Re: Use of copy_from_user in msm_gem_submit.c while holding a spin_lock

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Aug 17, 2016 at 11:08:46AM -0400, Rob Clark wrote:
> On Wed, Aug 17, 2016 at 7:40 AM, Vaishali Thakkar
> <vaishali.thakkar@oracle.com> wrote:
> > Hello,
> >
> > I was wondering about the call to copy_from_user in function submit_lookup_objects for drive
> > /gpu/drm/msm/msm_gem_submit.c  It calls copy_from_user[1] in a spin_lock, which is not normally
> > allowed, due to the possibility of a deadlock.
> >
> > Is there some reason that I am overlooking why it is OK in this case? Is there some code in the
> > same file which ensures that page fault will not occur when we are calling the function holding
> > spin_lock?
> 
> hmm, probably just that it isn't typical to use a swap file on these
> devices (and that lockdep/etc doesn't warn about it)..  I guess we
> probably need some sort of slow-path where we drop the lock and try
> again in case there would be a fault..

Sigh...  Folks, you don't need swap *at* *all* for copy_from_user() to block.
	/* get a zero-filled 64K buffer */
	addr = mmap(NULL, 65536, PROT_READ | PROT_WRITE,
		    MAP_ANONYMOUS | MAP_SHARED, -1, 0);
	if (addr < 0)
		piss off
	buffer = (void *)addr;
	....
	pass buf to a syscall
and copy_from_user() in that syscall will have to allocate pages (and possibly
page tables as well).  Which can block just fine, no swap involved.  Moreover,
if you modify some parts of the buffer first, you will get the pages containing
those modifications already present, but anything still untouched will
	a) act as if it had been zeroed first and
	b) possibly block on the first dereference, be it from kernel or from
userland.  Worse yet, there's nothing to stop libc from using the above for
calloc() and its ilk, with your application having no way to tell.  As far
as application is concerned, it has asked a library function to allocate and
zero a piece of memory, got one and yes, it does appear to be properly zeroed.

The bottom line is, copy_from_user() can realistically block, without
anything fishy going on in the userland setup.