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Subject: Re: [PATCH -next] bpf, test_run: fix alignment problem in
 bpf_prog_test_run_skb()
To:     Baisong Zhong <zhongbaisong@huawei.com>, edumazet@google.com,
        davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com
Cc:     linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        netdev@vger.kernel.org, ast@kernel.org, song@kernel.org,
        yhs@fb.com, haoluo@google.com,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>
References: <20221101040440.3637007-1-zhongbaisong@huawei.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <eca17bfb-c75f-5db1-f194-5b00c2a0c6f2@iogearbox.net>
Date:   Tue, 1 Nov 2022 17:45:02 +0100
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Precedence: bulk

[ +kfence folks ]

On 11/1/22 5:04 AM, Baisong Zhong wrote:
> Recently, we got a syzkaller problem because of aarch64
> alignment fault if KFENCE enabled.
> 
> When the size from user bpf program is an odd number, like
> 399, 407, etc, it will cause skb shard info's alignment access,
> as seen below:
> 
> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
> 
> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
>   bpf_prog_d3839dd9068ceb51+0x80/0x330
>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> 
> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, cache=kmalloc-512
> 
> allocated by task 15074 on cpu 0 at 1342.585390s:
>   kmalloc include/linux/slab.h:568 [inline]
>   kzalloc include/linux/slab.h:675 [inline]
>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
> 
> To fix the problem, we round up allocations with kmalloc_size_roundup()
> so that build_skb()'s use of kize() is always alignment and no special
> handling of the memory is needed by KFENCE.
> 
> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
> ---
>   net/bpf/test_run.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 13d578ce2a09..058b67108873 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
>   	if (user_size > size)
>   		return ERR_PTR(-EMSGSIZE);
>   
> +	size = kmalloc_size_roundup(size);
>   	data = kzalloc(size + headroom + tailroom, GFP_USER);

The fact that you need to do this roundup on call sites feels broken, no?
Was there some discussion / consensus that now all k*alloc() call sites
would need to be fixed up? Couldn't this be done transparently in k*alloc()
when KFENCE is enabled? I presume there may be lots of other such occasions
in the kernel where similar issue triggers, fixing up all call-sites feels
like ton of churn compared to api-internal, generic fix.

>   	if (!data)
>   		return ERR_PTR(-ENOMEM);
> 

Thanks,
Daniel