Subject: Re: memory leak in bpf

From: Rustam Kovhaev <[email-masked]>
Date: Wed, 7 Apr 2021 16:24:44 -0700

On Mon, Mar 01, 2021 at 09:43:00PM +0100, Dmitry Vyukov wrote:
> On Mon, Mar 1, 2021 at 9:39 PM Rustam Kovhaev <[email-masked]> 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 <[email-masked]> 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: [email-masked]
> > > > >
> > > > > 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?
thank you for your comments, they make sense, and indeed, the pointer
gets vmaped.
i should have looked into this sooner, becaused syzbot did trigger the
issue again, and Andrii had to look into the same bug, sorry about that.
if i am understanding this correctly here is what the fix should be:
---
 kernel/bpf/ringbuf.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
index f25b719ac786..30400e74abe2 100644
--- a/kernel/bpf/ringbuf.c
+++ b/kernel/bpf/ringbuf.c
@@ -8,6 +8,7 @@
 #include <linux/vmalloc.h>
 #include <linux/wait.h>
 #include <linux/poll.h>
+#include <linux/kmemleak.h>
 #include <uapi/linux/btf.h>
 
 #define RINGBUF_CREATE_FLAG_MASK (BPF_F_NUMA_NODE)
@@ -105,6 +106,7 @@ static struct bpf_ringbuf *bpf_ringbuf_area_alloc(size_t data_sz, int numa_node)
 	rb = vmap(pages, nr_meta_pages + 2 * nr_data_pages,
 		  VM_ALLOC | VM_USERMAP, PAGE_KERNEL);
 	if (rb) {
+		kmemleak_not_leak((void *) pages);
 		rb->pages = pages;
 		rb->nr_pages = nr_pages;
 		return rb;
-- 
2.30.2