netdev - Re: [PATCH] bpf: fix false positive kmemleak report in bpf_ringbuf_area

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Message-ID: <254ef541-6fd6-9ddf-3491-97b854a09554@iogearbox.net>
Date:   Mon, 28 Jun 2021 16:19:05 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Rustam Kovhaev <rkovhaev@...il.com>, ast@...nel.org,
        andrii@...nel.org, kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com, andrii@...nel.org
Subject: Re: [PATCH] bpf: fix false positive kmemleak report in
 bpf_ringbuf_area_alloc()

On 6/26/21 8:11 PM, Rustam Kovhaev wrote:
> kmemleak scans struct page, but it does not scan the page content.
> 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(), but part of struct bpf_ringbuf is
> mmaped to user space, and if struct bpf_ringbuf changes we would have to
> revisit and review size argument in kmemleak_alloc(), because we do not
> want kmemleak to scan the user space memory.
> let's simplify things and use kmemleak_not_leak() here.
> 
> Link: https://lore.kernel.org/lkml/YNTAqiE7CWJhOK2M@nuc10/
> Link: https://lore.kernel.org/lkml/20210615101515.GC26027@arm.com/
> Link: https://syzkaller.appspot.com/bug?extid=5d895828587f49e7fe9b
> Reported-and-tested-by: syzbot+5d895828587f49e7fe9b@...kaller.appspotmail.com
> Signed-off-by: Rustam Kovhaev <rkovhaev@...il.com>

Applied, thanks! (Also included Andrii's prior analysis as well to the commit
log so there's a bit more context if we need to revisit in future [0].)

   [0] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=ccff81e1d028bbbf8573d3364a87542386c707bf