[RFC,bpf-next,0/2] bpf: tnums: warn against the usage of tnum_in(tnum_range(), ...)

Message

Shung-Hsi Yu Aug. 31, 2022, 3:19 a.m. UTC

Commit a657182a5c51 ("bpf: Don't use tnum_range on array range checking
for poke descriptors") has shown that using tnum_range() as argument to
tnum_in() can lead to misleading code that looks like tight bound check
when in fact the actual allowed range is much wider.

This patchset is a follow up of the above commit. I've audited other
usage of tnum_in() in verifier and have concluded that all of either
provides a tight bound check, or is using reg->var_off as the first
argument, and thus safe.

To prevent the problematic tnum_in(tnum_range(), ...) usage, add
documentation in the tnum.h header file to warn against it.

This is sent as an RFC for two reasons:
1. Gather feedback on whether it's possible to prevent the problematic
   usage besides relying just on documentation. 

   One invasive option is to switch bound-checks done with
   tnum_in(tnum_range(), ...) to use reg->u{min,max}_value instead,
   which should always provide a tight bound check.

   Alternatively maybe problematic usage can be detected through
   development tool (sparse or Coccinelle?), but I know rather little
   about them.
   
2. Attach a proof for the claimed safe usage of tnum_in(tnum_range(), ...)
   found in patch 1, where the proof itself is not meant to be
   merged.

Shung-Hsi Yu (2):
  bpf: tnums: warn against the usage of tnum_in(tnum_range(), ...)
  proof for the safe usage of tnum_in()

 include/linux/tnum.h |  20 +++++-
 tnum_in.py           | 158 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 176 insertions(+), 2 deletions(-)
 create mode 100755 tnum_in.py