From: Dave Hansen <dave.han...@linux.intel.com> __ro_after_init data gets stuck in the .rodata section. That's normally fine because the kernel itself manages the R/W properties.
But, if we run __change_page_attr() on an area which is __ro_after_init, the .rodata checks will trigger and force the area to be immediately read-only, even if it is early-ish in boot. This caused problems when trying to clear the _PAGE_GLOBAL bit for these area in the PTI code: it cleared _PAGE_GLOBAL like I asked, but also took it up on itself to clear _PAGE_RW. The kernel then oopses the next time it wrote to a __ro_after_init data structure. To fix this, add the kernel_set_to_readonly check, just like we have for kernel text, just a few lines below in this function. Signed-off-by: Dave Hansen <dave.han...@linux.intel.com> Acked-by: Kees Cook <keesc...@chromium.org> Cc: Andrea Arcangeli <aarca...@redhat.com> Cc: Andy Lutomirski <l...@kernel.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Hugh Dickins <hu...@google.com> Cc: Juergen Gross <jgr...@suse.com> Cc: x...@kernel.org Cc: Nadav Amit <na...@vmware.com> --- b/arch/x86/mm/pageattr.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff -puN arch/x86/mm/pageattr.c~check-kernel_set_to_readonly arch/x86/mm/pageattr.c --- a/arch/x86/mm/pageattr.c~check-kernel_set_to_readonly 2018-04-06 10:47:57.711796120 -0700 +++ b/arch/x86/mm/pageattr.c 2018-04-06 10:47:57.714796120 -0700 @@ -298,9 +298,11 @@ static inline pgprot_t static_protection /* * The .rodata section needs to be read-only. Using the pfn - * catches all aliases. + * catches all aliases. This also includes __ro_after_init, + * so do not enforce until kernel_set_to_readonly is true. */ - if (within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT, + if (kernel_set_to_readonly && + within(pfn, __pa_symbol(__start_rodata) >> PAGE_SHIFT, __pa_symbol(__end_rodata) >> PAGE_SHIFT)) pgprot_val(forbidden) |= _PAGE_RW; _