On Tue, Dec 12, 2017 at 10:22 PM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote: > 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. #syz fix: kernel/relay.c: limit kmalloc size to KMALLOC_MAX_SIZE
