Re: [PATCH v2] x86/uaccess: Avoid check_object_size() in copy_from_user_nmi()

[Florian Lehner]

On Mon, Sep 19, 2022 at 01:16:48PM -0700, Kees Cook wrote:
> The check_object_size() helper under CONFIG_HARDENED_USERCOPY is
> designed to skip any checks where the length is known at compile time as
> a reasonable heuristic to avoid "likely known-good" cases. However, it can
> only do this when the copy_*_user() helpers are, themselves, inline too.
> 
> Using find_vmap_area() requires taking a spinlock. The check_object_size()
> helper can call find_vmap_area() when the destination is in vmap memory.
> If show_regs() is called in interrupt context, it will attempt a call to
> copy_from_user_nmi(), which may call check_object_size() and then
> find_vmap_area(). If something in normal context happens to be in the
> middle of calling find_vmap_area() (with the spinlock held), the interrupt
> handler will hang forever.
> 
> The copy_from_user_nmi() call is actually being called with a fixed-size
> length, so check_object_size() should never have been called in
> the first place. Given the narrow constraints, just replace the
> __copy_from_user_inatomic() call with an open-coded version that calls
> only into the sanitizers and not check_object_size(), followed by a call
> to raw_copy_from_user().
> 
> Reported-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@xxxxxxxxxxxxxx
> Reported-by: dev@xxxxxxxxxxx
> Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Fixes: 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> v2: drop the call explicitly instead of using inline to do it
> v1: https://lore.kernel.org/lkml/20220916135953.1320601-1-keescook@xxxxxxxxxxxx
> ---
>  arch/x86/lib/usercopy.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
> index ad0139d25401..d2aff9b176cf 100644
> --- a/arch/x86/lib/usercopy.c
> +++ b/arch/x86/lib/usercopy.c
> @@ -44,7 +44,8 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
>  	 * called from other contexts.
>  	 */
>  	pagefault_disable();
> -	ret = __copy_from_user_inatomic(to, from, n);
> +	instrument_copy_from_user(to, from, n);
> +	ret = raw_copy_from_user(to, from, n);
>  	pagefault_enable();
>  
>  	return ret;
> -- 
> 2.34.1
> 

Thanks!

Tested-by: Florian Lehner <dev@xxxxxxxxxxx>