[PATCH 0/7] fix segfaults with implicit-bool config

Carlos reported to the security list a case where you can cause Git to segfault by using an implicit bool like:

  [core]
  someVariable

when the parsing side for core.someVariable does not correctly check a NULL "value" string. This is mostly harmless, as anybody who can feed arbitrary config can already execute arbitrary code. There is one case of this when parsing .gitmodules (which we don't trust), but even there I don't think the security implications are that interesting. A malicious repo can get "clone --recurse-submodules" to segfault, but always with a strict NULL dereference, not any kind of controllable pointer. See patch 5 for more details.

I audited the whole code base for instances of the problem. It was fairly manual, so it's possible I missed a spot, but I think this should cover everything.

The first patch has vanilla cases, and the rest are instances where I thought it was worth calling out specific details.

  [1/7]: config: handle NULL value when parsing non-bools
  [2/7]: setup: handle NULL value when parsing extensions
  [3/7]: trace2: handle NULL values in tr2_sysenv config callback
  [4/7]: help: handle NULL value for alias.* config
  [5/7]: submodule: handle NULL value when parsing submodule.*.branch
  [6/7]: trailer: handle NULL value when parsing trailer-specific config
  [7/7]: fsck: handle NULL value when parsing message config

 builtin/blame.c        |  2 ++
 builtin/checkout.c     |  2 ++
 builtin/clone.c        |  2 ++
 builtin/log.c          |  5 ++++-
 builtin/pack-objects.c |  6 +++++-
 builtin/receive-pack.c | 11 +++++++----
 compat/mingw.c         |  2 ++
 config.c               |  8 ++++++++
 diff.c                 | 19 ++++++++++++++++---
 fetch-pack.c           | 12 ++++++++----
 fsck.c                 |  8 ++++++--
 help.c                 |  5 ++++-
 mailinfo.c             |  2 ++
 notes-utils.c          |  2 ++
 setup.c                |  2 ++
 submodule-config.c     |  4 +++-
 trace2/tr2_sysenv.c    |  2 ++
 trailer.c              |  8 ++++++++
 18 files changed, 85 insertions(+), 17 deletions(-)