Hi Liam, On Mon, Oct 7, 2024 at 7:19 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > * jeffxu@xxxxxxxxxxxx <jeffxu@xxxxxxxxxxxx> [241004 12:32]: > > From: Jeff Xu <jeffxu@xxxxxxxxxx> > > > > Seal vdso, vvar, sigpage, uprobes and vsyscall. > > > > Those mappings are readonly or executable only, sealing can protect > > them from ever changing during the life time of the process. > > > > System mappings such as vdso, vvar, and sigpage (for arm) are > > generated by the kernel during program initialization. These mappings > > are designated as non-writable, and sealing them will prevent them > > from ever becoming writeable. > > But it also means they cannot be unmapped, right? > > I'm not saying it's a thing people should, but recent conversations > with the ppc people seem to indicate that people do 'things' to the vdso > such as removing it. > > Won't this change mean they cannot do that, at least if mseal is enabled > on ppc64? In which case we would have a different special mapping for > powerpc, or any other platform that wants to be able to unmap the vdso > (or vvar or whatever else?) > > In fact, I came across people removing the vdso to catch callers to > those functions which they didn't want to allow. In this case enabling > the security of mseal would not allow them to stop applications from > vdso calls. Again, I'm not saying this is a good (or bad) idea but it > happening. > > > > > Unlike the aforementioned mappings, the uprobe mapping is not > > established during program startup. However, its lifetime is the same > > as the process's lifetime [1], thus sealable. > > > > The vdso, vvar, sigpage, and uprobe mappings all invoke the > > _install_special_mapping() function. As no other mappings utilize this > > function, it is logical to incorporate sealing logic within > > _install_special_mapping(). This approach avoids the necessity of > > modifying code across various architecture-specific implementations. > > > > The vsyscall mapping, which has its own initialization function, is > > sealed in the XONLY case, it seems to be the most common and secure > > case of using vsyscall. > > > > It is important to note that the CHECKPOINT_RESTORE feature (CRIU) may > > alter the mapping of vdso, vvar, and sigpage during restore > > operations. Consequently, this feature cannot be universally enabled > > across all systems. To address this, a kernel configuration option has > > been introduced to enable or disable this functionality. I tested > > CONFIG_SEAL_SYSTEM_MAPPINGS_ALWAYS with ChromeOS, which doesn’t use > > CHECKPOINT_RESTORE, to verify the sealing works. > > I am hesitant to say that CRIU is the only user of moving the vdso, as > the ppc people wanted the ability for the fallback methods to still > function when the vdso was unmapped. > > I am not sure we can change the user expected behaviour based on a > configuration option; users may be able to mmap/munmap but may not be > able to boot their own kernel, but maybe it's okay? > The text doesn't say CRIU is the **only** feature that is not compatible with this. The default config is "CONFIG_SEAL_SYSTEM_MAPPINGS_NEVER", and distribution needs to opt-in for this feature, such as ChromeOS or Android or other safe-by-default systems that doesn't allow to unmap or remap vdso in production build. Thanks -Jeff > > > > [1] https://lore.kernel.org/all/CABi2SkU9BRUnqf70-nksuMCQ+yyiWjo3fM4XkRkL-NrCZxYAyg@xxxxxxxxxxxxxx/ > > > > Jeff Xu (1): > > exec: seal system mappings > > > > .../admin-guide/kernel-parameters.txt | 9 ++++ > > arch/x86/entry/vsyscall/vsyscall_64.c | 9 +++- > > fs/exec.c | 53 +++++++++++++++++++ > > include/linux/fs.h | 1 + > > mm/mmap.c | 1 + > > security/Kconfig | 26 +++++++++ > > 6 files changed, 97 insertions(+), 2 deletions(-) > > > > -- > > 2.47.0.rc0.187.ge670bccf7e-goog > >
