On Fri, Jun 25, 2021 at 04:28:00PM +0200, Dmitry Vyukov wrote:

On Thu, Jun 24, 2021 at 7:28 PM Rustam Kovhaev [off-list ref] wrote:

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On Wed, Apr 7, 2021 at 4:24 PM Rustam Kovhaev [off-list ref] wrote:

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On Mon, Mar 01, 2021 at 09:43:00PM +0100, Dmitry Vyukov wrote:

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On Mon, Mar 1, 2021 at 9:39 PM Rustam Kovhaev [off-list ref] wrote:

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On Mon, Mar 01, 2021 at 08:05:42PM +0100, Dmitry Vyukov wrote:

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On Mon, Mar 1, 2021 at 5:21 PM Rustam Kovhaev [off-list ref] wrote:

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On Wed, Dec 09, 2020 at 10:58:10PM -0800, syzbot wrote:

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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?

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/CAEf4BzYk+dqs+jwu6VKXP-RttcTEGFe+ySTGWT9CRNkagDiJVA@mail.gmail.com/ (local)

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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.

Thanks for your heroic digging and persistence on this issue, Rustam!

thank you for the kind words, Dmitry, and thank you for asking the
questions, i learned quite a lot while trying to answer them.
i've just sent out a patch for this.

quoted

---
 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