From 13f0feafa6b8aead57a2a328e2fca6a5828bf286 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Tue, 23 Jun 2009 21:25:32 +0200 Subject: [PATCH 1/3] mm_for_maps: simplify, use ptrace_may_access() It would be nice to kill __ptrace_may_access(). It requires task_lock(), but this lock is only needed to read mm->flags in the middle. Convert mm_for_maps() to use ptrace_may_access(), this also simplifies the code a little bit. Also, we do not need to take ->mmap_sem in advance. In fact I think mm_for_maps() should not play with ->mmap_sem at all, the caller should take this lock. With or without this patch, without ->cred_guard_mutex held we can race with exec() and get the new ->mm but check old creds. Signed-off-by: Oleg Nesterov Reviewed-by: Serge Hallyn Signed-off-by: James Morris --- fs/proc/base.c | 23 +++++++++++------------ 1 file changed, 11 insertions(+), 12 deletions(-) diff --git a/fs/proc/base.c b/fs/proc/base.c index 3ce5ae9e3d2d..917f338a6739 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -237,20 +237,19 @@ 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) { + /* + * task->mm can be changed before security check, + * in that case we must notice the change after. + */ + if (!ptrace_may_access(task, PTRACE_MODE_READ) || + mm != task->mm) { + mmput(mm); + return NULL; + } + } down_read(&mm->mmap_sem); - task_lock(task); - if (task->mm != mm) - goto out; - if (task->mm != current->mm && - __ptrace_may_access(task, PTRACE_MODE_READ) < 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) From 00f89d218523b9bf6b522349c039d5ac80aa536d Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Fri, 10 Jul 2009 03:27:38 +0200 Subject: [PATCH 2/3] 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 Acked-by: Serge Hallyn Signed-off-by: James Morris --- fs/proc/base.c | 8 +++----- fs/proc/task_mmu.c | 1 + fs/proc/task_nommu.c | 1 + 3 files changed, 5 insertions(+), 5 deletions(-) diff --git a/fs/proc/base.c b/fs/proc/base.c index 917f338a6739..f3c2e4085fed 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -235,9 +235,8 @@ static int check_mem_permission(struct task_struct *task) 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 task_struct *task) if (!ptrace_may_access(task, PTRACE_MODE_READ) || mm != task->mm) { mmput(mm); - return NULL; + mm = NULL; } } - down_read(&mm->mmap_sem); return mm; } diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 6f61b7cc32e0..9bd8be1d235c 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m, loff_t *pos) 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; diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c index 64a72e2e7650..8f5c05d3dbd3 100644 --- a/fs/proc/task_nommu.c +++ b/fs/proc/task_nommu.c @@ -189,6 +189,7 @@ static void *m_start(struct seq_file *m, loff_t *pos) 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)) From 704b836cbf19e885f8366bccb2e4b0474346c02d Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Fri, 10 Jul 2009 03:27:40 +0200 Subject: [PATCH 3/3] mm_for_maps: take ->cred_guard_mutex to fix the race with exec The problem is minor, but without ->cred_guard_mutex held we can race with exec() and get the new ->mm but check old creds. Now we do not need to re-check task->mm after ptrace_may_access(), it can't be changed to the new mm under us. Strictly speaking, this also fixes another very minor problem. Unless security check fails or the task exits mm_for_maps() should never return NULL, the caller should get either old or new ->mm. Signed-off-by: Oleg Nesterov Acked-by: Serge Hallyn Signed-off-by: James Morris --- fs/proc/base.c | 22 +++++++++++----------- 1 file changed, 11 insertions(+), 11 deletions(-) diff --git a/fs/proc/base.c b/fs/proc/base.c index f3c2e4085fed..175db258942f 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -234,19 +234,19 @@ static int check_mem_permission(struct task_struct *task) struct mm_struct *mm_for_maps(struct task_struct *task) { - struct mm_struct *mm = get_task_mm(task); + struct mm_struct *mm; - if (mm && mm != current->mm) { - /* - * task->mm can be changed before security check, - * in that case we must notice the change after. - */ - if (!ptrace_may_access(task, PTRACE_MODE_READ) || - mm != task->mm) { - mmput(mm); - mm = NULL; - } + if (mutex_lock_killable(&task->cred_guard_mutex)) + return NULL; + + mm = get_task_mm(task); + if (mm && mm != current->mm && + !ptrace_may_access(task, PTRACE_MODE_READ)) { + mmput(mm); + mm = NULL; } + mutex_unlock(&task->cred_guard_mutex); + return mm; }