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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id l4-20020a17090b078400b002009db534d1sm375509pjz.24.2022.11.01.21.27.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Nov 2022 21:27:04 -0700 (PDT) Date: Tue, 1 Nov 2022 21:27:03 -0700 From: Kees Cook To: Jakub Kicinski Cc: zhongbaisong , Daniel Borkmann , edumazet@google.com, davem@davemloft.net, pabeni@redhat.com, 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 , Marco Elver , Dmitry Vyukov , Linux MM , kasan-dev@googlegroups.com Subject: Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb() Message-ID: <202211012121.47D68D0@keescook> References: <20221101040440.3637007-1-zhongbaisong@huawei.com> <20221101210542.724e3442@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20221101210542.724e3442@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 01, 2022 at 09:05:42PM -0700, Jakub Kicinski wrote: > On Wed, 2 Nov 2022 10:59:44 +0800 zhongbaisong wrote: > > On 2022/11/2 0:45, Daniel Borkmann wrote: > > > [ +kfence folks ] > > > > + cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov > > > > Do you have any suggestions about this problem? > > + Kees who has been sending similar patches for drivers > > > > 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 > > >> --- > > >>   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. I hope I answer this in more detail here: https://lore.kernel.org/lkml/202211010937.4631CB1B0E@keescook/ The problem is that ksize() should never have existed in the first place. :P Every runtime bounds checker has tripped over it, and with the addition of the __alloc_size attribute, I had to start ripping ksize() out: it can't be used to pretend an allocation grew in size. Things need to either preallocate more or go through *realloc() like everything else. Luckily, ksize() is rare. FWIW, the above fix doesn't look correct to me -- I would expect this to be: size_t alloc_size; ... alloc_size = kmalloc_size_roundup(size + headroom + tailroom); data = kzalloc(alloc_size, GFP_USER); -- Kees Cook