[PATCH 1/7] x86/mm/pti: clear Global bit more aggressively

Dave Hansen
2018-08-02T22:58:39Z

From: Dave Hansen <dave.hansen@linux.intel.com>

The kernel image starts out with the Global bit set across the entire
kernel image. The bit is cleared with set_memory_nonglobal() in the
configurations with PCIDs where we do not need the performance
benefits of the Global bit.

However, this is fragile. It means that we are stuck opting *out* of
the less-secure (Global bit set) configuration, which seems backwards.
Let's start more secure (Global bit clear) and then let things opt
back in if they want performance, or are truly mapping common data
between kernel and userspace.

This fixes a bug. Before this patch, there are areas that are
unmapped from the user page tables (like like everything above
0xffffffff82600000 in the example below). These have the hallmark of
being a wrong Global area: they are no identical in the
'current_kernel' and 'current_user' page table dumps. They are also
read-write, which means they're much more likely to contain secrets.

Before this patch:

current_kernel:---[ High Kernel Mapping ]---
current_kernel-0xffffffff80000000-0xffffffff81000000 16M pmd
current_kernel-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_kernel-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_kernel-0xffffffff81e11000-0xffffffff82000000 1980K RW GLB NX pte
current_kernel-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_kernel-0xffffffff82600000-0xffffffff82c00000 6M RW PSE GLB NX pmd
current_kernel-0xffffffff82c00000-0xffffffff82e00000 2M RW GLB NX pte
current_kernel-0xffffffff82e00000-0xffffffff83200000 4M RW PSE GLB NX pmd
current_kernel-0xffffffff83200000-0xffffffffa0000000 462M pmd

current_user:---[ High Kernel Mapping ]---
current_user-0xffffffff80000000-0xffffffff81000000 16M pmd
current_user-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_user-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_user-0xffffffff81e11000-0xffffffff82000000 1980K RW GLB NX pte
current_user-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_user-0xffffffff82600000-0xffffffffa0000000 474M pmd

After this patch:

current_kernel:---[ High Kernel Mapping ]---
current_kernel-0xffffffff80000000-0xffffffff81000000 16M pmd
current_kernel-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_kernel-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_kernel-0xffffffff81e11000-0xffffffff82000000 1980K RW NX pte
current_kernel-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_kernel-0xffffffff82600000-0xffffffff82c00000 6M RW PSE NX pmd
current_kernel-0xffffffff82c00000-0xffffffff82e00000 2M RW NX pte
current_kernel-0xffffffff82e00000-0xffffffff83200000 4M RW PSE NX pmd
current_kernel-0xffffffff83200000-0xffffffffa0000000 462M pmd

current_user:---[ High Kernel Mapping ]---
current_user-0xffffffff80000000-0xffffffff81000000 16M pmd
current_user-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd
current_user-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte
current_user-0xffffffff81e11000-0xffffffff82000000 1980K RW NX pte
current_user-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd
current_user-0xffffffff82600000-0xffffffffa0000000 474M pmd

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reported-by: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <keescook@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
---

b/arch/x86/mm/pageattr.c | 6 ++++++
b/arch/x86/mm/pti.c | 34 ++++++++++++++++++++++++----------
2 files changed, 30 insertions(+), 10 deletions(-)

diff -puN arch/x86/mm/pageattr.c~pti-non-pcid-clear-more-global-in-kernel-mapping arch/x86/mm/pageattr.c
--- a/arch/x86/mm/pageattr.c~pti-non-pcid-clear-more-global-in-kernel-mapping 2018-08-02 14:42:35.905479118 -0700
+++ b/arch/x86/mm/pageattr.c 2018-08-02 14:42:35.912479118 -0700
@@ -1784,6 +1784,12 @@ int set_memory_nonglobal(unsigned long a
__pgprot(_PAGE_GLOBAL), 0);
}

+int set_memory_global(unsigned long addr, int numpages)
+{
+ return change_page_attr_set(&addr, numpages,
+ __pgprot(_PAGE_GLOBAL), 0);
+}
+
static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
{
struct cpa_data cpa;
diff -puN arch/x86/mm/pti.c~pti-non-pcid-clear-more-global-in-kernel-mapping arch/x86/mm/pti.c
--- a/arch/x86/mm/pti.c~pti-non-pcid-clear-more-global-in-kernel-mapping 2018-08-02 14:42:35.909479118 -0700
+++ b/arch/x86/mm/pti.c 2018-08-02 14:42:35.913479118 -0700
@@ -435,6 +435,13 @@ static inline bool pti_kernel_image_glob
}

/*
+ * This is the only user for these and it is not arch-generic
+ * like the other set_memory.h functions. Just extern them.
+ */
+extern int set_memory_nonglobal(unsigned long addr, int numpages);
+extern int set_memory_global(unsigned long addr, int numpages);
+
+/*
* For some configurations, map all of kernel text into the user page
* tables. This reduces TLB misses, especially on non-PCID systems.
*/
@@ -446,7 +453,8 @@ void pti_clone_kernel_text(void)
* clone the areas past rodata, they might contain secrets.
*/
unsigned long start = PFN_ALIGN(_text);
- unsigned long end = (unsigned long)__end_rodata_hpage_align;
+ unsigned long end_clone = (unsigned long)__end_rodata_hpage_align;
+ unsigned long end_global = PFN_ALIGN((unsigned long)__stop___ex_table);

if (!pti_kernel_image_global_ok())
return;
@@ -458,14 +466,18 @@ void pti_clone_kernel_text(void)
* pti_set_kernel_image_nonglobal() did to clear the
* global bit.
*/
- pti_clone_pmds(start, end, _PAGE_RW);
+ pti_clone_pmds(start, end_clone, _PAGE_RW);
+
+ /*
+ * pti_clone_pmds() will set the global bit in any PMDs
+ * that it clones, but we also need to get any PTEs in
+ * the last level for areas that are not huge-page-aligned.
+ */
+
+ /* Set the global bit for normal non-__init kernel text: */
+ set_memory_global(start, (end_global - start) >> PAGE_SHIFT);
}

-/*
- * This is the only user for it and it is not arch-generic like
- * the other set_memory.h functions. Just extern it.
- */
-extern int set_memory_nonglobal(unsigned long addr, int numpages);
void pti_set_kernel_image_nonglobal(void)
{
/*
@@ -477,9 +489,11 @@ void pti_set_kernel_image_nonglobal(void
unsigned long start = PFN_ALIGN(_text);
unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);

- if (pti_kernel_image_global_ok())
- return;
-
+ /*
+ * This clears _PAGE_GLOBAL from the entire kernel image.
+ * pti_clone_kernel_text() map put _PAGE_GLOBAL back for
+ * areas that are mapped to userspace.
+ */
set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
}

_