Re: regression in 6.6.46; arch/x86/mm/pti.c

[Jamie Heilman <jamie@audible.transient.net>]

Thomas Gleixner wrote:
> On Mon, Sep 09 2024 at 05:03, Jamie Heilman wrote:
> > 3db03fb4995e ("x86/mm: Fix pti_clone_entry_text() for i386") which got
> > landed in 6.6.46, has introduced two back to back warnings on boot on
> > my 32bit system (found on 6.6.50):
> 
> Right.
> 
> > Reverting that commit removes the warnings (tested against 6.6.50).
> > The follow-on commit of c48b5a4cf312 ("x86/mm: Fix PTI for i386 some
> > more") doesn't apply cleanly to 6.6.50, but I did try out a build of
> > 6.11-rc7 and that works fine too with no warnings on boot.
> 
> See backport below.

Yep, that tests out fine for me too.  Thanks!

> ---
> From: Thomas Gleixner <tglx@linutronix.de>
> Date:   Tue Aug 6 20:48:43 2024 +0200
> Subject: [PATCH 6.6.y, 6.1.y, 5.10.y] x86/mm: Fix PTI for i386 some more
> 
> commit c48b5a4cf3125adb679e28ef093f66ff81368d05 upstream.
> 
> 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>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> 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(-)
> 
> --- a/arch/x86/mm/pti.c
> +++ b/arch/x86/mm/pti.c
> @@ -241,7 +241,7 @@ static pmd_t *pti_user_pagetable_walk_pm
>   *
>   * 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_pt
>  	if (!pmd)
>  		return NULL;
>  
> -	/* We can't do anything sensible if we hit a large mapping. */
> +	/* Large PMD mapping found */
>  	if (pmd_large(*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(vo
>  	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, u
>  				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;
>  
> @@ -453,7 +458,7 @@ static void __init pti_clone_user_shared
>  		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;
>  
> @@ -476,7 +481,7 @@ static void __init pti_clone_user_shared
>  	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 */
>  
> @@ -493,11 +498,11 @@ static void __init pti_setup_espfix64(vo
>  /*
>   * 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);
>  }
>  
>  /*
> @@ -572,7 +577,7 @@ 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
> @@ -639,8 +644,15 @@ 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();
>  }
> @@ -657,10 +669,11 @@ 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();
> 

-- 
Jamie Heilman                     http://audible.transient.net/~jamie/