bpf: Fix indirect var_off stack access support

v2->v3:
- sanity check max value for variable offset.

v1->v2:
- rely on meta = NULL to reject var_off stack access to uninit buffer.

This patch set is a follow-up for discussion [1].

It fixes variable offset stack access handling for raw and unprivileged mode, rejecting both of them, and sanity checks max variable offset value.

Patch 1 handles raw (uninitialized) mode.
Patch 2 adds test for raw mode.
Patch 3 handles unprivileged mode.
Patch 4 adds test for unprivileged mode.
Patch 5 adds sanity check for max value of variable offset. Patch 6 adds test for variable offset max value checking. Patch 7 is a minor fix in verbose log.

Unprivileged mode is an interesting case since one (and only?) way to come up with variable offset is to use pointer arithmetics. Though pointer arithmetics is already prohibited for unprivileged mode. I'm not sure if it's enough though and it seems like a good idea to still reject variable offset for unpriv in check_stack_boundary(). Please see patches 3 and 4 for more details on this.

[1] https://marc.info/?l=linux-netdev&m=155419526427742&w=2

Andrey Ignatov (7):
bpf: Reject indirect var_off stack access in raw mode selftests/bpf: Test indirect var_off stack access in raw mode
bpf: Reject indirect var_off stack access in unpriv mode selftests/bpf: Test indirect var_off stack access in unpriv mode
bpf: Sanity check max value for var_off stack access selftests/bpf: Test unbounded var_off stack access
bpf: Add missed newline in verifier verbose log

 kernel/bpf/verifier.c                         |  45 ++++++-
 .../testing/selftests/bpf/verifier/var_off.c  | 111 +++++++++++++++++-
 2 files changed, 150 insertions(+), 6 deletions(-)

2.17.1