Re: [PATCH v3] mm: memdup_user*() should use same gfp flags

[Michal Hocko <mhocko@suse.com>]

On Wed 27-01-21 19:55:38, Tetsuo Handa wrote:
> syzbot is reporting that memdup_user_nul() which receives user-controlled
> size (which can be up to (INT_MAX & PAGE_MASK)) via vfs_write() will hit
> order >= MAX_ORDER path [1].
> 
> Making costly allocations (order > PAGE_ALLOC_COSTLY_ORDER) naturally fail
> should be better than trying to enforce PAGE_SIZE upper limit, for some of
> callers accept space-delimited list arguments.
> 
> Therefore, let's add __GFP_NOWARN to memdup_user_nul() as with
> commit 6c8fcc096be9d02f ("mm: don't let userspace spam allocations
> warnings"). Also use GFP_USER as with other userspace-controllable
> allocations like memdup_user().

I absolutely detest hiding this behind __GFP_NOWARN. There should be no
reason to even try hard for memdup_user_nul. Can you explain why this
cannot use kvmalloc instead?

> [1] https://syzkaller.appspot.com/bug?id=8bf7efb3db19101b4008dc9198522ef977d098a6
> 
> Reported-by: syzbot <syzbot+a71a442385a0b2815497@syzkaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  mm/util.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 8c9b7d1e7c49..265b40a86856 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -252,12 +252,7 @@ void *memdup_user_nul(const void __user *src, size_t len)
>  {
>  	char *p;
>  
> -	/*
> -	 * Always use GFP_KERNEL, since copy_from_user() can sleep and
> -	 * cause pagefault, which makes it pointless to use GFP_NOFS
> -	 * or GFP_ATOMIC.
> -	 */
> -	p = kmalloc_track_caller(len + 1, GFP_KERNEL);
> +	p = kmalloc_track_caller(len + 1, GFP_USER | __GFP_NOWARN);
>  	if (!p)
>  		return ERR_PTR(-ENOMEM);
>  
> -- 
> 2.18.4
> 

-- 
Michal Hocko
SUSE Labs