Re: memory leak in bpf

[Rustam Kovhaev <rkovhaev@gmail.com>]

On Mon, Mar 01, 2021 at 08:05:42PM +0100, Dmitry Vyukov wrote:
> On Mon, Mar 1, 2021 at 5:21 PM Rustam Kovhaev <rkovhaev@gmail.com> wrote:
> >
> > On Wed, Dec 09, 2020 at 10:58:10PM -0800, syzbot wrote:
> > > syzbot has found a reproducer for the following issue on:
> > >
> > > HEAD commit:    a68a0262 mm/madvise: remove racy mm ownership check
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=11facf17500000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=4305fa9ea70c7a9f
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=f3694595248708227d35
> > > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=159a9613500000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11bf7123500000
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+f3694595248708227d35@syzkaller.appspotmail.com
> > >
> > > Debian GNU/Linux 9 syzkaller ttyS0
> > > Warning: Permanently added '10.128.0.9' (ECDSA) to the list of known hosts.
> > > executing program
> > > executing program
> > > executing program
> > > BUG: memory leak
> > > unreferenced object 0xffff88810efccc80 (size 64):
> > >   comm "syz-executor334", pid 8460, jiffies 4294945724 (age 13.850s)
> > >   hex dump (first 32 bytes):
> > >     c0 cb 14 04 00 ea ff ff c0 c2 11 04 00 ea ff ff  ................
> > >     c0 56 3f 04 00 ea ff ff 40 18 38 04 00 ea ff ff  .V?.....@.8.....
> > >   backtrace:
> > >     [<0000000036ae98a7>] kmalloc_node include/linux/slab.h:575 [inline]
> > >     [<0000000036ae98a7>] bpf_ringbuf_area_alloc kernel/bpf/ringbuf.c:94 [inline]
> > >     [<0000000036ae98a7>] bpf_ringbuf_alloc kernel/bpf/ringbuf.c:135 [inline]
> > >     [<0000000036ae98a7>] ringbuf_map_alloc kernel/bpf/ringbuf.c:183 [inline]
> > >     [<0000000036ae98a7>] ringbuf_map_alloc+0x1be/0x410 kernel/bpf/ringbuf.c:150
> > >     [<00000000d2cb93ae>] find_and_alloc_map kernel/bpf/syscall.c:122 [inline]
> > >     [<00000000d2cb93ae>] map_create kernel/bpf/syscall.c:825 [inline]
> > >     [<00000000d2cb93ae>] __do_sys_bpf+0x7d0/0x30a0 kernel/bpf/syscall.c:4381
> > >     [<000000008feaf393>] do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> > >     [<00000000e1f53cfd>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > >
> > >
> >
> > i am pretty sure that this one is a false positive
> > the problem with reproducer is that it does not terminate all of the
> > child processes that it spawns
> >
> > i confirmed that it is a false positive by tracing __fput() and
> > bpf_map_release(), i ran reproducer, got kmemleak report, then i
> > manually killed those running leftover processes from reproducer and
> > then both functions were executed and memory was freed
> >
> > i am marking this one as:
> > #syz invalid
> 
> Hi Rustam,
> 
> Thanks for looking into this.
> 
> I wonder how/where are these objects referenced? If they are not
> leaked and referenced somewhere, KMEMLEAK should not report them as
> leaks.
> So even if this is a false positive for BPF, this is a true positive
> bug and something to fix for KMEMLEAK ;)
> And syzbot will probably re-create this bug report soon as this still
> happens and is not a one-off thing.

hi Dmitry, i haven't thought of it this way, but i guess you are right,
it is a kmemleak bug, ideally kmemleak should be aware that there are
still running processes holding references to bpf fd/anonymous inodes
which in their turn hold references to allocated bpf maps