[PATCH v4 0/6] MDWE without inheritance

Florent Revest
2023-08-28T15:10:04Z

Joey recently introduced a Memory-Deny-Write-Executable (MDWE) prctl which tags
current with a flag that prevents pages that were previously not executable from
becoming executable.
This tag always gets inherited by children tasks. (it's in MMF_INIT_MASK)

At Google, we've been using a somewhat similar downstream patch for a few years
now. To make the adoption of this feature easier, we've had it support a mode in
which the W^X flag does not propagate to children. For example, this is handy if
a C process which wants W^X protection suspects it could start children
processes that would use a JIT.

I'd like to align our features with the upstream prctl. This series proposes a
new NO_INHERIT flag to the MDWE prctl to make this kind of adoption easier. It
sets a different flag in current that is not in MMF_INIT_MASK and which does not
propagate.

As part of looking into MDWE, I also fixed a couple of things in the MDWE test.

This series applies on the mm-everything-2023-08-25-20-06 tag of the mm tree:
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/

Diff since v3:
- Added a bunch of Reviewed-by, Acked-by and Tested-by. Thanks everyone!
- Reworded patch 2's description for clarity
- Removed an unnecessary int cast
- Added test coverage for errnos of invalid prctls (EPERM/EINVAL)
- Added test coverage for can_keep_no_flags and can_keep_both_flags

Diff since v2:
- Turned the MMF_INIT_FLAGS macro into a mmf_init_flags function as suggested by
David Hildenbrand
- Removed the ability to transition from to PR_MDWE_REFUSE_EXEC_GAIN from
(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT) which also significantly
simplifies the prctl_set_mdwe logic
- Cc-ed -stable on patch 3 as suggested by Alexey Izbyshev
- Added a handful of Reviewed-by/Acked-by trailers

Diff since v1:
- MMF_HAS_MDWE_NO_INHERIT clears MMF_HAS_MDWE in the fork path as part of a
MMF_INIT_FLAGS macro (suggested by Catalin)
- PR_MDWE_* are defined as unsigned long rather than int (suggested by Andrey)

Florent Revest (6):
kselftest: vm: Fix tabs/spaces inconsistency in the mdwe test
kselftest: vm: Fix mdwe's mmap_FIXED test case
kselftest: vm: Check errnos in mdwe_test
mm: Make PR_MDWE_REFUSE_EXEC_GAIN an unsigned long
mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl
kselftest: vm: Add tests for no-inherit memory-deny-write-execute

include/linux/sched/coredump.h | 10 ++
include/uapi/linux/prctl.h | 3 +-
kernel/fork.c | 2 +-
kernel/sys.c | 32 ++++--
tools/include/uapi/linux/prctl.h | 3 +-
tools/testing/selftests/mm/mdwe_test.c | 137 ++++++++++++++++++++++---
6 files changed, 163 insertions(+), 24 deletions(-)

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2.42.0.rc2.253.gd59a3bf2b4-goog