[PATCH] x86/mm: fix _pgd_alloc() for Xen PV mode

[PATCH] x86/mm: fix _pgd_alloc() for Xen PV mode

[Juergen Gross]

Recently _pgd_alloc() was switched from using __get_free_pages() to
pagetable_alloc_noprof(), which might return a compound page in case
the allocation order is larger than 0.

On x86 this will be the case if CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
is set, even if PTI has been disabled at runtime.

When running as a Xen PV guest (this will always disable PTI), using
a compound page for a PGD will result in VM_BUG_ON_PGFLAGS being
triggered when the Xen code tries to pin the PGD.

Fix the Xen issue together with the not needed 8k allocation for a
PGD with PTI disabled by using a variable holding the PGD allocation
order in case CONFIG_MITIGATION_PAGE_TABLE_ISOLATION is set.

Reported-by: Petr Vaněk <arkamar@atlas.cz>
Fixes: a9b3c355c2e6 ("asm-generic: pgalloc: provide generic __pgd_{alloc,free}")
Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/include/asm/pgalloc.h | 7 ++++++-
 arch/x86/mm/pgtable.c          | 4 ++++
 arch/x86/mm/pti.c              | 3 +++
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgalloc.h b/arch/x86/include/asm/pgalloc.h
index a33147520044..754f95bddf98 100644
--- a/arch/x86/include/asm/pgalloc.h
+++ b/arch/x86/include/asm/pgalloc.h
@@ -34,8 +34,13 @@ static inline void paravirt_release_p4d(unsigned long pfn) {}
  * Instead of one PGD, we acquire two PGDs.  Being order-1, it is
  * both 8k in size and 8k-aligned.  That lets us just flip bit 12
  * in a pointer to swap between the two 4k halves.
+ *
+ * As PTI can be runtime disabled (either via boot parameter or due to
+ * running as a Xen PV guest), store the actually needed allocation
+ * order in a global variable.
  */
-#define PGD_ALLOCATION_ORDER 1
+#define PGD_ALLOCATION_ORDER pgd_allocation_order
+extern unsigned int pgd_allocation_order;
 #else
 #define PGD_ALLOCATION_ORDER 0
 #endif
diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index a05fcddfc811..f61b2d6be311 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -12,6 +12,10 @@ phys_addr_t physical_mask __ro_after_init = (1ULL << __PHYSICAL_MASK_SHIFT) - 1;
 EXPORT_SYMBOL(physical_mask);
 #endif
 
+#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
+unsigned int pgd_allocation_order = 0;
+#endif
+
 pgtable_t pte_alloc_one(struct mm_struct *mm)
 {
 	return __pte_alloc_one(mm, GFP_PGTABLE_USER);
diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 5f0d579932c6..44b7120c63e3 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -38,6 +38,7 @@
 #include <asm/desc.h>
 #include <asm/sections.h>
 #include <asm/set_memory.h>
+#include <asm/pgalloc.h>
 
 #undef pr_fmt
 #define pr_fmt(fmt)     "Kernel/User page tables isolation: " fmt
@@ -97,6 +98,8 @@ void __init pti_check_boottime_disable(void)
 	if (pti_mode == PTI_AUTO && !boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
 		return;
 
+	pgd_allocation_order = 1;
+
 	setup_force_cpu_cap(X86_FEATURE_PTI);
 }
 
-- 
2.43.0

Re: [PATCH] x86/mm: fix _pgd_alloc() for Xen PV mode

[Dave Hansen]

On 4/17/25 07:48, Juergen Gross wrote:
> -#define PGD_ALLOCATION_ORDER 1
> +#define PGD_ALLOCATION_ORDER pgd_allocation_order
> +extern unsigned int pgd_allocation_order;
>  #else
>  #define PGD_ALLOCATION_ORDER 0
>  #endif

Instead of hiding a variable behind a macro-looking name and a bunch of
#ifdefs, can we please fix this properly?

static inline pgd_allocation_order(void)
{
	if (cpu_feature_enabled(X86_FEATURE_PTI))
		return 1;
	return 0;
}

and then s/PGD_ALLOCATION_ORDER/pgd_allocation_order()/.

Wouldn't that be a billion times better?

Re: [PATCH] x86/mm: fix _pgd_alloc() for Xen PV mode

[Petr Vaněk]

On Thu, Apr 17, 2025 at 04:48:08PM +0200, Juergen Gross wrote:
> Recently _pgd_alloc() was switched from using __get_free_pages() to
> pagetable_alloc_noprof(), which might return a compound page in case
> the allocation order is larger than 0.
> 
> On x86 this will be the case if CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
> is set, even if PTI has been disabled at runtime.
> 
> When running as a Xen PV guest (this will always disable PTI), using
> a compound page for a PGD will result in VM_BUG_ON_PGFLAGS being
> triggered when the Xen code tries to pin the PGD.

> Fix the Xen issue together with the not needed 8k allocation for a
> PGD with PTI disabled by using a variable holding the PGD allocation
> order in case CONFIG_MITIGATION_PAGE_TABLE_ISOLATION is set.
> 
> Reported-by: Petr Vaněk <arkamar@atlas.cz>
> Fixes: a9b3c355c2e6 ("asm-generic: pgalloc: provide generic __pgd_{alloc,free}")
> Cc: stable@vger.kernel.org
> Signed-off-by: Juergen Gross <jgross@suse.com>

I have runtime tested this patch, and it fixes the reported issue. The
following trailers can be appended to the commit message (as per [1]):

Closes: https://lore.kernel.org/lkml/202541612720-Z_-deOZTOztMXHBh-arkamar@atlas.cz/
Tested-by: Petr Vaněk <arkamar@atlas.cz>

Cheers,
Petr

[1] https://docs.kernel.org/process/5.Posting.html#patch-formatting-and-changelogs