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x86/mm: Fix _pgd_alloc() for Xen PV mode

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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 replacing PGD_ALLOCATION_ORDER with an
inline helper returning the needed order for PGD allocations.

Fixes: a9b3c355c2e6 ("asm-generic: pgalloc: provide generic __pgd_{alloc,free}")
Reported-by: Petr Vaněk <arkamar@atlas.cz>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Petr Vaněk <arkamar@atlas.cz>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20250422131717.25724-1-jgross%40suse.com
This commit is contained in:
Juergen Gross 2025-04-22 15:17:17 +02:00 committed by Dave Hansen
parent 83b2d345e1
commit 4ce385f564
4 changed files with 17 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
arch/x86/include/asm/pgalloc.h
View File

@ -6,6 +6,8 @@
#include <linux/mm.h> /* for struct page */
#include <linux/pagemap.h>
#include <asm/cpufeature.h>
#define __HAVE_ARCH_PTE_ALLOC_ONE
#define __HAVE_ARCH_PGD_FREE
#include <asm-generic/pgalloc.h>
@ -29,16 +31,17 @@ static inline void paravirt_release_pud(unsigned long pfn) {}
static inline void paravirt_release_p4d(unsigned long pfn) {}
#endif
#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
/*
* 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.
* In case of Page Table Isolation active, we acquire two PGDs instead of one.
* 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.
*/
#define PGD_ALLOCATION_ORDER 1
#else
#define PGD_ALLOCATION_ORDER 0
#endif
static inline unsigned int pgd_allocation_order(void)
{
if (cpu_feature_enabled(X86_FEATURE_PTI))
return 1;
return 0;
}
/*
* Allocate and free page tables.

4
arch/x86/kernel/machine_kexec_32.c
View File

@ -42,7 +42,7 @@ static void load_segments(void)
static void machine_kexec_free_page_tables(struct kimage *image)
{
free_pages((unsigned long)image->arch.pgd, PGD_ALLOCATION_ORDER);
free_pages((unsigned long)image->arch.pgd, pgd_allocation_order());
image->arch.pgd = NULL;
#ifdef CONFIG_X86_PAE
free_page((unsigned long)image->arch.pmd0);
@ -59,7 +59,7 @@ static void machine_kexec_free_page_tables(struct kimage *image)
static int machine_kexec_alloc_page_tables(struct kimage *image)
{
image->arch.pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
PGD_ALLOCATION_ORDER);
pgd_allocation_order());
#ifdef CONFIG_X86_PAE
image->arch.pmd0 = (pmd_t *)get_zeroed_page(GFP_KERNEL);
image->arch.pmd1 = (pmd_t *)get_zeroed_page(GFP_KERNEL);

4
arch/x86/mm/pgtable.c
View File

@ -360,7 +360,7 @@ static inline pgd_t *_pgd_alloc(struct mm_struct *mm)
* We allocate one page for pgd.
*/
if (!SHARED_KERNEL_PMD)
return __pgd_alloc(mm, PGD_ALLOCATION_ORDER);
return __pgd_alloc(mm, pgd_allocation_order());
/*
* Now PAE kernel is not running as a Xen domain. We can allocate
@ -380,7 +380,7 @@ static inline void _pgd_free(struct mm_struct *mm, pgd_t *pgd)
static inline pgd_t *_pgd_alloc(struct mm_struct *mm)
{
return __pgd_alloc(mm, PGD_ALLOCATION_ORDER);
return __pgd_alloc(mm, pgd_allocation_order());
}
static inline void _pgd_free(struct mm_struct *mm, pgd_t *pgd)

4
arch/x86/platform/efi/efi_64.c
View File

@ -73,7 +73,7 @@ int __init efi_alloc_page_tables(void)
gfp_t gfp_mask;
gfp_mask = GFP_KERNEL | __GFP_ZERO;
efi_pgd = (pgd_t *)__get_free_pages(gfp_mask, PGD_ALLOCATION_ORDER);
efi_pgd = (pgd_t *)__get_free_pages(gfp_mask, pgd_allocation_order());
if (!efi_pgd)
goto fail;
@ -96,7 +96,7 @@ free_p4d:
if (pgtable_l5_enabled())
free_page((unsigned long)pgd_page_vaddr(*pgd));
free_pgd:
free_pages((unsigned long)efi_pgd, PGD_ALLOCATION_ORDER);
free_pages((unsigned long)efi_pgd, pgd_allocation_order());
fail:
return -ENOMEM;
}