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

[Zhengchao Shao <shaozhengchao@xxxxxxxxxx>]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Zhengchao Shao <shaozhengchao@xxxxxxxxxx>
---
 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);
+			if (!true_data)
+				return ERR_PTR(-ENOMEM);
+		data = true_data;
+	}
+
 	if (copy_from_user(data + headroom, data_in, user_size)) {
 		kfree(data);
 		return ERR_PTR(-EFAULT);
-- 
2.17.1