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Date: Mon, 28 May 2018 11:33:33 +0200
From: Salvatore Mesoraca <s.mesoraca16@...il.com>
To: Jann Horn <jannh@...gle.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
2018-05-28 11:06 GMT+02:00 Jann Horn <jannh@...gle.com>:
> 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)`.
You are absolutely right,
Thank you,
Salvatore
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