linux-kernel - [RFC bpf-next 0/2] bpf: tnums: warn against the usage of tnum_in(tnum

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 31 Aug 2022 11:19:05 +0800
From:   Shung-Hsi Yu <shung-hsi.yu@...e.com>
To:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "Alexei Starovoitov" <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "John Fastabend" <john.fastabend@...il.com>,
        Shung-Hsi Yu <shung-hsi.yu@...e.com>
Subject: [RFC bpf-next 0/2] bpf: tnums: warn against the usage of tnum_in(tnum_range(), ...)

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

-- 
2.37.2