Re: [PATCH v8 0/7] mseal system mappings

Lorenzo Stoakes
2025-03-03T11:51:10Z

Great nice descriptions thanks!

On Mon, Mar 03, 2025 at 05:09:14AM +0000, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@google.com>
>
> This is V8 version, addressing comments from V7, without code logic
> change.
>
> -------------------------------------------------------------------
> As discussed during mseal() upstream process [1], mseal() protects
> the VMAs of a given virtual memory range against modifications, such
> as the read/write (RW) and no-execute (NX) bits. For complete
> descriptions of memory sealing, please see mseal.rst [2].
>
> The mseal() is useful to mitigate memory corruption issues where a
> corrupted pointer is passed to a memory management system. For
> example, such an attacker primitive can break control-flow integrity
> guarantees since read-only memory that is supposed to be trusted can
> become writable or .text pages can get remapped.
>
> The system mappings are readonly only, memory sealing can protect
> them from ever changing to writable or unmmap/remapped as different
> attributes.
>
> System mappings such as vdso, vvar, vvar_vclock,
> vectors (arm compact-mode), sigpage (arm compact-mode),
> are created by the kernel during program initialization, and could
> be sealed after creation.
>
> Unlike the aforementioned mappings, the uprobe mapping is not
> established during program startup. However, its lifetime is the same
> as the process's lifetime [3]. It could be sealed from creation.
>
> The vsyscall on x86-64 uses a special address (0xffffffffff600000),
> which is outside the mm managed range. This means mprotect, munmap, and
> mremap won't work on the vsyscall. Since sealing doesn't enhance
> the vsyscall's security, it is skipped in this patch. If we ever seal
> the vsyscall, it is probably only for decorative purpose, i.e. showing
> the 'sl' flag in the /proc/pid/smaps. For this patch, it is ignored.
>
> It is important to note that the CHECKPOINT_RESTORE feature (CRIU) may
> alter the system mappings during restore operations. UML(User Mode Linux)
> and gVisor, rr are also known to change the vdso/vvar mappings.
> Consequently, this feature cannot be universally enabled across all
> systems. As such, CONFIG_MSEAL_SYSTEM_MAPPINGS is disabled by default.
>
> To support mseal of system mappings, architectures must define
> CONFIG_ARCH_SUPPORTS_MSEAL_SYSTEM_MAPPINGS and update their special
> mappings calls to pass mseal flag. Additionally, architectures must
> confirm they do not unmap/remap system mappings during the process
> lifetime. The existence of this flag for an architecture implies that
> it does not require the remapping of thest system mappings during
> process lifetime, so sealing these mappings is safe from a kernel
> perspective.
>
> This version covers x86-64 and arm64 archiecture as minimum viable feature.
>
> While no specific CPU hardware features are required for enable this
> feature on an archiecture, memory sealing requires a 64-bit kernel. Other
> architectures can choose whether or not to adopt this feature. Currently,
> I'm not aware of any instances in the kernel code that actively
> munmap/mremap a system mapping without a request from userspace. The PPC
> does call munmap when _install_special_mapping fails for vdso; however,
> it's uncertain if this will ever fail for PPC - this needs to be
> investigated by PPC in the future [4]. The UML kernel can add this support
> when KUnit tests require it [5].
>
> In this version, we've improved the handling of system mapping sealing from
> previous versions, instead of modifying the _install_special_mapping
> function itself, which would affect all architectures, we now call
> _install_special_mapping with a sealing flag only within the specific
> architecture that requires it. This targeted approach offers two key
> advantages: 1) It limits the code change's impact to the necessary
> architectures, and 2) It aligns with the software architecture by keeping
> the core memory management within the mm layer, while delegating the
> decision of sealing system mappings to the individual architecture, which
> is particularly relevant since 32-bit architectures never require sealing.
>
> Prior to this patch series, we explored sealing special mappings from
> userspace using glibc's dynamic linker. This approach revealed several
> issues:
> - The PT_LOAD header may report an incorrect length for vdso, (smaller
> than its actual size). The dynamic linker, which relies on PT_LOAD
> information to determine mapping size, would then split and partially
> seal the vdso mapping. Since each architecture has its own vdso/vvar
> code, fixing this in the kernel would require going through each
> archiecture. Our initial goal was to enable sealing readonly mappings,
> e.g. .text, across all architectures, sealing vdso from kernel since
> creation appears to be simpler than sealing vdso at glibc.
> - The [vvar] mapping header only contains address information, not length
> information. Similar issues might exist for other special mappings.
