Re: [PATCH 0/4] Fix misused kernel_read_file() enums

Kees Cook
2020-07-07T21:45:17Z

On Tue, Jul 07, 2020 at 11:36:04AM -0400, Mimi Zohar wrote:
> Hi Kees,
>
> On Tue, 2020-07-07 at 01:19 -0700, Kees Cook wrote:
> > Hi,
> >
> > In looking for closely at the additions that got made to the
> > kernel_read_file() enums, I noticed that FIRMWARE_PREALLOC_BUFFER
> > and FIRMWARE_EFI_EMBEDDED were added, but they are not appropriate
> > *kinds* of files for the LSM to reason about. They are a "how" and
> > "where", respectively. Remove these improper aliases and refactor the
> > code to adapt to the changes.
>
> Thank you for adding the missing calls and the firmware pre allocated
> buffer comment update.
>
> >
> > Additionally adds in missing calls to security_kernel_post_read_file()
> > in the platform firmware fallback path (to match the sysfs firmware
> > fallback path) and in module loading. I considered entirely removing
> > security_kernel_post_read_file() hook since it is technically unused,
> > but IMA probably wants to be able to measure EFI-stored firmware images,
> > so I wired it up and matched it for modules, in case anyone wants to
> > move the module signature checks out of the module core and into an LSM
> > to avoid the current layering violations.
>
> IMa has always verified kernel module signatures.  Recently appended

Right, but not through the kernel_post_read_file() hook, nor via
out-of-band hooks in kernel/module.c. I was just meaning that future
work could be done here to regularize module_sig_check() into an actual
LSM (which could, in theory, be extended to kexec() to avoid code
duplication there, as kimage_validate_signature() has some overlap with
mod_verify_sig()). into a bit more normal of an LSM.

As far as IMA and regularizing things, though, what about fixing IMA to
not manually stack:

$ grep -B3 ima_ security/security.c
ret = call_int_hook(bprm_check_security, 0, bprm);
if (ret)
return ret;
return ima_bprm_check(bprm);
--
ret = call_int_hook(file_mprotect, 0, vma, reqprot, prot);
if (ret)
return ret;
return ima_file_mprotect(vma, prot);
...

Can IMA implement a hook-last method to join the regular stacking routines?

> kernel module signature support was added to IMA.  The same appended
> signature format is also being used to sign and verify the kexec
> kernel image.
>
> With IMA's new kernel module appended signature support and patch 4/4
> in this series, IMA won't be limit to the finit_module syscall, but
> could support the init_module syscall as well.

Exactly.

> > This touches several trees, and I suspect it would be best to go through
> > James's LSM tree.
>
> Sure.

Is this an "Acked-by"? :)

--
Kees Cook