This is a note to let you know that I've just added the patch titled

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

to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: x86-mm-64-fix-vmapped-stack-syncing-on-very-large-memory-4-level-systems.patch and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From 5beda7d54eafece4c974cfa9fbb9f60fb18fd20a Mon Sep 17 00:00:00 2001

From: Andy Lutomirski luto@kernel.org Date: Thu, 25 Jan 2018 13:12:14 -0800 Subject: x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems

From: Andy Lutomirski luto@kernel.org

commit 5beda7d54eafece4c974cfa9fbb9f60fb18fd20a upstream.

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@datacore.com Signed-off-by: Andy Lutomirski luto@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: Konstantin Khlebnikov khlebnikov@yandex-team.ru Cc: Dave Hansen dave.hansen@intel.com Cc: Borislav Petkov bp@alien8.de Link: https://lkml.kernel.org/r/346541c56caed61abbe693d7d2742b4a380c5001.151691452... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

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

--- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -151,6 +151,34 @@ void switch_mm(struct mm_struct *prev, s 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 * 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 */

Patches currently in stable-queue which might be from luto@kernel.org are

queue-4.14/x86-mm-64-fix-vmapped-stack-syncing-on-very-large-memory-4-level-systems.patch