> - Mappings like uprobe are not covered by the dynamic linker,
> and there is no effective solution for them.
>
> This feature's security enhancements will benefit ChromeOS, Android,
> and other high security systems.
>
> Testing:
> This feature was tested on ChromeOS and Android for both x86-64 and ARM64.
> - Enable sealing and verify vdso/vvar, sigpage, vector are sealed properly,
> i.e. "sl" shown in the smaps for those mappings, and mremap is blocked.
> - Passing various automation tests (e.g. pre-checkin) on ChromeOS and
> Android to ensure the sealing doesn't affect the functionality of
> Chromebook and Android phone.
>
> I also tested the feature on Ubuntu on x86-64:
> - With config disabled, vdso/vvar is not sealed,
> - with config enabled, vdso/vvar is sealed, and booting up Ubuntu is OK,
> normal operations such as browsing the web, open/edit doc are OK.
>
> Link: https://lore.kernel.org/all/20240415163527.626541-1-jeffxu@chromium.org/ [1]
> Link: Documentation/userspace-api/mseal.rst [2]
> Link: https://lore.kernel.org/all/CABi2SkU9BRUnqf70-nksuMCQ+yyiWjo3fM4XkRkL-NrCZxYAyg@mail.gmail.com/ [3]
> Link: https://lore.kernel.org/all/CABi2SkV6JJwJeviDLsq9N4ONvQ=EFANsiWkgiEOjyT9TQSt+HA@mail.gmail.com/ [4]
> Link: https://lore.kernel.org/all/202502251035.239B85A93@keescook/ [5]
>
> -------------------------------------------
> History:
>
> V8:
> - Change ARCH_SUPPORTS_MSEAL_X to ARCH_SUPPORTS_MSEAL_X (Liam R. Howlett)
> - Update comments in Kconfig and mseal.rst (Lorenzo Stoakes, Liam R. Howlett)
> - Change patch header perfix to "mseal sysmap" (Lorenzo Stoakes)
> - Remove "vm_flags =" (Kees Cook, Liam R. Howlett, Oleg Nesterov)
> - Drop uml architecture (Lorenzo Stoakes, Kees Cook)
> - Add a selftest to verify system mappings are sealed (Lorenzo Stoakes)
>
> V7:
> https://lore.kernel.org/all/20250224225246.3712295-1-jeffxu@google.com/
> - Remove cover letter from the first patch (Liam R. Howlett)
> - Change macro name to VM_SEALED_SYSMAP (Liam R. Howlett)
> - logging and fclose() in selftest (Liam R. Howlett)
>
> V6:
> https://lore.kernel.org/all/20250224174513.3600914-1-jeffxu@google.com/
> - mseal.rst: fix a typo (Randy Dunlap)
> - security/Kconfig: add rr into note (Liam R. Howlett)
> - remove mseal_system_mappings() and use macro instead (Liam R. Howlett)
> - mseal.rst: add incompatible userland software (Lorenzo Stoakes)
> - remove RFC from title (Kees Cook)
>
> V5
> https://lore.kernel.org/all/20250212032155.1276806-1-jeffxu@google.com/
> - Remove kernel cmd line (Lorenzo Stoakes)
> - Add test info (Lorenzo Stoakes)
> - Add threat model info (Lorenzo Stoakes)
> - Fix x86 selftest: test_mremap_vdso
> - Restrict code change to ARM64/x86-64/UM arch only.
> - Add userprocess.h to include seal_system_mapping().
> - Remove sealing vsyscall.
> - Split the patch.
>
> V4:
> https://lore.kernel.org/all/20241125202021.3684919-1-jeffxu@google.com/
> - ARCH_HAS_SEAL_SYSTEM_MAPPINGS (Lorenzo Stoakes)
> - test info (Lorenzo Stoakes)
> - Update mseal.rst (Liam R. Howlett)
> - Update test_mremap_vdso.c (Liam R. Howlett)
> - Misc. style, comments, doc update (Liam R. Howlett)
>
> V3:
> https://lore.kernel.org/all/20241113191602.3541870-1-jeffxu@google.com/
> - Revert uprobe to v1 logic (Oleg Nesterov)
> - use CONFIG_SEAL_SYSTEM_MAPPINGS instead of _ALWAYS/_NEVER (Kees Cook)
> - Move kernel cmd line from fs/exec.c to mm/mseal.c and
> misc. (Liam R. Howlett)
>
> V2:
> https://lore.kernel.org/all/20241014215022.68530-1-jeffxu@google.com/
> - Seal uprobe always (Oleg Nesterov)
> - Update comments and description (Randy Dunlap, Liam R.Howlett, Oleg Nesterov)
> - Rebase to linux_main
>
> V1:
> - https://lore.kernel.org/all/20241004163155.3493183-1-jeffxu@google.com/
>
> --------------------------------------------------
>
>
> Jeff Xu (7):
> mseal sysmap: kernel config and header change
> selftests: x86: test_mremap_vdso: skip if vdso is msealed
> mseal sysmap: enable x86-64
> mseal sysmap: enable arm64
> mseal sysmap: uprobe mapping
> mseal sysmap: update mseal.rst
> selftest: test system mappings are sealed.
>
> Documentation/userspace-api/mseal.rst | 20 ++++
> arch/arm64/Kconfig | 1 +
> arch/arm64/kernel/vdso.c | 12 +-
> arch/x86/Kconfig | 1 +
> arch/x86/entry/vdso/vma.c | 7 +-
> include/linux/mm.h | 10 ++
> init/Kconfig | 22 ++++
> kernel/events/uprobes.c | 3 +-
> security/Kconfig | 21 ++++
> .../mseal_system_mappings/.gitignore | 2 +
> .../selftests/mseal_system_mappings/Makefile | 6 +
> .../selftests/mseal_system_mappings/config | 1 +
> .../mseal_system_mappings/sysmap_is_sealed.c | 113 ++++++++++++++++++
> .../testing/selftests/x86/test_mremap_vdso.c | 43 +++++++
> 14 files changed, 254 insertions(+), 8 deletions(-)
> create mode 100644 tools/testing/selftests/mseal_system_mappings/.gitignore
> create mode 100644 tools/testing/selftests/mseal_system_mappings/Makefile
> create mode 100644 tools/testing/selftests/mseal_system_mappings/config
> create mode 100644 tools/testing/selftests/mseal_system_mappings/sysmap_is_sealed.c
>
> --
> 2.48.1.711.g2feabab25a-goog
>