linux-kernel - Re: [git pull] uaccess-related bits of vfs.git

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Date:   Sat, 13 May 2017 22:25:39 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [git pull] uaccess-related bits of vfs.git

On Sat, May 13, 2017 at 01:52:29PM -0700, Linus Torvalds wrote:
> I wouldn't change the existing "memdup_user()" interface itself, but
> if there really are users that can validly pass in a maxbyte value,
> why not add a new helper:
> 
>   void *memdup_user_limit(userptr, nmember, nsize, maxsize);
> 
> and then have
> 
>   #define memdup_user(ptr,size) memdup_user_limit(ptr, size, 1, -1)
> 
> or something. I definitely see a couple of memdup_user() people who do
> that "num*size" multiplication by hand, and it's very easy to get
> wrong and have an overflow.
> 
> And for a kvmalloc/kvfree() interface, you *definitely* want that
> maxsize thing, since it absolutely needs an upper limit.

*nod*

Speaking of insanities around open-coded memdup_user()...  Enjoy:

                ias_opt = kmalloc(sizeof(struct irda_ias_set), GFP_ATOMIC);
                if (ias_opt == NULL) {
                        err = -ENOMEM;
                        goto out;
                }

                /* Copy query to the driver. */
                if (copy_from_user(ias_opt, optval, optlen)) {

Can't have it block, sir, has to be GFP_ATOMIC...  Whaddya mean, "what
if copy_from_user() blocks?"  As far as I can see, that came in circa
2.4.6, when local sturct irda_ias_set got switched to dynamic allocation...