2009-07-10 01:30:52

by Oleg Nesterov

Subject: [PATCH 1/2] mm_for_maps: shift down_read(mmap_sem) to the caller

mm_for_maps() takes ->mmap_sem after security checks, this looks
strange and obfuscates the locking rules. Move this lock to its
single caller, m_start().

Signed-off-by: Oleg Nesterov <[email protected]>
---

fs/proc/task_mmu.c | 1 +
fs/proc/task_nommu.c | 1 +
fs/proc/base.c | 8 +++-----
3 files changed, 5 insertions(+), 5 deletions(-)

--- WAIT/fs/proc/task_mmu.c~1_MOVE_MMAP_SEM 2009-05-03 17:15:27.000000000 +0200
+++ WAIT/fs/proc/task_mmu.c 2009-07-10 02:00:56.000000000 +0200
@@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m,
mm = mm_for_maps(priv->task);
if (!mm)
return NULL;
+ down_read(&mm->mmap_sem);

tail_vma = get_gate_vma(priv->task);
priv->tail_vma = tail_vma;
--- WAIT/fs/proc/task_nommu.c~1_MOVE_MMAP_SEM 2009-04-09 00:39:09.000000000 +0200
+++ WAIT/fs/proc/task_nommu.c 2009-07-10 02:01:25.000000000 +0200
@@ -189,6 +189,7 @@ static void *m_start(struct seq_file *m,
priv->task = NULL;
return NULL;
}
+ down_read(&mm->mmap_sem);

/* start from the Nth VMA */
for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))
--- WAIT/fs/proc/base.c~1_MOVE_MMAP_SEM 2009-07-09 17:56:27.000000000 +0200
+++ WAIT/fs/proc/base.c 2009-07-10 02:05:14.000000000 +0200
@@ -235,9 +235,8 @@ static int check_mem_permission(struct t
struct mm_struct *mm_for_maps(struct task_struct *task)
{
struct mm_struct *mm = get_task_mm(task);
- if (!mm)
- return NULL;
- if (mm != current->mm) {
+
+ if (mm && mm != current->mm) {
/*
* task->mm can be changed before security check,
* in that case we must notice the change after.
@@ -245,10 +244,9 @@ struct mm_struct *mm_for_maps(struct tas
if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
mm != task->mm) {
mmput(mm);
- return NULL;
+ mm = NULL;
}
}
- down_read(&mm->mmap_sem);
return mm;
}

2009-07-10 03:06:42

by Serge E. Hallyn

Subject: Re: [PATCH 1/2] mm_for_maps: shift down_read(mmap_sem) to the caller

Quoting Oleg Nesterov ([email protected]):
> mm_for_maps() takes ->mmap_sem after security checks, this looks
> strange and obfuscates the locking rules. Move this lock to its
> single caller, m_start().
>
> Signed-off-by: Oleg Nesterov <[email protected]>

Looks sane.

Acked-by: Serge Hallyn <[email protected]>

> ---
>
> fs/proc/task_mmu.c | 1 +
> fs/proc/task_nommu.c | 1 +
> fs/proc/base.c | 8 +++-----
> 3 files changed, 5 insertions(+), 5 deletions(-)
>
> --- WAIT/fs/proc/task_mmu.c~1_MOVE_MMAP_SEM 2009-05-03 17:15:27.000000000 +0200
> +++ WAIT/fs/proc/task_mmu.c 2009-07-10 02:00:56.000000000 +0200
> @@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m,
> mm = mm_for_maps(priv->task);
> if (!mm)
> return NULL;
> + down_read(&mm->mmap_sem);
>
> tail_vma = get_gate_vma(priv->task);
> priv->tail_vma = tail_vma;
> --- WAIT/fs/proc/task_nommu.c~1_MOVE_MMAP_SEM 2009-04-09 00:39:09.000000000 +0200
> +++ WAIT/fs/proc/task_nommu.c 2009-07-10 02:01:25.000000000 +0200
> @@ -189,6 +189,7 @@ static void *m_start(struct seq_file *m,
> priv->task = NULL;
> return NULL;
> }
> + down_read(&mm->mmap_sem);
>
> /* start from the Nth VMA */
> for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))
> --- WAIT/fs/proc/base.c~1_MOVE_MMAP_SEM 2009-07-09 17:56:27.000000000 +0200
> +++ WAIT/fs/proc/base.c 2009-07-10 02:05:14.000000000 +0200
> @@ -235,9 +235,8 @@ static int check_mem_permission(struct t
> struct mm_struct *mm_for_maps(struct task_struct *task)
> {
> struct mm_struct *mm = get_task_mm(task);
> - if (!mm)
> - return NULL;
> - if (mm != current->mm) {
> +
> + if (mm && mm != current->mm) {
> /*
> * task->mm can be changed before security check,
> * in that case we must notice the change after.
> @@ -245,10 +244,9 @@ struct mm_struct *mm_for_maps(struct tas
> if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
> mm != task->mm) {
> mmput(mm);
> - return NULL;
> + mm = NULL;
> }
> }
> - down_read(&mm->mmap_sem);
> return mm;
> }
>