fs/proc: Stop trying to report thread stacks

This reverts more of:

  b76437579d13 ("procfs: mark thread stack correctly in proc/<pid>/maps")

... which was partially reverted by:

  65376df58217 ("proc: revert /proc/<pid>/maps [stack:TID] annotation")

Originally, /proc/PID/task/TID/maps was the same as /proc/TID/maps.

In current kernels, /proc/PID/maps (or /proc/TID/maps even for
threads) shows "[stack]" for VMAs in the mm's stack address range.

In contrast, /proc/PID/task/TID/maps uses KSTK_ESP to guess the
target thread's stack's VMA.  This is racy, probably returns garbage
and, on arches with CONFIG_TASK_INFO_IN_THREAD=y, is also crash-prone:
KSTK_ESP is not safe to use on tasks that aren't known to be running
ordinary process-context kernel code.

This patch removes the difference and just shows "[stack]" for VMAs
in the mm's stack range.  This is IMO much more sensible -- the
actual "stack" address really is treated specially by the VM code,
and the current thread stack isn't even well-defined for programs
that frequently switch stacks on their own.

Reported-by: Jann Horn <jann@thejh.net>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux API <linux-api@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tycho Andersen <tycho.andersen@canonical.com>
Link: http://lkml.kernel.org/r/3e678474ec14e0a0ec34c611016753eea2e1b8ba.1475257877.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 20 insertions, 37 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 6909582ce5e5..35b92d81692f 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -266,24 +266,15 @@ static int do_maps_open(struct inode *inode, struct file *file,
  * /proc/PID/maps that is the stack of the main task.
  */
 static int is_stack(struct proc_maps_private *priv,
-                    struct vm_area_struct *vma, int is_pid)
+                    struct vm_area_struct *vma)
 {
-        int stack = 0;
-
-        if (is_pid) {
-                stack = vma->vm_start <= vma->vm_mm->start_stack &&
-                        vma->vm_end >= vma->vm_mm->start_stack;
-        } else {
-                struct inode *inode = priv->inode;
-                struct task_struct *task;
-
-                rcu_read_lock();
-                task = pid_task(proc_pid(inode), PIDTYPE_PID);
-                if (task)
-                        stack = vma_is_stack_for_task(vma, task);
-                rcu_read_unlock();
-        }
-        return stack;
+        /*
+         * We make no effort to guess what a given thread considers to be
+         * its "stack".  It's not even well-defined for programs written
+         * languages like Go.
+         */
+        return vma->vm_start <= vma->vm_mm->start_stack &&
+                vma->vm_end >= vma->vm_mm->start_stack;
 }
 
 static void
@@ -354,7 +345,7 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma, int is_pid)
                         goto done;
                 }
 
-                if (is_stack(priv, vma, is_pid))
+                if (is_stack(priv, vma))
                         name = "[stack]";
         }
 
@@ -1669,7 +1660,7 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
                 seq_file_path(m, file, "\n\t= ");
         } else if (vma->vm_start <= mm->brk && vma->vm_end >= mm->start_brk) {
                 seq_puts(m, " heap");
-        } else if (is_stack(proc_priv, vma, is_pid)) {
+        } else if (is_stack(proc_priv, vma)) {
                 seq_puts(m, " stack");
         }
 
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index faacb0c0d857..37175621e890 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -124,25 +124,17 @@ unsigned long task_statm(struct mm_struct *mm,
 }
 
 static int is_stack(struct proc_maps_private *priv,
-                    struct vm_area_struct *vma, int is_pid)
+                    struct vm_area_struct *vma)
 {
         struct mm_struct *mm = vma->vm_mm;
-        int stack = 0;
-
-        if (is_pid) {
-                stack = vma->vm_start <= mm->start_stack &&
-                        vma->vm_end >= mm->start_stack;
-        } else {
-                struct inode *inode = priv->inode;
-                struct task_struct *task;
-
-                rcu_read_lock();
-                task = pid_task(proc_pid(inode), PIDTYPE_PID);
-                if (task)
-                        stack = vma_is_stack_for_task(vma, task);
-                rcu_read_unlock();
-        }
-        return stack;
+
+        /*
+         * We make no effort to guess what a given thread considers to be
+         * its "stack".  It's not even well-defined for programs written
+         * languages like Go.
+         */
+        return vma->vm_start <= mm->start_stack &&
+                vma->vm_end >= mm->start_stack;
 }
 
 /*
@@ -184,7 +176,7 @@ static int nommu_vma_show(struct seq_file *m, struct vm_area_struct *vma,
         if (file) {
                 seq_pad(m, ' ');
                 seq_file_path(m, file, "");
-        } else if (mm && is_stack(priv, vma, is_pid)) {
+        } else if (mm && is_stack(priv, vma)) {
                 seq_pad(m, ' ');
                 seq_printf(m, "[stack]");
         }