restrict reading from /proc/<pid>/maps to those who share ->mm or can ptrace pid

Contents of /proc/*/maps is sensitive and may become sensitive after
open() (e.g.  if target originally shares our ->mm and later does exec
on suid-root binary).

Check at read() (actually, ->start() of iterator) time that mm_struct
we'd grabbed and locked is
 - still the ->mm of target
 - equal to reader's ->mm or the target is ptracable by reader.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -202,6 +202,26 @@ static int proc_root_link(struct inode *inode, struct dentry **dentry, struct vf
 	 (task->state == TASK_STOPPED || task->state == TASK_TRACED) && \
 	 security_ptrace(current,task) == 0))
 
+struct mm_struct *mm_for_maps(struct task_struct *task)
+{
+	struct mm_struct *mm = get_task_mm(task);
+	if (!mm)
+		return NULL;
+	down_read(&mm->mmap_sem);
+	task_lock(task);
+	if (task->mm != mm)
+		goto out;
+	if (task->mm != current->mm && __ptrace_may_attach(task) < 0)
+		goto out;
+	task_unlock(task);
+	return mm;
+out:
+	task_unlock(task);
+	up_read(&mm->mmap_sem);
+	mmput(mm);
+	return NULL;
+}
+
 static int proc_pid_cmdline(struct task_struct *task, char * buffer)
 {
 	int res = 0;

fs/proc/internal.h

@@ -27,6 +27,8 @@ struct vmalloc_info {
 	unsigned long	largest_chunk;
 };
 
+extern struct mm_struct *mm_for_maps(struct task_struct *);
+
 #ifdef CONFIG_MMU
 #define VMALLOC_TOTAL (VMALLOC_END - VMALLOC_START)
 extern void get_vmalloc_info(struct vmalloc_info *vmi);

fs/proc/task_mmu.c

@@ -397,12 +397,11 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	if (!priv->task)
 		return NULL;
 
-	mm = get_task_mm(priv->task);
+	mm = mm_for_maps(priv->task);
 	if (!mm)
 		return NULL;
 
 	priv->tail_vma = tail_vma = get_gate_vma(priv->task);
-	down_read(&mm->mmap_sem);
 
 	/* Start with last addr hint */
 	if (last_addr && (vma = find_vma(mm, last_addr))) {

fs/proc/task_nommu.c

@@ -165,15 +165,13 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	if (!priv->task)
 		return NULL;
 
-	mm = get_task_mm(priv->task);
+	mm = mm_for_maps(priv->task);
 	if (!mm) {
 		put_task_struct(priv->task);
 		priv->task = NULL;
 		return NULL;
 	}
 
-	down_read(&mm->mmap_sem);
-
 	/* start from the Nth VMA */
 	for (vml = mm->context.vmlist; vml; vml = vml->next)
 		if (n-- == 0)

include/linux/ptrace.h

@@ -97,6 +97,7 @@ extern void __ptrace_link(struct task_struct *child,
 extern void __ptrace_unlink(struct task_struct *child);
 extern void ptrace_untrace(struct task_struct *child);
 extern int ptrace_may_attach(struct task_struct *task);
+extern int __ptrace_may_attach(struct task_struct *task);
 
 static inline void ptrace_link(struct task_struct *child,
 			       struct task_struct *new_parent)

kernel/ptrace.c

@@ -120,7 +120,7 @@ int ptrace_check_attach(struct task_struct *child, int kill)
 	return ret;
 }
 
-static int may_attach(struct task_struct *task)
+int __ptrace_may_attach(struct task_struct *task)
 {
 	/* May we inspect the given task?
 	 * This check is used both for attaching with ptrace
@@ -154,7 +154,7 @@ int ptrace_may_attach(struct task_struct *task)
 {
 	int err;
 	task_lock(task);
-	err = may_attach(task);
+	err = __ptrace_may_attach(task);
 	task_unlock(task);
 	return !err;
 }