[PATCH v2 0/4] arm64, riscv: release shadow stacks on failed fork

[Iván Ezequiel Rodriguez]

This is not a port of the x86 TLS fix. That leak was inside
copy_thread() when set_new_tls() failed, so copy_process() skipped
exit_thread(). Here copy_thread() returns 0 and a later
copy_process() step fails. The concrete hole is clone3() with
CLONE_VM|CLONE_PIDFD and an unwritable pidfd pointer: put_user()
returns -EFAULT after the architecture has mapped a shadow stack.
exit_thread() does run, but arm64 and riscv did not implement it, so
that mapping stays in the parent's mm.

Normal exit already frees via deactivate_mm(). The failed-fork path
never reaches that. The free helpers already refuse a foreign mm
and a cleared base, so wiring them to exit_thread() does not unmap
the parent on !CLONE_VM or double-free on a normal exit. A
successful clone3() child with the same flags exits through
deactivate_mm() and then exit_thread(); that is the usual teardown
order this series also enables.

riscv's shstk_release() comment already described this
exit_thread() call. The architecture never selected
HAVE_EXIT_THREAD, so the documented path was not connected.

Do not add cleanup for a copy_thread() allocation error:
copy_process() skips exit_thread() when copy_thread() fails. arm64
clears inherited allocation tracking before attempting the
allocation; on riscv a failed allocation can still leave the
inherited parent base in place. Tasks created through the args->fn
path do not allocate a user shadow stack in copy_thread(); with
CLONE_VM a later failed copy_process() still shares the caller's
mm, so that path clears inherited tracking before return.

x86 does not need the same args->fn clear: copy_thread() runs
shstk_alloc_thread_stack() before both the PF_KTHREAD and later
args->fn return paths, so CLONE_VM replaces the inherited tracking
first.

The selftests assume /proc is mounted. They use kselftest.h / KTAP.
Feature and clone3 availability skips run before ksft_set_plan().
The plan is three tests: a valid clone3 writes a pidfd, sixteen
bad-pidfd clones return -EFAULT, and the shadow-stack VMA count is
unchanged.

Based on commit f47d223627f8 ("Merge tag 'vfs-7.4-rc1.sync_close' of
git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs").

arm64: QEMU 11.1.2 -machine virt -cpu max, CONFIG_ARM64_GCS, one
vCPU. Unpatched VMA count 1->17. Patched 1->1, 3/3 KTAP pass.

riscv: QEMU 11.1.2 -machine virt
-cpu rv64,zimop=on,zcmop=on,zicfiss=on, CONFIG_RISCV_USER_CFI, one
vCPU. Unpatched VMA count 1->17. Patched 1->1, 3/3 KTAP pass.

Changes since v1:
- arm64: introduce gcs_reset_thread() (empty stub without
  CONFIG_ARM64_GCS). Document why x86 does not need the same
  args->fn clear. Comment the property of the args->fn path, not
  only kernel_thread().
- riscv: same args->fn wording; clarify that normal exit reaches
  shstk_release() through deactivate_mm() while failed-fork did not.
- selftests: rewrite on kselftest.h / KTAP. Drop open-coded
  [OK]/[FAIL], exclusive-end VMA matching, ss_kb, depth(16), and
  parent GCSPR/SSP ping checks. Count only VmFlags lines with " ss".
  Skip before ksft_set_plan(). Factor my_syscall2 into gcs-util.h.

Assisted-by: LLM

Iván Ezequiel Rodriguez (4):
  arm64/gcs: release GCS on failed fork
  riscv/shstk: release shadow stack on failed fork
  selftests/arm64: GCS must not leak on a failed clone3
  selftests/riscv: shadow stack must not leak on a failed clone3

 arch/arm64/Kconfig                            |   1 +
 arch/arm64/include/asm/gcs.h                  |   8 +
 arch/arm64/kernel/process.c                   |  19 ++
 arch/riscv/Kconfig                            |   1 +
 arch/riscv/kernel/process.c                   |  20 ++
 tools/testing/selftests/arm64/gcs/Makefile    |   3 +-
 .../selftests/arm64/gcs/gcs-clone-pidfd.c     | 168 +++++++++++++++++
 .../testing/selftests/arm64/gcs/gcs-locking.c |  20 --
 tools/testing/selftests/arm64/gcs/gcs-util.h  |  24 +++
 tools/testing/selftests/arm64/gcs/libc-gcs.c  |  20 --
 tools/testing/selftests/riscv/cfi/Makefile    |   5 +-
 .../selftests/riscv/cfi/shstk-clone-pidfd.c   | 173 ++++++++++++++++++
 12 files changed, 420 insertions(+), 42 deletions(-)
 create mode 100644 tools/testing/selftests/arm64/gcs/gcs-clone-pidfd.c
 create mode 100644 tools/testing/selftests/riscv/cfi/shstk-clone-pidfd.c


base-commit: f47d223627f80cb7a1eb14c585fe6cb35de5e984
-- 
2.43.0