[PATCH bpf-next v2 0/4] bpf: Cancel special fields in resizable hashtab on recycle

[chenyuan_fl]

From: Yuan Chen <chenyuan@xxxxxxxxxx>

Posted in reply to Kumar Kartikeya Dwivedi's review [1] of Nuoqi Gui's
overlapping series [2]: switching rhtab to bpf_obj_cancel_fields() alone
leaks referenced kptrs, because rhtab_map_update_elem() zeroes the kptr
slot of a recycled element via check_and_init_map_value() before the
allocator destructor can release it.

The resizable hashtab still eagerly calls bpf_obj_free_fields() on element
delete/replace, which runs kptr destructors from the caller's execution
context (unsafe in NMI). Patch 1 applies the cancel semantics like hash/array
maps (a3a81d247651) and fixes the recycle-path kptr leak with
rhtab_init_map_value(); patch 2 fixes a program-BTF use-after-free in the
mem-alloc destructor found while testing; patches 3-4 add regression tests
(NMI update, and per-field delete/re-insert cycles).

Verified in QEMU (KASAN, current bpf-next): rhtab_kptr, rhtab_fields (4/4)
and rhash pass with the fix; on the unfixed kernel the kptr subtests fail at
the recycle assertions and the program-BTF UAF reproduces.

Changes since v1: fix the recycle-path kptr leak, extend the selftests with
delete/re-insert recycle coverage, add per-field combination tests, fix the
program-BTF UAF (patch 2).

[1] https://lore.kernel.org/bpf/DKEVG8ZVJDDQ.2G40FTOIZKU57@xxxxxxxxx/
[2] https://lore.kernel.org/bpf/20260726-f01-23-rhash-cancel-bpf-next-v1-0-6e5e1131d885@xxxxxxxxxxxxxxxxxxxxx/

Yuan Chen (4):
  bpf: Cancel special fields in resizable hashtab on recycle
  bpf: Fix use-after-free of program BTF in mem-alloc destructor
  selftests/bpf: Test rhtab kptr recycle from NMI context
  selftests/bpf: Test rhtab special-field combinations

 kernel/bpf/hashtab.c                          | 115 ++++++-
 .../selftests/bpf/prog_tests/rhtab_fields.c   | 213 ++++++++++++
 .../selftests/bpf/prog_tests/rhtab_kptr.c     | 146 +++++++++
 .../selftests/bpf/progs/rhtab_fields.c        | 305 ++++++++++++++++++
 .../testing/selftests/bpf/progs/rhtab_kptr.c  | 132 ++++++++
 5 files changed, 901 insertions(+), 10 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/rhtab_fields.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/rhtab_kptr.c
 create mode 100644 tools/testing/selftests/bpf/progs/rhtab_fields.c
 create mode 100644 tools/testing/selftests/bpf/progs/rhtab_kptr.c

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2.54.0