NULL pointer dereference in coredump code

[Thibaut Sautereau]

 I hit a kernel NULL pointer dereference caused by the following call chain:

do_coredump()
  file_start_write(cprm.file) # cprm.file is NULL
    file_inode(file) # NULL ptr deref

The `ispipe` path is followed in do_coredump(), and:
    # cat /proc/sys/kernel/core_pattern
    |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h

It seems that cprm.file can be NULL after the call to the usermode
helper, especially when setting CONFIG_STATIC_USERMODEHELPER=y and
CONFIG_STATIC_USERMODEHELPER_PATH="", which is the case for me.

One may say it's a strange combination of configuration options but I
think it should not crash the kernel anyway. As I don't know much about
coredumps in general and this code, I don't know what's the best way to
fix this issue in a clean and comprehensive way.

I attached the patch I used to temporarily work around this issue, if
that can clarify anything.

Thanks,

-- 
Thibaut Sautereau
CLIP OS developer