[tip:x86/urgent] x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems

[tip-bot for Andy Lutomirski]

Commit-ID:  5beda7d54eafece4c974cfa9fbb9f60fb18fd20a
Gitweb:     https://git.kernel.org/tip/5beda7d54eafece4c974cfa9fbb9f60fb18fd20a
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Thu, 25 Jan 2018 13:12:14 -0800
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 26 Jan 2018 15:56:23 +0100

x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems

Neil Berrington reported a double-fault on a VM with 768GB of RAM that uses
large amounts of vmalloc space with PTI enabled.

The cause is that load_new_mm_cr3() was never fixed to take the 5-level pgd
folding code into account, so, on a 4-level kernel, the pgd synchronization
logic compiles away to exactly nothing.

Interestingly, the problem doesn't trigger with nopti.  I assume this is
because the kernel is mapped with global pages if we boot with nopti.  The
sequence of operations when we create a new task is that we first load its
mm while still running on the old stack (which crashes if the old stack is
unmapped in the new mm unless the TLB saves us), then we call
prepare_switch_to(), and then we switch to the new stack.
prepare_switch_to() pokes the new stack directly, which will populate the
mapping through vmalloc_fault().  I assume that we're getting lucky on
non-PTI systems -- the old stack's TLB entry stays alive long enough to
make it all the way through prepare_switch_to() and switch_to() so that we
make it to a valid stack.

Fixes: b50858ce3e2a ("x86/mm/vmalloc: Add 5-level paging support")
Reported-and-tested-by: Neil Berrington <neil.berrington@xxxxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Link: https://lkml.kernel.org/r/346541c56caed61abbe693d7d2742b4a380c5001.1516914529.git.luto@xxxxxxxxxx

---
 arch/x86/mm/tlb.c | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index a156195..5bfe61a 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -151,6 +151,34 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	local_irq_restore(flags);
 }
 
+static void sync_current_stack_to_mm(struct mm_struct *mm)
+{
+	unsigned long sp = current_stack_pointer;
+	pgd_t *pgd = pgd_offset(mm, sp);
+
+	if (CONFIG_PGTABLE_LEVELS > 4) {
+		if (unlikely(pgd_none(*pgd))) {
+			pgd_t *pgd_ref = pgd_offset_k(sp);
+
+			set_pgd(pgd, *pgd_ref);
+		}
+	} else {
+		/*
+		 * "pgd" is faked.  The top level entries are "p4d"s, so sync
+		 * the p4d.  This compiles to approximately the same code as
+		 * the 5-level case.
+		 */
+		p4d_t *p4d = p4d_offset(pgd, sp);
+
+		if (unlikely(p4d_none(*p4d))) {
+			pgd_t *pgd_ref = pgd_offset_k(sp);
+			p4d_t *p4d_ref = p4d_offset(pgd_ref, sp);
+
+			set_p4d(p4d, *p4d_ref);
+		}
+	}
+}
+
 void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 			struct task_struct *tsk)
 {
@@ -226,11 +254,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 			 * mapped in the new pgd, we'll double-fault.  Forcibly
 			 * map it.
 			 */
-			unsigned int index = pgd_index(current_stack_pointer);
-			pgd_t *pgd = next->pgd + index;
-
-			if (unlikely(pgd_none(*pgd)))
-				set_pgd(pgd, init_mm.pgd[index]);
+			sync_current_stack_to_mm(next);
 		}
 
 		/* Stop remote flushes for the previous mm */