Subject: Re: memory leak in bpf

From: Rustam Kovhaev <[email-masked]>
Date: Thu, 24 Jun 2021 10:28:10 -0700

On Sun, Jun 13, 2021 at 08:17:04AM -0700, Rustam Kovhaev wrote:
> On Thu, Apr 08, 2021 at 11:56:15AM -0700, Rustam Kovhaev wrote:
> > On Wed, Apr 07, 2021 at 04:35:34PM -0700, Andrii Nakryiko wrote:
> > > On Wed, Apr 7, 2021 at 4:24 PM Rustam Kovhaev <[email-masked]> wrote:
> > > >
> > > > 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.
> > > 
> > > No worries! I actually forgot about this thread :) Let's leave the
> > > link to my today's investigation ([0]) just for completeness.
> > > 
> > >   [0] https://lore.kernel.org/bpf/[email-masked]/
> > > 
> > > > 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);
> > > 
> > > If that makes kmemleak happy, I have no problems with this. But maybe
> > > leave some comment explaining why this is needed at all?
> > > 
> > > And for my understanding, how vmap changes anything? Those pages are
> > > still referenced from rb, which is referenced from some struct file in
> > > the system. Sorry if that's a naive question.
> > > 
> > valid question, it does look like kmemleak should be scanning
> > vmalloc()/vmap() memory, i will research this further
> 
> a quick update, i see a problem in kmemleak code, and i have simplified
> the reproducer by getting rid of a vmap().
> i will reach out to maintainer and mm and afterwards i will update this
> bug, cheers!
> 

Andrii, we have discovered that kmemleak scans struct page, but it does
not scan page contents and this is by design. if we allocate some memory
with kmalloc(), then allocate page with alloc_page(), and if we put
kmalloc pointer somewhere inside that page, kmemleak will report kmalloc
pointer as a false positive.
we can instruct kmemleak to scan the memory area by calling
kmemleak_alloc()/kmemleak_free() as shown below. if we don't need that
memory to be scanned then we can use kmemleak_not_leak().
if we use the former then i guess we need to be careful since we do not
want/need to scan the memory that is being used by user-space.

---
 kernel/bpf/ringbuf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
index 84b3b35fc0d0..cf7ce10b4fb1 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_alloc(rb, PAGE_SIZE, 1, flags);
 		rb->pages = pages;
 		rb->nr_pages = nr_pages;
 		return rb;
@@ -184,6 +186,7 @@ static void bpf_ringbuf_free(struct bpf_ringbuf *rb)
 	struct page **pages = rb->pages;
 	int i, nr_pages = rb->nr_pages;
 
+	kmemleak_free(rb);
 	vunmap(rb);
 	for (i = 0; i < nr_pages; i++)
 		__free_page(pages[i]);
-- 
2.30.2