[Xen-devel] [PATCH AUTOSEL 4.19 30/97] x86/mm: Fix guard hole handling

[Sasha Levin <sashal@xxxxxxxxxx>]

From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>

[ Upstream commit 16877a5570e0c5f4270d5b17f9bab427bcae9514 ]

There is a guard hole at the beginning of the kernel address space, also
used by hypervisors. It occupies 16 PGD entries.

This reserved range is not defined explicitely, it is calculated relative
to other entities: direct mapping and user space ranges.

The calculation got broken by recent changes of the kernel memory layout:
LDT remap range is now mapped before direct mapping and makes the
calculation invalid.

The breakage leads to crash on Xen dom0 boot[1].

Define the reserved range explicitely. It's part of kernel ABI (hypervisors
expect it to be stable) and must not depend on changes in the rest of
kernel memory layout.

[1] https://lists.xenproject.org/archives/html/xen-devel/2018-11/msg03313.html

Fixes: d52888aa2753 ("x86/mm: Move LDT remap out of KASLR region on 5-level 
paging")
Reported-by: Hans van Kranenburg <hans.van.kranenburg@xxxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Hans van Kranenburg <hans.van.kranenburg@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Cc: bp@xxxxxxxxx
Cc: hpa@xxxxxxxxx
Cc: dave.hansen@xxxxxxxxxxxxxxx
Cc: luto@xxxxxxxxxx
Cc: peterz@xxxxxxxxxxxxx
Cc: boris.ostrovsky@xxxxxxxxxx
Cc: bhe@xxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Link: 
https://lkml.kernel.org/r/20181130202328.65359-2-kirill.shutemov@xxxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/include/asm/pgtable_64_types.h |  5 +++++
 arch/x86/mm/dump_pagetables.c           |  8 ++++----
 arch/x86/xen/mmu_pv.c                   | 11 ++++++-----
 3 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/pgtable_64_types.h 
b/arch/x86/include/asm/pgtable_64_types.h
index 84bd9bdc1987..88bca456da99 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -111,6 +111,11 @@ extern unsigned int ptrs_per_p4d;
  */
 #define MAXMEM                 (1UL << MAX_PHYSMEM_BITS)
 
+#define GUARD_HOLE_PGD_ENTRY   -256UL
+#define GUARD_HOLE_SIZE                (16UL << PGDIR_SHIFT)
+#define GUARD_HOLE_BASE_ADDR   (GUARD_HOLE_PGD_ENTRY << PGDIR_SHIFT)
+#define GUARD_HOLE_END_ADDR    (GUARD_HOLE_BASE_ADDR + GUARD_HOLE_SIZE)
+
 #define LDT_PGD_ENTRY          -240UL
 #define LDT_BASE_ADDR          (LDT_PGD_ENTRY << PGDIR_SHIFT)
 #define LDT_END_ADDR           (LDT_BASE_ADDR + PGDIR_SIZE)
diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index a12afff146d1..073755c89126 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -493,11 +493,11 @@ static inline bool is_hypervisor_range(int idx)
 {
 #ifdef CONFIG_X86_64
        /*
-        * ffff800000000000 - ffff87ffffffffff is reserved for
-        * the hypervisor.
+        * A hole in the beginning of kernel address space reserved
+        * for a hypervisor.
         */
-       return  (idx >= pgd_index(__PAGE_OFFSET) - 16) &&
-               (idx <  pgd_index(__PAGE_OFFSET));
+       return  (idx >= pgd_index(GUARD_HOLE_BASE_ADDR)) &&
+               (idx <  pgd_index(GUARD_HOLE_END_ADDR));
 #else
        return false;
 #endif
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 2c84c6ad8b50..c8f011e07a15 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -640,19 +640,20 @@ static int __xen_pgd_walk(struct mm_struct *mm, pgd_t 
*pgd,
                          unsigned long limit)
 {
        int i, nr, flush = 0;
-       unsigned hole_low, hole_high;
+       unsigned hole_low = 0, hole_high = 0;
 
        /* The limit is the last byte to be touched */
        limit--;
        BUG_ON(limit >= FIXADDR_TOP);
 
+#ifdef CONFIG_X86_64
        /*
         * 64-bit has a great big hole in the middle of the address
-        * space, which contains the Xen mappings.  On 32-bit these
-        * will end up making a zero-sized hole and so is a no-op.
+        * space, which contains the Xen mappings.
         */
-       hole_low = pgd_index(USER_LIMIT);
-       hole_high = pgd_index(PAGE_OFFSET);
+       hole_low = pgd_index(GUARD_HOLE_BASE_ADDR);
+       hole_high = pgd_index(GUARD_HOLE_END_ADDR);
+#endif
 
        nr = pgd_index(limit) + 1;
        for (i = 0; i < nr; i++) {
-- 
2.19.1


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