Re: memory leak in bpf

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Mar 1, 2021 at 9:39 PM Rustam Kovhaev <rkovhaev@gmail.com> wrote:
>
> 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

KMEMLEAK scans whole memory, so if there are pointers to the object
anywhere in memory, KMEMLEAK should not report them as leaked. Running
processes have no direct effect on KMEMLEAK logic.
So the question is: where are these pointers to these objects? If we
answer this, we can check how/why KMEMLEAK misses them. Are they
mangled in some way?