On MMU systems, during exec, argument and environment strings are copied
into the new address space held by struct linux_binprm before that address
space is installed on the task_struct. Existing eBPF user memory helpers
only support reading from the old address space (i.e. current process),
and for this reason programs cannot access these strings from the
bprm_check_security LSM hook.

This series adds two sleepable BPF kfuncs for copying bytes or
NUL-terminated strings from a trusted struct mm_struct. It also marks
linux_binprm->mm as trusted-or-null, allowing BPF LSM programs to pass it
to the kfuncs after a NULL check and inspect exec arguments before
allowing the exec to continue.

Changing bprm->mm to trusted-or-null would otherwise reject existing BPF
programs that read through it without a NULL check. Preserve that behavior
by allowing fault-protected reads through trusted-or-null BTF pointers.
Pointer arithmetic, writes, and passing the pointer to a kfunc that
requires a non-NULL trusted argument continue to require an explicit
NULL check.

On NOMMU systems, exec argument and environment strings remain in
bprm->page[] until they are transferred to the new process stack. They
cannot be accessed through bprm->mm at the bprm_check_security hook.
The new kfuncs remain available on NOMMU for address ranges represented
by a supplied struct mm_struct.

The series also adds selftests covering both kfuncs when reading argument
and environment strings, and verifier tests covering trusted-or-null BTF
pointer reads.

Changes in v3:
- Replace the linux_binprm-specific kfuncs with generic struct mm_struct
  kfuncs, as suggested by Andrii Nakryiko.
- Move the kfuncs next to the existing user memory helpers and make the
  task-based variants delegate to the new mm-based implementations, as
  suggested by Andrii Nakryiko.
- Clear bprm->mm before dropping its reference on exec error paths.
- Mark linux_binprm->mm as trusted-or-null.
- Allow fault-protected reads through trusted-or-null BTF pointers to
  preserve compatibility with existing BPF programs, as suggested by
  Andrii Nakryiko.
- Add verifier and runtime selftests for trusted-or-null BTF pointer
  reads.
- Rename __copy_remote_vm_str() to __copy_remote_mm_str(), as suggested
  by Andrii Nakryiko.
- Clarify that reading exec strings through bprm->mm is limited to MMU
  systems, while the generic mm-based kfuncs remain available on NOMMU.

Changes in v2:
- Register the kfuncs on NOMMU systems and return -EOPNOTSUPP when called,
  as suggested by Justin Suess.
- Add selftest coverage for reading environment strings, as suggested by
  Justin Suess.
- Clarify that copy_remote_mm_str() leaves the destination untouched when
  called with a zero-length buffer.
- Use sizeof() instead of hardcoded argument lengths in the selftests.
- Use ~0ULL for invalid-flags checks in the selftests.

v2:
https://lore.kernel.org/bpf/20260820131801.68759-1-tasos.papagiannnis@gmail.com/

v1:
https://lore.kernel.org/bpf/20260812111140.7762-1-tasos.papagiannnis@gmail.com/

Anastasios Papagiannis (7):
  mm: Add copy_remote_mm_str()
  bpf: Add user memory access kfuncs for mm_struct
  exec: Clear bprm->mm before dropping its reference
  bpf: Allow reads through trusted-or-null BTF pointers
  selftests/bpf: Cover trusted-or-null BTF pointer reads
  bpf: Mark linux_binprm->mm as trusted-or-null
  selftests/bpf: Test mm_struct user memory kfuncs with linux_binprm

 fs/exec.c                                     |   7 +-
 include/linux/bpf_verifier.h                  |   9 +-
 include/linux/mm.h                            |   2 +
 kernel/bpf/helpers.c                          | 115 ++++++++++++++--
 kernel/bpf/verifier.c                         |  17 ++-
 mm/memory.c                                   |  33 ++++-
 mm/nommu.c                                    |  33 ++++-
 .../selftests/bpf/prog_tests/bpf_iter.c       |   6 +-
 .../bpf/prog_tests/copy_from_user_bprm.c      |  72 ++++++++++
 .../prog_tests/test_struct_ops_maybe_null.c   |  12 +-
 .../bpf/prog_tests/tp_btf_nullable.c          |  28 ++++
 .../selftests/bpf/progs/copy_from_user_bprm.c | 123 ++++++++++++++++++
 .../selftests/bpf/progs/raw_tp_null_fail.c    |  63 ++++++++-
 .../bpf/progs/test_tp_btf_nullable.c          |  17 ++-
 .../bpf/progs/test_tp_btf_nullable_runtime.c  |  35 +++++
 .../selftests/bpf/progs/verifier_lsm.c        |  19 ++-
 .../selftests/bpf/progs/verifier_vfs_accept.c |  13 ++
 .../selftests/bpf/progs/verifier_vfs_reject.c |  14 --
 .../selftests/bpf/test_kmods/bpf_testmod.c    |   1 +
 .../testing/selftests/sched_ext/maybe_null.c  |   6 +-
 20 files changed, 566 insertions(+), 59 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/copy_from_user_bprm.c
 create mode 100644 tools/testing/selftests/bpf/progs/copy_from_user_bprm.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_tp_btf_nullable_runtime.c


base-commit: d761934c9483ecde93fe99d8705282f716dfee50
-- 
2.55.0