fs/proc/task_mmu.c: shift mm_access() from m_start() to proc_maps_open()

A simple test-case from Kirill Shutemov

	cat /proc/self/maps >/dev/null
	chmod +x /proc/self/net/packet
	exec /proc/self/net/packet

makes lockdep unhappy, cat/exec take seq_file->lock + cred_guard_mutex in
the opposite order.

It's a false positive and probably we should not allow "chmod +x" on proc
files. Still I think that we should avoid mm_access() and cred_guard_mutex
in sys_read() paths, security checking should happen at open time. Besides,
this doesn't even look right if the task changes its ->mm between m_stop()
and m_start().

Add the new "mm_struct *mm" member into struct proc_maps_private and change
proc_maps_open() to initialize it using proc_mem_open(). Change m_start() to
use priv->mm if atomic_inc_not_zero(mm_users) succeeds or return NULL (eof)
otherwise.

The only complication is that proc_maps_open() users should additionally do
mmdrop() in fop->release(), add the new proc_map_release() helper for that.

Note: this is the user-visible change, if the task execs after open("maps")
the new ->mm won't be visible via this file. I hope this is fine, and this
matches /proc/pid/mem bahaviour.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: "Kirill A. Shutemov" <kirill@shutemov.name>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 29 insertions, 9 deletions

diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 3c685563406f..d27182854a28 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -270,6 +270,7 @@ extern int proc_remount(struct super_block *, int *, char *);
 struct proc_maps_private {
         struct pid *pid;
         struct task_struct *task;
+        struct mm_struct *mm;
 #ifdef CONFIG_MMU
         struct vm_area_struct *tail_vma;
 #endif
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 4d716a09d500..a1454dac7e0a 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -165,9 +165,9 @@ static void *m_start(struct seq_file *m, loff_t *pos)
         if (!priv->task)
                 return ERR_PTR(-ESRCH);
 
-        mm = mm_access(priv->task, PTRACE_MODE_READ);
-        if (!mm || IS_ERR(mm))
-                return mm;
+        mm = priv->mm;
+        if (!mm || !atomic_inc_not_zero(&mm->mm_users))
+                return NULL;
         down_read(&mm->mmap_sem);
 
         tail_vma = get_gate_vma(mm);
@@ -240,9 +240,28 @@ static int proc_maps_open(struct inode *inode, struct file *file,
                 return -ENOMEM;
 
         priv->pid = proc_pid(inode);
+        priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
+        if (IS_ERR(priv->mm)) {
+                int err = PTR_ERR(priv->mm);
+
+                seq_release_private(inode, file);
+                return err;
+        }
+
         return 0;
 }
 
+static int proc_map_release(struct inode *inode, struct file *file)
+{
+        struct seq_file *seq = file->private_data;
+        struct proc_maps_private *priv = seq->private;
+
+        if (priv->mm)
+                mmdrop(priv->mm);
+
+        return seq_release_private(inode, file);
+}
+
 static int do_maps_open(struct inode *inode, struct file *file,
                         const struct seq_operations *ops)
 {
@@ -398,14 +417,14 @@ const struct file_operations proc_pid_maps_operations = {
         .open           = pid_maps_open,
         .read           = seq_read,
         .llseek         = seq_lseek,
-        .release        = seq_release_private,
+        .release        = proc_map_release,
 };
 
 const struct file_operations proc_tid_maps_operations = {
         .open           = tid_maps_open,
         .read           = seq_read,
         .llseek         = seq_lseek,
-        .release        = seq_release_private,
+        .release        = proc_map_release,
 };
 
 /*
@@ -680,14 +699,14 @@ const struct file_operations proc_pid_smaps_operations = {
         .open           = pid_smaps_open,
         .read           = seq_read,
         .llseek         = seq_lseek,
-        .release        = seq_release_private,
+        .release        = proc_map_release,
 };
 
 const struct file_operations proc_tid_smaps_operations = {
         .open           = tid_smaps_open,
         .read           = seq_read,
         .llseek         = seq_lseek,
-        .release        = seq_release_private,
+        .release        = proc_map_release,
 };
 
 /*
@@ -1544,13 +1563,13 @@ const struct file_operations proc_pid_numa_maps_operations = {
         .open           = pid_numa_maps_open,
         .read           = seq_read,
         .llseek         = seq_lseek,
-        .release        = seq_release_private,
+        .release        = proc_map_release,
 };
 
 const struct file_operations proc_tid_numa_maps_operations = {
         .open           = tid_numa_maps_open,
         .read           = seq_read,
         .llseek         = seq_lseek,
-        .release        = seq_release_private,
+        .release        = proc_map_release,
 };
 #endif /* CONFIG_NUMA */