Re: [PATCH 4/4] x86/kasan: Instrument user memory access API

[Ingo Molnar]

* Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:

> Exchange between user and kernel memory is coded in assembly language.
> Which means that such accesses won't be spotted by KASAN as a compiler
> instruments only C code.
> Add explicit KASAN checks to user memory access API to ensure that
> userspace writes to (or reads from) a valid kernel memory.
> 
> Note: Unlike others strncpy_from_user() is written mostly in C and KASAN
> sees memory accesses in it. However, it makes sense to add explicit check
> for all @count bytes that *potentially* could be written to the kernel.
> 
> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> ---
>  arch/x86/include/asm/uaccess.h    | 5 +++++
>  arch/x86/include/asm/uaccess_64.h | 7 +++++++
>  lib/strncpy_from_user.c           | 2 ++
>  3 files changed, 14 insertions(+)

[...]

> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index 3384032..e3472b0 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -1,5 +1,6 @@
>  #include <linux/compiler.h>
>  #include <linux/export.h>
> +#include <linux/kasan-checks.h>
>  #include <linux/uaccess.h>
>  #include <linux/kernel.h>
>  #include <linux/errno.h>
> @@ -103,6 +104,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
>  	if (unlikely(count <= 0))
>  		return 0;
>  
> +	kasan_check_write(dst, count);
>  	max_addr = user_addr_max();
>  	src_addr = (unsigned long)src;
>  	if (likely(src_addr < max_addr)) {

Please do the check inside the condition, before the user_access_begin(), because 
where you've put the check we might still fail and not do a user copy and -EFAULT 
out.

With that fixed:

Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>

Thanks,

	Ingo