From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Eduardo Valentin <eduval@xxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: aliguori@xxxxxxxxxx
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: hughd@xxxxxxxxxx
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: David Laight <David.Laight@xxxxxxxxxx>
Cc: Andy Lutomirsky <luto@xxxxxxxxxx>
Cc: keescook@xxxxxxxxxx
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Borislav Petkov <bpetkov@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: dan.j.williams@xxxxxxxxx
Cc: kirill.shutemov@xxxxxxxxxxxxxxx
---
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(¤t_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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