On Mon, Dec 04, 2017 at 12:26:32PM +0300, Dan Carpenter wrote: > On Mon, Dec 04, 2017 at 09:18:05AM +0100, Dmitry Vyukov wrote: > > On Mon, Dec 4, 2017 at 9:14 AM, Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote: > > > On Sun, Dec 03, 2017 at 12:16:08PM -0800, Eric Biggers wrote: > > >> Looks like BLKTRACESETUP doesn't limit the '.buf_nr' parameter, allowing anyone > > >> who can open a block device to cause an extremely large kmalloc. Here's a > > >> simplified reproducer: > > >> > > > > > > There are lots of places which allow people to allocate as much as they > > > want. With Syzcaller, you might want to just hard code a __GFP_NOWARN > > > in to disable it. > > > > Hi, > > > > Hard code it where? > > My idea was to just make warn_alloc() a no-op. > > > > > User-controllable allocation are supposed to use __GFP_NOWARN. > > No that's not right. What we don't want is unprivileged users to use > all the memory and we don't want unprivileged users to spam > /var/log/messages. But you have to have slightly elevated permissions > to open block devices right? The warning is helpful. Admins should > "don't do that" if they don't want the warning. WARN_ON() should only be used for kernel bugs. printk can be a different story. If it's a "userspace shouldn't do this" kind of thing, then if there is any message at all it should be a rate-limited printk that actually explains what the problem is, not a random WARN_ON() that can only be interpreted by kernel developers. And yes, the fact that anyone with read access to any block device, even e.g. a loop device, can cause the kernel to do an unbounded kmalloc *is* a bug. It needs to have a reasonable limit. It is not a problem on all systems, but on some systems "the admin" might give users read access to some block devices. Eric
