[PATCH] proc: prevent a task from writing on its own /proc/*/mem

[Salvatore Mesoraca]

2018-05-28 11:06 GMT+02:00 Jann Horn <jannh at google.com>:
> On Sat, May 26, 2018 at 4:50 PM, Salvatore Mesoraca
> <s.mesoraca16 at gmail.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 at gmail.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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