Re: [tip: x86/urgent] x86/mm: Fix pti_clone_entry_text() for i386

Guenter Roeck
2024-08-06T19:12:56Z

On 8/6/24 11:48, Peter Zijlstra wrote:
> On Tue, Aug 06, 2024 at 09:42:23AM -0700, Guenter Roeck wrote:
>> On 8/6/24 08:59, Peter Zijlstra wrote:
>>> On Tue, Aug 06, 2024 at 05:46:53PM +0200, Peter Zijlstra wrote:
>>>> On Tue, Aug 06, 2024 at 05:05:15PM +0200, Peter Zijlstra wrote:
>>>>> On Tue, Aug 06, 2024 at 04:56:32PM +0200, Peter Zijlstra wrote:
>>>>>> On Tue, Aug 06, 2024 at 07:25:42AM -0700, Guenter Roeck wrote:
>>>>>>
>>>>>>> I created http://server.roeck-us.net/qemu/x86-v6.11-rc2/ with all
>>>>>>> the relevant information. Please let me know if you need anything else.
>>>>>>
>>>>>> So I grabbed that config, stuck it in the build dir I used last time and
>>>>>> upgraded gcc-13 from 13.2 ro 13.3. But alas, my build runs successfully
>>>>>> :/
>>>>>>
>>>>>> Is there anything else special I missed?
>>>>>
>>>>> run.sh is not exacrlty the same this time, different CPU model, that
>>>>> made it go.
>>>>>
>>>>> OK, lemme poke at this.
>>>>
>>>> Urgh, so crypto's late_initcall() does user-mode-helper based modprobe
>>>> looking for algorithms before we kick off /bin/init :/
>>>>
>>>> This makes things difficult.
>>>>
>>>> Urgh.
>>>
>>> So the problem is that mark_readonly() splits a code PMD due to NX. Then
>>> the second pti_clone_entry_text() finds a kernel PTE but a user PMD
>>> mapping for the same address (from the early clone) and gets upset.
>>>
>>> And we can't run mark_readonly() sooner, because initcall expect stuff
>>> to be RW. But initcalls do modprobe, which runs user crap before we're
>>> done initializing everything.
>>>
>>> This is a right mess, and I really don't know what to do.
>>
>> And there was me thinking this one should be easy to solve. Oh well.
>>
>> Maybe Linus has an idea ? I am getting a bit wary to reporting all those
>> weird problems to him, though.
>
> While I had dinner with the family, Thomas cooked up the below, which
> seems to make it happy.
>

I'll give it a try for all emulations (both x86 and x86_64), just to be sure,
but it does indeed fix the problem for the test with the affected CPU/build.

Thanks!

Guenter

> It basically splits the PMD on the late copy.
>
> ---
> diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
> index bfdf5f45b137..b6ebbc9c23b1 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
> @@ -639,7 +644,7 @@ 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();
> + pti_clone_entry_text(false);
> pti_setup_espfix64();
> pti_setup_vsyscall();
> }
> @@ -659,7 +664,7 @@ void pti_finalize(void)
> * We need to clone everything (again) that maps parts of the
> * kernel image.
> */
> - pti_clone_entry_text();
> + pti_clone_entry_text(true);
> pti_clone_kernel_text();
>
> debug_checkwx_user();