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Message-ID: <CAG48ez2N8tjyjGbdh+927uf2A_Xtsie=+DL+GZbvBniiO8jNHw@mail.gmail.com>
Date: Mon, 28 May 2018 11:06:38 +0200
From: Jann Horn <jannh@...gle.com>
To: Salvatore Mesoraca <s.mesoraca16@...il.com>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-security-module <linux-security-module@...r.kernel.org>,
kernel list <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>, Alexey Dobriyan <adobriyan@...il.com>,
Akinobu Mita <akinobu.mita@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>,
Arnd Bergmann <arnd@...db.de>, Davidlohr Bueso <dave@...olabs.net>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] proc: prevent a task from writing on its own /proc/*/mem
On Sat, May 26, 2018 at 4:50 PM, Salvatore Mesoraca
<s.mesoraca16@...il.com> wrote:
> Prevent a task from opening, in "write" mode, any /proc/*/mem
> file that operates on the task's mm.
> /proc/*/mem is mainly a debugging means and, as such, it shouldn't
> be used by the inspected process itself.
> Current implementation always allow a task to access its own
> /proc/*/mem file.
> A process can use it to overwrite read-only memory, making
> pointless the use of security_file_mprotect() or other ways to
> enforce RO memory.
>
> Signed-off-by: Salvatore Mesoraca <s.mesoraca16@...il.com>
> ---
> fs/proc/base.c | 25 ++++++++++++++++++-------
> fs/proc/internal.h | 3 ++-
> fs/proc/task_mmu.c | 4 ++--
> fs/proc/task_nommu.c | 2 +-
> 4 files changed, 23 insertions(+), 11 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 1a76d75..01ecfec 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -762,8 +762,9 @@ static int proc_single_open(struct inode *inode, struct file *filp)
> .release = single_release,
> };
>
> -
> -struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
> +struct mm_struct *proc_mem_open(struct inode *inode,
> + unsigned int mode,
> + fmode_t f_mode)
> {
> struct task_struct *task = get_proc_task(inode);
> struct mm_struct *mm = ERR_PTR(-ESRCH);
> @@ -773,10 +774,20 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
> put_task_struct(task);
>
> if (!IS_ERR_OR_NULL(mm)) {
> - /* ensure this mm_struct can't be freed */
> - mmgrab(mm);
> - /* but do not pin its memory */
> - mmput(mm);
> + /*
> + * Prevent this interface from being used as a mean
> + * to bypass memory restrictions, including those
> + * imposed by LSMs.
> + */
> + if (mm == current->mm &&
> + f_mode & FMODE_WRITE)
> + mm = ERR_PTR(-EACCES);
> + else {
> + /* ensure this mm_struct can't be freed */
> + mmgrab(mm);
> + /* but do not pin its memory */
> + mmput(mm);
> + }
> }
> }
I don't have an opinion on the overall patch, but this part looks
buggy: In the error path, you set `mm` to an error pointer, but you
still own the reference that mm_access() took on the old `mm`. The
error path needs to call `mmput(mm)`.
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