kfence: fix memory leak when cat kfence objects

In the Linux kernel, the following vulnerability has been resolved:

kfence: fix memory leak when cat kfence objects

Hulk robot reported a kmemleak problem:

unreferenced object 0xffff93d1d8cc02e8 (size 248):

comm "cat", pid 23327, jiffies 4624670141 (age 495992.217s)

hex dump (first 32 bytes):

00 40 85 19 d4 93 ff ff 00 10 00 00 00 00 00 00 .@..............

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

backtrace:

seq_open+0x2a/0x80

full_proxy_open+0x167/0x1e0

do_dentry_open+0x1e1/0x3a0

path_openat+0x961/0xa20

do_filp_open+0xae/0x120

do_sys_openat2+0x216/0x2f0

do_sys_open+0x57/0x80

do_syscall_64+0x33/0x40

entry_SYSCALL_64_after_hwframe+0x44/0xa9

unreferenced object 0xffff93d419854000 (size 4096):

comm "cat", pid 23327, jiffies 4624670141 (age 495992.217s)

hex dump (first 32 bytes):

6b 66 65 6e 63 65 2d 23 32 35 30 3a 20 30 78 30 kfence-#250: 0x0

30 30 30 30 30 30 30 37 35 34 62 64 61 31 32 2d 0000000754bda12-

backtrace:

seq_read_iter+0x313/0x440

seq_read+0x14b/0x1a0

full_proxy_read+0x56/0x80

vfs_read+0xa5/0x1b0

ksys_read+0xa0/0xf0

do_syscall_64+0x33/0x40

entry_SYSCALL_64_after_hwframe+0x44/0xa9

I find that we can easily reproduce this problem with the following

commands:

cat /sys/kernel/debug/kfence/objects

echo scan > /sys/kernel/debug/kmemleak

cat /sys/kernel/debug/kmemleak

The leaked memory is allocated in the stack below:

do_syscall_64

do_sys_open

do_dentry_open

full_proxy_open

seq_open ---> alloc seq_file

vfs_read

full_proxy_read

seq_read

seq_read_iter

traverse ---> alloc seq_buf

And it should have been released in the following process:

do_syscall_64

syscall_exit_to_user_mode

exit_to_user_mode_prepare

task_work_run

____fput

__fput

full_proxy_release ---> free here

However, the release function corresponding to file_operations is not

implemented in kfence. As a result, a memory leak occurs. Therefore,

the solution to this problem is to implement the corresponding release

function.