[tip: x86/urgent] x86/mm: Fix PTI for i386 some more

tip-bot2 for Thomas Gleixner
2024-08-07T13:51:30Z

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: c48b5a4cf3125adb679e28ef093f66ff81368d05
Gitweb: https://git.kernel.org/tip/c48b5a4cf3125adb679e28ef093f66ff81368d05
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 06 Aug 2024 20:48:43 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 07 Aug 2024 15:35:01 +02:00

x86/mm: Fix PTI for i386 some more

So it turns out that we have to do two passes of
pti_clone_entry_text(), once before initcalls, such that device and
late initcalls can use user-mode-helper / modprobe and once after
free_initmem() / mark_readonly().

Now obviously mark_readonly() can cause PMD splits, and
pti_clone_pgtable() doesn't like that much.

Allow the late clone to split PMDs so that pagetables stay in sync.

[peterz: Changelog and comments]
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lkml.kernel.org/r/20240806184843.GX37996@noisy.programming.kicks-ass.net
---
arch/x86/mm/pti.c | 45 +++++++++++++++++++++++++++++----------------
1 file changed, 29 insertions(+), 16 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index bfdf5f4..851ec8f 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -241,7 +241,7 @@ static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
*
* Returns a pointer to a PTE on success, or NULL on failure.
*/
-static pte_t *pti_user_pagetable_walk_pte(unsigned long address)
+static pte_t *pti_user_pagetable_walk_pte(unsigned long address, bool late_text)
{
gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
pmd_t *pmd;
@@ -251,10 +251,15 @@ static pte_t *pti_user_pagetable_walk_pte(unsigned long address)
if (!pmd)
return NULL;

- /* We can't do anything sensible if we hit a large mapping. */
+ /* Large PMD mapping found */
if (pmd_leaf(*pmd)) {
- WARN_ON(1);
- return NULL;
+ /* Clear the PMD if we hit a large mapping from the first round */
+ if (late_text) {
+ set_pmd(pmd, __pmd(0));
+ } else {
+ WARN_ON_ONCE(1);
+ return NULL;
+ }
}

if (pmd_none(*pmd)) {
@@ -283,7 +288,7 @@ static void __init pti_setup_vsyscall(void)
if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
return;

- target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
+ target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR, false);
if (WARN_ON(!target_pte))
return;

@@ -301,7 +306,7 @@ enum pti_clone_level {

static void
pti_clone_pgtable(unsigned long start, unsigned long end,
- enum pti_clone_level level)
+ enum pti_clone_level level, bool late_text)
{
unsigned long addr;

@@ -390,7 +395,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
return;

/* Allocate PTE in the user page-table */
- target_pte = pti_user_pagetable_walk_pte(addr);
+ target_pte = pti_user_pagetable_walk_pte(addr, late_text);
if (WARN_ON(!target_pte))
return;

@@ -452,7 +457,7 @@ static void __init pti_clone_user_shared(void)
phys_addr_t pa = per_cpu_ptr_to_phys((void *)va);
pte_t *target_pte;

- target_pte = pti_user_pagetable_walk_pte(va);
+ target_pte = pti_user_pagetable_walk_pte(va, false);
if (WARN_ON(!target_pte))
return;

@@ -475,7 +480,7 @@ static void __init pti_clone_user_shared(void)
start = CPU_ENTRY_AREA_BASE;
end = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);

- pti_clone_pgtable(start, end, PTI_CLONE_PMD);
+ pti_clone_pgtable(start, end, PTI_CLONE_PMD, false);
}
#endif /* CONFIG_X86_64 */

@@ -492,11 +497,11 @@ static void __init pti_setup_espfix64(void)
/*
* Clone the populated PMDs of the entry text and force it RO.
*/
-static void pti_clone_entry_text(void)
+static void pti_clone_entry_text(bool late)
{
pti_clone_pgtable((unsigned long) __entry_text_start,
(unsigned long) __entry_text_end,
- PTI_LEVEL_KERNEL_IMAGE);
+ PTI_LEVEL_KERNEL_IMAGE, late);
}

/*
@@ -571,7 +576,7 @@ static void pti_clone_kernel_text(void)
* pti_set_kernel_image_nonglobal() did to clear the
* global bit.
*/
- pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE);
+ pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE, false);

/*
* pti_clone_pgtable() will set the global bit in any PMDs
@@ -638,8 +643,15 @@ void __init pti_init(void)

/* Undo all global bits from the init pagetables in head_64.S: */
pti_set_kernel_image_nonglobal();
+
/* Replace some of the global bits just for shared entry text: */
- pti_clone_entry_text();
+ /*
+ * This is very early in boot. Device and Late initcalls can do
+ * modprobe before free_initmem() and mark_readonly(). This
+ * pti_clone_entry_text() allows those user-mode-helpers to function,
+ * but notably the text is still RW.
+ */
+ pti_clone_entry_text(false);
pti_setup_espfix64();
pti_setup_vsyscall();
}
@@ -656,10 +668,11 @@ void pti_finalize(void)
if (!boot_cpu_has(X86_FEATURE_PTI))
return;
/*
- * We need to clone everything (again) that maps parts of the
- * kernel image.
+ * This is after free_initmem() (all initcalls are done) and we've done
+ * mark_readonly(). Text is now NX which might've split some PMDs
+ * relative to the early clone.
*/
- pti_clone_entry_text();
+ pti_clone_entry_text(true);
pti_clone_kernel_text();

debug_checkwx_user();