[RFC PATCH bpf 0/6] bpf: Disallow interpreter fallback for interpreter-unsupported insns

[Leon Hwang <leon.hwang@linux.dev>]

Sashiko reported two potential issues about interpreter fallback [1]
[2].

After verifying them by patch #7, I think they are real issues. With
LLM assistance, the interpreter does not support the internal
BPF_PROBE_ATOMIC insn and the gotox insn (used for indirect jumps),
either.

1) the user BPF_ADDR_SPACE_CAST insn
   the interpreter just ignores it.

2) the arena ST/STX/LDX insn
   the interpreter could hit the BUG_ON() in ___bpf_prog_run().

3) the BPF_MOV64_PERCPU_REG insn
   the interpreter could hit page fault, due to loading memory from
   invalid __percpu pointer.

4) the internal BPF_PROBE_ATOMIC insn
   the interpreter could hit the BUG_ON() in ___bpf_prog_run().

5) the gotox insn used for indirect jumps
   the interpreter could hit the BUG_ON() in ___bpf_prog_run(), too.

Reject these insns on interpreter fallback path in
__bpf_prog_select_runtime().

This series is built on
"bpf: Fix unaligned interpreter panic on JIT fallback path" [3]. The
patch #7 is also able to verify the issue of un-JITed helper.

However, The patch #7 aims to verify the issues. I think it is not
proper to be applied to upstream, because it adds a stub
'bpf_jit_test_fail_task' to bpf_prog_jit_compile() for the tests.

I'd like to drop the patch #7 in the next revision.

Link:
[1] https://lore.kernel.org/bpf/20260608151347.2C77D1F00893@smtp.kernel.org/
[2] https://lore.kernel.org/bpf/20260622150759.EC9071F000E9@smtp.kernel.org/
[3] https://lore.kernel.org/bpf/20260615025316.24429-1-yangtiezhu@loongson.cn/

Leon Hwang (6):
  bpf: Disallow interpreter fallback for user BPF_ADDR_SPACE_CAST insn
  bpf: Disallow interpreter fallback for arena insn
  bpf: Disallow interpreter fallback for BPF_MOV64_PERCPU_REG insn
  bpf: Disallow interpreter fallback for internal BPF_PROBE_ATOMIC insn
  bpf: Disallow interpreter fallback for gotox insn
  lib/test_bpf: Add interpreter-fallback tests

 include/linux/bpf.h                      |   1 +
 include/linux/filter.h                   |   4 +
 kernel/bpf/core.c                        |  69 +-
 lib/test_bpf.c                           | 800 ++++++++++++++++++++++-
 tools/lib/bpf/skel_internal.h            |   2 +
 tools/testing/selftests/bpf/test_kmod.sh |  39 +-
 6 files changed, 903 insertions(+), 12 deletions(-)

--
2.54.0