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Subject[PATCH 4.14 15/74] x86/ldt: Rework locking
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@infradead.org>

commit c2b3496bb30bd159e9de42e5c952e1f1f33c9a77 upstream.

The LDT is duplicated on fork() and on exec(), which is wrong as exec()
should start from a clean state, i.e. without LDT. To fix this the LDT
duplication code will be moved into arch_dup_mmap() which is only called
for fork().

This introduces a locking problem. arch_dup_mmap() holds mmap_sem of the
parent process, but the LDT duplication code needs to acquire
mm->context.lock to access the LDT data safely, which is the reverse lock
order of write_ldt() where mmap_sem nests into context.lock.

Solve this by introducing a new rw semaphore which serializes the
read/write_ldt() syscall operations and use context.lock to protect the
actual installment of the LDT descriptor.

So context.lock stabilizes mm->context.ldt and can nest inside of the new
semaphore or mmap_sem.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: dan.j.williams@intel.com
Cc: hughd@google.com
Cc: keescook@google.com
Cc: kirill.shutemov@linux.intel.com
Cc: linux-mm@kvack.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/include/asm/mmu.h | 4 +++-
arch/x86/include/asm/mmu_context.h | 2 ++
arch/x86/kernel/ldt.c | 33 +++++++++++++++++++++------------
3 files changed, 26 insertions(+), 13 deletions(-)

--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -3,6 +3,7 @@
#define _ASM_X86_MMU_H

#include <linux/spinlock.h>
+#include <linux/rwsem.h>
#include <linux/mutex.h>
#include <linux/atomic.h>

@@ -27,7 +28,8 @@ typedef struct {
atomic64_t tlb_gen;

#ifdef CONFIG_MODIFY_LDT_SYSCALL
- struct ldt_struct *ldt;
+ struct rw_semaphore ldt_usr_sem;
+ struct ldt_struct *ldt;
#endif

#ifdef CONFIG_X86_64
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -132,6 +132,8 @@ void enter_lazy_tlb(struct mm_struct *mm
static inline int init_new_context(struct task_struct *tsk,
struct mm_struct *mm)
{
+ mutex_init(&mm->context.lock);
+
mm->context.ctx_id = atomic64_inc_return(&last_mm_ctx_id);
atomic64_set(&mm->context.tlb_gen, 0);

--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -5,6 +5,11 @@
* Copyright (C) 2002 Andi Kleen
*
* This handles calls from both 32bit and 64bit mode.
+ *
+ * Lock order:
+ * contex.ldt_usr_sem
+ * mmap_sem
+ * context.lock
*/

#include <linux/errno.h>
@@ -42,7 +47,7 @@ static void refresh_ldt_segments(void)
#endif
}

-/* context.lock is held for us, so we don't need any locking. */
+/* context.lock is held by the task which issued the smp function call */
static void flush_ldt(void *__mm)
{
struct mm_struct *mm = __mm;
@@ -99,15 +104,17 @@ static void finalize_ldt_struct(struct l
paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
}

-/* context.lock is held */
-static void install_ldt(struct mm_struct *current_mm,
- struct ldt_struct *ldt)
+static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
{
+ mutex_lock(&mm->context.lock);
+
/* Synchronizes with READ_ONCE in load_mm_ldt. */
- smp_store_release(&current_mm->context.ldt, ldt);
+ smp_store_release(&mm->context.ldt, ldt);

- /* Activate the LDT for all CPUs using current_mm. */
- on_each_cpu_mask(mm_cpumask(current_mm), flush_ldt, current_mm, true);
+ /* Activate the LDT for all CPUs using currents mm. */
+ on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);
+
+ mutex_unlock(&mm->context.lock);
}

static void free_ldt_struct(struct ldt_struct *ldt)
@@ -133,7 +140,8 @@ int init_new_context_ldt(struct task_str
struct mm_struct *old_mm;
int retval = 0;

- mutex_init(&mm->context.lock);
+ init_rwsem(&mm->context.ldt_usr_sem);
+
old_mm = current->mm;
if (!old_mm) {
mm->context.ldt = NULL;
@@ -180,7 +188,7 @@ static int read_ldt(void __user *ptr, un
unsigned long entries_size;
int retval;

- mutex_lock(&mm->context.lock);
+ down_read(&mm->context.ldt_usr_sem);

if (!mm->context.ldt) {
retval = 0;
@@ -209,7 +217,7 @@ static int read_ldt(void __user *ptr, un
retval = bytecount;

out_unlock:
- mutex_unlock(&mm->context.lock);
+ up_read(&mm->context.ldt_usr_sem);
return retval;
}

@@ -269,7 +277,8 @@ static int write_ldt(void __user *ptr, u
ldt.avl = 0;
}

- mutex_lock(&mm->context.lock);
+ if (down_write_killable(&mm->context.ldt_usr_sem))
+ return -EINTR;

old_ldt = mm->context.ldt;
old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
@@ -291,7 +300,7 @@ static int write_ldt(void __user *ptr, u
error = 0;

out_unlock:
- mutex_unlock(&mm->context.lock);
+ up_write(&mm->context.ldt_usr_sem);
out:
return error;
}

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