proc: fix NULL dereference when reading /proc/<pid>/auxv

proc: fix NULL dereference when reading /proc/<pid>/auxv

Reading auxv of any kernel thread results in NULL pointer dereferencing
in auxv_read() where mm can be NULL.  Fix that by checking for NULL mm
and bailing out early.  This is also the original behavior changed by
recent commit c5317167854e ("proc: switch auxv to use of __mem_open()").

  # cat /proc/2/auxv
  Unable to handle kernel NULL pointer dereference at virtual address 000000a8
  Internal error: Oops: 17 [#1] PREEMPT SMP ARM
  CPU: 3 PID: 113 Comm: cat Not tainted 4.9.0-rc1-ARCH+ #1
  Hardware name: BCM2709
  task: ea3b0b00 task.stack: e99b2000
  PC is at auxv_read+0x24/0x4c
  LR is at do_readv_writev+0x2fc/0x37c
  Process cat (pid: 113, stack limit = 0xe99b2210)
  Call chain:
    auxv_read
    do_readv_writev
    vfs_readv
    default_file_splice_read
    splice_direct_to_actor
    do_splice_direct
    do_sendfile
    SyS_sendfile64
    ret_fast_syscall

Fixes: c5317167854e ("proc: switch auxv to use of __mem_open()")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Leon Yu <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Janis Danisevskis <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>