Re: [PATCH] module: Taint the kernel when write-protecting ro_after_init fails

Luis Chamberlain
2025-03-06T16:57:26Z

+ linux-mm since we're adding TAINT_BAD_PAGE

On Thu, Mar 06, 2025 at 11:36:55AM +0100, Petr Pavlu wrote:
> In the unlikely case that setting ro_after_init data to read-only fails, it
> is too late to cancel loading of the module. The loader then issues only
> a warning about the situation. Given that this reduces the kernel's
> protection, it was suggested to make the failure more visible by tainting
> the kernel.
>
> Allow TAINT_BAD_PAGE to be set per-module and use it in this case. The flag
> is set in similar situations and has the following description in
> Documentation/admin-guide/tainted-kernels.rst: "bad page referenced or some
> unexpected page flags".
>
> Adjust the warning that reports the failure to avoid references to internal
> functions and to add information about the kernel being tainted, both to
> match the style of other messages in the file. Additionally, merge the
> message on a single line because checkpatch.pl recommends that for the
> ability to grep for the string.
>
> Suggested-by: Kees Cook <kees@kernel.org>
> Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
> ---
> I opted to use TAINT_BAD_PAGE for now because it seemed unnecessary to me
> to introduce a new flag only for this specific case. However, if we end up
> similarly checking set_memory_*() in the boot context, a separate flag
> would be probably better.
> ---
> kernel/module/main.c | 7 ++++---
> kernel/panic.c | 2 +-
> 2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 1fb9ad289a6f..8f424a107b92 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -3030,10 +3030,11 @@ static noinline int do_init_module(struct module *mod)
> rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
> #endif
> ret = module_enable_rodata_ro_after_init(mod);
> - if (ret)
> - pr_warn("%s: module_enable_rodata_ro_after_init() returned %d, "
> - "ro_after_init data might still be writable\n",
> + if (ret) {
> + pr_warn("%s: write-protecting ro_after_init data failed with %d, the data might still be writable - tainting kernel\n",
> mod->name, ret);
> + add_taint_module(mod, TAINT_BAD_PAGE, LOCKDEP_STILL_OK);
> + }
>
> mod_tree_remove_init(mod);
> module_arch_freeing_init(mod);
> diff --git a/kernel/panic.c b/kernel/panic.c
> index d8635d5cecb2..794c443bfb5c 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -497,7 +497,7 @@ const struct taint_flag taint_flags[TAINT_FLAGS_COUNT] = {
> TAINT_FLAG(CPU_OUT_OF_SPEC, 'S', ' ', false),
> TAINT_FLAG(FORCED_RMMOD, 'R', ' ', false),
> TAINT_FLAG(MACHINE_CHECK, 'M', ' ', false),
> - TAINT_FLAG(BAD_PAGE, 'B', ' ', false),
> + TAINT_FLAG(BAD_PAGE, 'B', ' ', true),
> TAINT_FLAG(USER, 'U', ' ', false),
> TAINT_FLAG(DIE, 'D', ' ', false),
> TAINT_FLAG(OVERRIDDEN_ACPI_TABLE, 'A', ' ', false),

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>

For our needs this makes sense, however I am curious if TAINT_BAD_PAGE
is too broadly generic, and also if we're going to add it, if there are
other mm uses for such a thing.

Luis