Re: [PATCH bpf-next] bpf, test_run: fix alignment problem in bpf_test_init()

[John Fastabend <john.fastabend@gmail.com>]

shaozhengchao wrote:
> 
> 
> On 2022/12/7 3:58, John Fastabend wrote:
> > Zhengchao Shao wrote:
> >> The problem reported by syz is as follows:
> >> BUG: KASAN: slab-out-of-bounds in __build_skb_around+0x230/0x330
> >> Write of size 32 at addr ffff88807ec6b2c0 by task bpf_repo/6711
> >> Call Trace:
> >> <TASK>
> >> dump_stack_lvl+0x8e/0xd1
> >> print_report+0x155/0x454
> >> kasan_report+0xba/0x1f0
> >> kasan_check_range+0x35/0x1b0
> >> memset+0x20/0x40
> >> __build_skb_around+0x230/0x330
> >> build_skb+0x4c/0x260
> >> bpf_prog_test_run_skb+0x2fc/0x1ce0
> >> __sys_bpf+0x1798/0x4b60
> >> __x64_sys_bpf+0x75/0xb0
> >> do_syscall_64+0x35/0x80
> >> entry_SYSCALL_64_after_hwframe+0x46/0xb0
> >> </TASK>
> >>
> >> Allocated by task 6711:
> >> kasan_save_stack+0x1e/0x40
> >> kasan_set_track+0x21/0x30
> >> __kasan_kmalloc+0xa1/0xb0
> >> __kmalloc+0x4e/0xb0
> >> bpf_test_init.isra.0+0x77/0x100
> >> bpf_prog_test_run_skb+0x219/0x1ce0
> >> __sys_bpf+0x1798/0x4b60
> >> __x64_sys_bpf+0x75/0xb0
> >> do_syscall_64+0x35/0x80
> >> entry_SYSCALL_64_after_hwframe+0x46/0xb0
> >>
> >> The process is as follows:
> >> bpf_prog_test_run_skb()
> >> 	bpf_test_init()
> >> 		data = kzalloc()	//The length of input is 576.
> >> 					//The actual allocated memory
> >> 					//size is 1024.
> >> 	build_skb()
> >> 		__build_skb_around()
> >> 			size = ksize(data)//size = 1024
> >> 			size -= SKB_DATA_ALIGN(
> >> 					sizeof(struct skb_shared_info));
> >> 					//size = 704
> >> 			skb_set_end_offset(skb, size);
> >> 			shinfo = skb_shinfo(skb);//shinfo = data + 704
> >> 			memset(shinfo...)	//Write out of bounds
> >>
> >> In bpf_test_init(), the accessible space allocated to data is 576 bytes,
> >> and the memory allocated to data is 1024 bytes. In __build_skb_around(),
> >> shinfo indicates the offset of 704 bytes of data, which triggers the issue
> >> of writing out of bounds.
> >>
> >> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
> >> Reported-by: syzbot+fda18eaa8c12534ccb3b@syzkaller.appspotmail.com
> >> Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
> >> ---
> >>   net/bpf/test_run.c | 10 ++++++++++
> >>   1 file changed, 10 insertions(+)
> >>
> >> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> >> index fcb3e6c5e03c..fbd5337b8f68 100644
> >> --- a/net/bpf/test_run.c
> >> +++ b/net/bpf/test_run.c
> >> @@ -766,6 +766,8 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
> >>   			   u32 size, u32 headroom, u32 tailroom)
> >>   {
> >>   	void __user *data_in = u64_to_user_ptr(kattr->test.data_in);
> >> +	unsigned int true_size;
> >> +	void *true_data;
> >>   	void *data;
> >>   
> >>   	if (size < ETH_HLEN || size > PAGE_SIZE - headroom - tailroom)
> >> @@ -779,6 +781,14 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
> >>   	if (!data)
> >>   		return ERR_PTR(-ENOMEM);
> >>   
> >> +	true_size = ksize(data);
> >> +	if (size + headroom + tailroom < true_size) {
> >> +		true_data = krealloc(data, true_size, GFP_USER | __GFP_ZERO);
> > 
> > This comes from a kzalloc, should we zero realloc'd memory as well?
> > 
> >> +			if (!true_data)
> >> +				return ERR_PTR(-ENOMEM);
> > 
> > I think its worth fixing the extra tab here.
> > 
> 
> Hi John:
> 	Thank you for your review. Your suggestion looks good to me. And I 
> found Kees Cook also focus on this issue.
> https://patchwork.kernel.org/project/netdevbpf/patch/20221206231659.never.929-kees@kernel.org/
> Perhaps his solution will be more common?

Maybe, seems ksize should not be used either.