CVE-2021-47089

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.

Weakness

The product does not sufficiently track and release allocated memory after it has been used, which slowly consumes remaining memory.

Affected Software

Potential Mitigations

• Choose a language or tool that provides automatic memory management, or makes manual memory management less error-prone.
• For example, glibc in Linux provides protection against free of invalid pointers.
• When using Xcode to target OS X or iOS, enable automatic reference counting (ARC) [REF-391].
• To help correctly and consistently manage memory when programming in C++, consider using a smart pointer class such as std::auto_ptr (defined by ISO/IEC ISO/IEC 14882:2003), std::shared_ptr and std::unique_ptr (specified by an upcoming revision of the C++ standard, informally referred to as C++ 1x), or equivalent solutions such as Boost.

References

• https://git.kernel.org/stable/c/0129ab1f268b6cf88825eae819b9b84aa0a85634
• https://git.kernel.org/stable/c/2f06c8293d27f6337f907042c602c9c953988c48
• https://git.kernel.org/stable/c/0129ab1f268b6cf88825eae819b9b84aa0a85634
• https://git.kernel.org/stable/c/2f06c8293d27f6337f907042c602c9c953988c48