On Tue, Dec 12, 2017 at 10:22 PM, Eric Biggers [off-list ref] wrote:

On Mon, Dec 04, 2017 at 12:26:32PM +0300, Dan Carpenter wrote:

quoted

On Mon, Dec 04, 2017 at 09:18:05AM +0100, Dmitry Vyukov wrote:

quoted

On Mon, Dec 4, 2017 at 9:14 AM, Dan Carpenter [off-list ref] wrote:

quoted

On Sun, Dec 03, 2017 at 12:16:08PM -0800, Eric Biggers wrote:

quoted

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.

quoted

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