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Date: Mon, 9 Nov 2009 14:47:17 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc: Americo Wang <xiyou.wangcong@...il.com>,
Timo Sirainen <tss@....fi>, Bryan Donlan <bdonlan@...il.com>,
Ulrich Drepper <drepper@...hat.com>,
LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org
Subject: Re: [PATCH v5] Added PR_SET_PROCTITLE_AREA option for prctl()
On Wed, 4 Nov 2009 00:26:44 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> ========================================
>
> Subject: [PATCH v5] Added PR_SET_PROCTITLE_AREA option for prctl()
> From: Timo Sirainen <tss@....fi>
>
> Currently glibc2 doesn't have setproctitle(3), so several userland
> daemons attempt to emulate it by doing some brutal stack modifications.
> This works most of the time, but it has problems. For example:
>
> % ps -ef |grep avahi-daemon
> avahi 1679 1 0 09:20 ? 00:00:00 avahi-daemon: running [kosadesk.local]
>
> # cat /proc/1679/cmdline
> avahi-daemon: running [kosadesk.local]
>
> This looks good, but the process has also overwritten its environment
> area and made the environ file useless:
>
> # cat /proc/1679/environ
> adesk.local]
>
> Another problem is that the process title length is limited by the size of
> the environment. Security conscious people try to avoid potential information
> leaks by clearing most of the environment before running a daemon:
>
> # env - MINIMUM_NEEDED_VAR=foo /path/to/daemon
>
> The resulting environment size may be too small to fit the wanted process
> titles.
>
> This patch makes it possible for userspace to implement setproctitle()
> cleanly. It adds a new PR_SET_PROCTITLE_AREA option for prctl(), which
> updates task's mm_struct->arg_start and arg_end to the given area.
>
> test_setproctitle.c
> ================================================
> #include <string.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <sys/prctl.h>
>
> #define ERR(str) (perror(str), exit(1))
>
> void settitle(char* title){
> int err;
>
> err = prctl(34, title, strlen(title)+1);
> if (err < 0)
> ERR("prctl ");
> }
>
> void main(void){
> long i;
> char buf[1024];
>
> for (i = 0; i < 10000000000LL; i++){
> sprintf(buf, "loooooooooooooooooooooooong string %d",i);
> settitle(buf);
> }
> }
What happens if userspace unmaps the memory after telling the kernel to
use it?
Will processes which try to read the command line get an error reading
/proc? If so, do all the commandline-reading programs in the world
handle this in an appropriate fashion?
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 837469a..ac800b4 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -255,32 +255,45 @@ static int proc_pid_cmdline(struct task_struct *task, char * buffer)
> int res = 0;
> unsigned int len;
> struct mm_struct *mm = get_task_mm(task);
> +
> if (!mm)
> goto out;
> +
> + /* The process was not constructed yet? */
> if (!mm->arg_end)
> goto out_mm; /* Shh! No looking before we're done */
>
> - len = mm->arg_end - mm->arg_start;
> -
> + mutex_lock(&mm->arg_lock);
> + len = mm->arg_end - mm->arg_start;
> if (len > PAGE_SIZE)
> len = PAGE_SIZE;
> -
> +
> res = access_process_vm(task, mm->arg_start, buffer, len, 0);
> + if (mm->arg_end != mm->env_start)
> + /* prctl(PR_SET_PROCTITLE_AREA) used */
> + goto out_unlock;
>
> - // If the nul at the end of args has been overwritten, then
> - // assume application is using setproctitle(3).
> + /*
> + * If the nul at the end of args has been overwritten, then assume
> + * application is using sendmail's SPT_REUSEARGV style argv override.
> + */
> if (res > 0 && buffer[res-1] != '\0' && len < PAGE_SIZE) {
> len = strnlen(buffer, res);
> - if (len < res) {
> - res = len;
> - } else {
> + if (len < res)
> + res = len;
> + else {
> len = mm->env_end - mm->env_start;
> if (len > PAGE_SIZE - res)
> len = PAGE_SIZE - res;
> - res += access_process_vm(task, mm->env_start, buffer+res, len, 0);
> + res += access_process_vm(task, mm->env_start,
> + buffer+res, len, 0);
> res = strnlen(buffer, res);
> }
> }
> +
> +out_unlock:
> + mutex_unlock(&mm->arg_lock);
> +
> out_mm:
> mmput(mm);
> out:
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 84a524a..3e2a346 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -12,6 +12,7 @@
> #include <linux/completion.h>
> #include <linux/cpumask.h>
> #include <linux/page-debug-flags.h>
> +#include <linux/mutex.h>
> #include <asm/page.h>
> #include <asm/mmu.h>
>
> @@ -236,6 +237,7 @@ struct mm_struct {
> unsigned long stack_vm, reserved_vm, def_flags, nr_ptes;
> unsigned long start_code, end_code, start_data, end_data;
> unsigned long start_brk, brk, start_stack;
> + struct mutex arg_lock;
> unsigned long arg_start, arg_end, env_start, env_end;
>
> unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
Please document the role of arg_lock with a code comment here.
> diff --git a/include/linux/prctl.h b/include/linux/prctl.h
> index 9311505..da47542 100644
> --- a/include/linux/prctl.h
> +++ b/include/linux/prctl.h
> @@ -90,4 +90,7 @@
>
> #define PR_MCE_KILL 33
>
> +/* Set process title memory area for setproctitle() */
> +#define PR_SET_PROCTITLE_AREA 34
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 4c20fff..881a6b4 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -459,6 +459,7 @@ static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
> mm->cached_hole_size = ~0UL;
> mm_init_aio(mm);
> mm_init_owner(mm, p);
> + mutex_init(&mm->arg_lock);
>
> if (likely(!mm_alloc_pgd(mm))) {
> mm->def_flags = 0;
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 255475d..bde6957 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1564,6 +1564,28 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> error = 0;
> break;
>
> + case PR_SET_PROCTITLE_AREA: {
> + struct mm_struct *mm = current->mm;
> + unsigned long addr = arg2;
> + unsigned long len = arg3;
> + unsigned long end = arg2 + arg3;
> +
> + if (len > PAGE_SIZE)
> + return -EINVAL;
> +
> + if (addr >= end)
> + return -EINVAL;
> +
> + if (!access_ok(VERIFY_READ, addr, len))
> + return -EFAULT;
It's unobvious (to me) why this access_ok() check is here. If that
wasn't totally dumb of me, please add a comment so the next reader
won't be similarly mystified.
> + mutex_lock(&mm->arg_lock);
> + mm->arg_start = addr;
> + mm->arg_end = end;
> + mutex_unlock(&mm->arg_lock);
> +
> + return 0;
> + }
> default:
> error = -EINVAL;
> break;
--
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