bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

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 the struct skb_shared_info's
unaligned 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-torvalds#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-torvalds#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 adjust @SiZe so that (@SiZe + @hearoom) is a
multiple of SMP_CACHE_BYTES. So we make sure the struct skb_shared_info
is aligned to a cache line.

Fixes: 1cf1cae ("bpf: introduce BPF_PROG_TEST_RUN command")
Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>

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 = SKB_DATA_ALIGN(size);
 	data = kzalloc(size + headroom + tailroom, GFP_USER);
 	if (!data)
 		return ERR_PTR(-ENOMEM);