From: Eric W. Biederman In process of getting proc_fd_access_allowed to work it has developed a few warts. In particular the special case that always allows introspection and the special case to allow inspection of kernel threads. The special case for introspection is needed for /proc/self/mem. The special case for kernel threads really should be overridable by security modules. So consolidate these checks into ptrace.c:may_attach(). The check to always allow introspection is trivial. The check to allow access to kernel threads, and zombies is a little trickier. mem_read and mem_write already verify an mm exists so it isn't needed twice. proc_fd_access_allowed only doesn't want a check to verify task->mm exits, s it prevents all access to kernel threads. So just move the task->mm check into ptrace_attach where it is needed for practical reasons. I did a quick audit and none of the security modules in the kernel seem to care if they are passed a task without an mm into security_ptrace. So the above move should be safe and it allows security modules to come up with more restrictive policy. Signed-off-by: Eric W. Biederman Cc: Stephen Smalley Cc: Chris Wright Cc: James Morris Signed-off-by: Andrew Morton --- fs/proc/base.c | 26 ++++++-------------------- kernel/ptrace.c | 20 +++++++++++++++++--- 2 files changed, 23 insertions(+), 23 deletions(-) diff -puN fs/proc/base.c~proc-cleanup-proc_fd_access_allowed fs/proc/base.c --- devel/fs/proc/base.c~proc-cleanup-proc_fd_access_allowed 2006-06-09 15:18:14.000000000 -0700 +++ devel-akpm/fs/proc/base.c 2006-06-09 15:18:14.000000000 -0700 @@ -532,29 +532,15 @@ static int proc_fd_access_allowed(struct { struct task_struct *task; int allowed = 0; - /* Allow access to a task's file descriptors if either we may - * use ptrace attach to the process and find out that - * information, or if the task cannot possibly be ptraced - * allow access if we have the proper capability. + /* Allow access to a task's file descriptors if it is us or we + * may use ptrace attach to the process and find out that + * information. */ task = get_proc_task(inode); - if (task == current) - allowed = 1; - if (task && !allowed) { - int alive; - - task_lock(task); - alive = !!task->mm; - task_unlock(task); - if (alive) - /* For a living task obey ptrace_may_attach */ - allowed = ptrace_may_attach(task); - else - /* For a special task simply check the capability */ - allowed = capable(CAP_SYS_PTRACE); - } - if (task) + if (task) { + allowed = ptrace_may_attach(task); put_task_struct(task); + } return allowed; } diff -puN kernel/ptrace.c~proc-cleanup-proc_fd_access_allowed kernel/ptrace.c --- devel/kernel/ptrace.c~proc-cleanup-proc_fd_access_allowed 2006-06-09 15:18:14.000000000 -0700 +++ devel-akpm/kernel/ptrace.c 2006-06-09 15:18:14.000000000 -0700 @@ -120,8 +120,18 @@ int ptrace_check_attach(struct task_stru static int may_attach(struct task_struct *task) { - if (!task->mm) - return -EPERM; + /* May we inspect the given task? + * This check is used both for attaching with ptrace + * and for allowing access to sensitive information in /proc. + * + * ptrace_attach denies several cases that /proc allows + * because setting up the necessary parent/child relationship + * or halting the specified task is impossible. + */ + int dumpable = 0; + /* Don't let security modules deny introspection */ + if (task == current) + return 0; if (((current->uid != task->euid) || (current->uid != task->suid) || (current->uid != task->uid) || @@ -130,7 +140,9 @@ static int may_attach(struct task_struct (current->gid != task->gid)) && !capable(CAP_SYS_PTRACE)) return -EPERM; smp_rmb(); - if (!task->mm->dumpable && !capable(CAP_SYS_PTRACE)) + if (task->mm) + dumpable = task->mm->dumpable; + if (!dumpable && !capable(CAP_SYS_PTRACE)) return -EPERM; return security_ptrace(current, task); @@ -176,6 +188,8 @@ repeat: goto repeat; } + if (!task->mm) + goto bad; /* the same process cannot be attached many times */ if (task->ptrace & PT_PTRACED) goto bad; _