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

Qian Cai
2020-09-17T18:46:25Z

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