Re: slab-out-of-bounds in iov_iter_revert()

[Qian Cai]

On Thu, 2020-09-17 at 17:44 +0100, Al Viro wrote:
> On Thu, Sep 17, 2020 at 10:10:27AM -0400, Qian Cai wrote:
> 
> > [   81.942909]  generic_file_read_iter+0x23b/0x4b0
> > [   81.942918]  fuse_file_read_iter+0x280/0x4e0 [fuse]
> > [   81.942931]  ? fuse_direct_IO+0xd30/0xd30 [fuse]
> > [   81.942949]  ? _raw_spin_lock_irqsave+0x80/0xe0
> > [   81.942957]  ? timerqueue_add+0x15e/0x280
> > [   81.942960]  ? _raw_spin_lock_irqsave+0x80/0xe0
> > [   81.942966]  new_sync_read+0x3b7/0x620
> > [   81.942968]  ? __ia32_sys_llseek+0x2e0/0x2e0
> 
> Interesting...  Basic logics in there:
> 	->direct_IO() might consume more (on iov_iter_get_pages()
> and friends) than it actually reads.  We want to revert the
> excess.  Suppose by the time we call ->direct_IO() we had
> N bytes already consumed and C bytes left.  We expect that
> after ->direct_IO() returns K, we have C' bytes left, N + (C - C')
> consumed and N + K out of those actually read.  So we revert by
> C - K - C'.  You end up trying to revert beyond the beginning.
> 
> 	Use of iov_iter_truncate() is problematic here, since it
> changes the amount of data left without having consumed anything.
> Basically, it changes the position of end, and the logics in the
> caller expects that to remain unchanged.  iov_iter_reexpand() use
> should restore the position of end.
> 
> 	How much IO does it take to trigger that on your reproducer?

I can even reproduce this with a single child of the trinity:

https://people.redhat.com/qcai/iov_iter_revert/single/

[   77.841021] BUG: KASAN: stack-out-of-bounds in iov_iter_revert+0x693/0x8c0
[   77.842055] Read of size 8 at addr ffff8886efe47d98 by task trinity-c0/1449