Re: [syzbot] KASAN: use-after-free Read in ucma_destroy_private_ctx

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Jan 05, 2022 at 07:45:21PM +0800, Hillf Danton wrote:
> On Mon, 3 Jan 2022 20:47:02 +0200 Leon Romanovsky wrote:
> > On Mon, Jan 03, 2022 at 09:05:16AM -0800, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > > 
> > > HEAD commit:    a8ad9a2434dc Merge tag 'efi-urgent-for-v5.16-2' of git://g..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=10cf5253b00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1a86c22260afac2f
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=e3f96c43d19782dd14a7
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > 
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > > 
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+e3f96c43d19782dd14a7@syzkaller.appspotmail.com
> > > 
> > > ==================================================================
> > > BUG: KASAN: use-after-free in ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
> > > BUG: KASAN: use-after-free in ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
> > > Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529
> > 
> > Jason,
> > 
> > Can it be race between ucma_process_join() and "if (refcount_read(&ctx->ref))"
> > check in ucma_destroy_private_ctx()?
> 
> Given cmpxchg in both ucma_close() and ucma_destroy_id(),
> ucma_destroy_private_ctx() can not run more than once, in addition to what
> is more weird is that the ucma_fops.release either is running in parallel
> to a writer or completes with a writer left behind. Light on if that weirdness
> is down to anything other than syzbot is highly appreciated.

It is a stupid mistake, it is because

	xa_for_each(&multicast_table, index, mc) {
		if (mc->ctx != ctx)
                   ^^^^^^^

Nothing is locking mc here, this needed to hold the xa_lock to be
correct.

It is caused by this:

commit 95fe51096b7adf1d1e7315c49c75e2f75f162584
Author: Jason Gunthorpe <jgg@ziepe.ca>
Date:   Tue Aug 18 15:05:17 2020 +0300

    RDMA/ucma: Remove mc_list and rely on xarray
    
    It is not really necessary to keep a linked list of mcs associated with
    each context when we can just scan the xarray to find the right things.
    
    The removes another overloading of file->mut by relying on the xarray
    locking for mc instead.
    
    Link: https://lore.kernel.org/r/20200818120526.702120-6-leon@kernel.org
    Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
    Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>


So it is solved either by putting the linked list back, but locking it
using the xa_lock, or by holding the xa_lock when doing the
xa_for_each()

A list is probably the better choice.

Jason