Re: memory leak in bpf

Dmitry Vyukov
2021-03-01T19:11:30Z

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.