointer_string(buf, end, ptr, spec);
> +
>  	/* When debugging early boot use non-cryptographically secure hash. */
>  	if (unlikely(debug_boot_weak_hash)) {
>  		hashval = hash_long((unsigned long)ptr, 32);
> -- 
> 2.19.2
> 

-- 
Kees Cook