4.14-stable review patch. If anyone has any objections, please let me know.
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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(¤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; }