Re: [PATCH v1 3/7] kernel/fork: always deny write access to current MM exe_file

[David Hildenbrand <david@redhat.com>]

On 12.08.21 12:05, Christian Brauner wrote:
> [+Cc Andrei]
> 
> On Thu, Aug 12, 2021 at 10:43:44AM +0200, David Hildenbrand wrote:
>> We want to remove VM_DENYWRITE only currently only used when mapping the
>> executable during exec. During exec, we already deny_write_access() the
>> executable, however, after exec completes the VMAs mapped
>> with VM_DENYWRITE effectively keeps write access denied via
>> deny_write_access().
>>
>> Let's deny write access when setting the MM exe_file. With this change, we
>> can remove VM_DENYWRITE for mapping executables.
>>
>> This represents a minor user space visible change:
>> sys_prctl(PR_SET_MM_EXE_FILE) can now fail if the file is already
>> opened writable. Also, after sys_prctl(PR_SET_MM_EXE_FILE), the file
> 
> Just for completeness, this also affects PR_SET_MM_MAP when exe_fd is
> set.

Correct.

> 
>> cannot be opened writable. Note that we can already fail with -EACCES if
>> the file doesn't have execute permissions.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
> 
> The biggest user I know and that I'm involved in is CRIU which heavily
> uses PR_SET_MM_MAP (with a fallback to PR_SET_MM_EXE_FILE on older
> kernels) during restore. Afair, criu opens the exe fd as an O_PATH
> during dump and thus will use the same flag during restore when
> opening it. So that should be fine.

Yes.

> 
> However, if I understand the consequences of this change correctly, a
> problem could be restoring workloads that hold a writable fd open to
> their exe file at dump time which would mean that during restore that fd
> would be reopened writable causing CRIU to fail when setting the exe
> file for the task to be restored.

If it's their exe file, then the existing VM_DENYWRITE handling would 
have forbidden these workloads to open the fd of their exe file 
writable, right? At least before doing any 
PR_SET_MM_MAP/PR_SET_MM_EXE_FILE. But that should rule out quite a lot 
of cases we might be worried about, right?

> 
> Which honestly, no idea how many such workloads exist. (I know at least
> of runC and LXC need to sometimes reopen to rexec themselves (weird bug
> to protect against attacking the exe file) and thus re-open
> /proc/self/exe but read-only.)
> 
>>   kernel/fork.c | 39 ++++++++++++++++++++++++++++++++++-----
>>   1 file changed, 34 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 6bd2e52bcdfb..5d904878f19b 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -476,6 +476,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>>   {
>>   	struct vm_area_struct *mpnt, *tmp, *prev, **pprev;
>>   	struct rb_node **rb_link, *rb_parent;
>> +	struct file *exe_file;
>>   	int retval;
>>   	unsigned long charge;
>>   	LIST_HEAD(uf);
>> @@ -493,7 +494,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>>   	mmap_write_lock_nested(mm, SINGLE_DEPTH_NESTING);
>>   
>>   	/* No ordering required: file already has been exposed. */
>> -	RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));
>> +	exe_file = get_mm_exe_file(oldmm);
>> +	RCU_INIT_POINTER(mm->exe_file, exe_file);
>> +	if (exe_file)
>> +		deny_write_access(exe_file);
>>   
>>   	mm->total_vm = oldmm->total_vm;
>>   	mm->data_vm = oldmm->data_vm;
>> @@ -638,8 +642,13 @@ static inline void mm_free_pgd(struct mm_struct *mm)
>>   #else
>>   static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
>>   {
>> +	struct file *exe_file;
>> +
>>   	mmap_write_lock(oldmm);
>> -	RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));
>> +	exe_file = get_mm_exe_file(oldmm);
>> +	RCU_INIT_POINTER(mm->exe_file, exe_file);
>> +	if (exe_file)
>> +		deny_write_access(exe_file);
>>   	mmap_write_unlock(oldmm);
>>   	return 0;
>>   }
>> @@ -1163,11 +1172,19 @@ void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
>>   	 */
>>   	old_exe_file = rcu_dereference_raw(mm->exe_file);
>>   
>> -	if (new_exe_file)
>> +	if (new_exe_file) {
>>   		get_file(new_exe_file);
>> +		/*
>> +		 * exec code is required to deny_write_access() successfully,
>> +		 * so this cannot fail
>> +		 */
>> +		deny_write_access(new_exe_file);
>> +	}
>>   	rcu_assign_pointer(mm->exe_file, new_exe_file);
>> -	if (old_exe_file)
>> +	if (old_exe_file) {
>> +		allow_write_access(old_exe_file);
>>   		fput(old_exe_file);
>> +	}
>>   }
>>   
>>   int atomic_set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
>> @@ -1194,10 +1211,22 @@ int atomic_set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
>>   	}
>>   
>>   	/* set the new file, lockless */
>> +	ret = deny_write_access(new_exe_file);
>> +	if (ret)
>> +		return -EACCES;
>>   	get_file(new_exe_file);
>> +
>>   	old_exe_file = xchg(&mm->exe_file, new_exe_file);
>> -	if (old_exe_file)
>> +	if (old_exe_file) {
>> +		/*
>> +		 * Don't race with dup_mmap() getting the file and disallowing
>> +		 * write access while someone might open the file writable.
>> +		 */
>> +		mmap_read_lock(mm);
>> +		allow_write_access(old_exe_file);
>>   		fput(old_exe_file);
>> +		mmap_read_unlock(mm);
>> +	}
>>   	return 0;
>>   }
>>   
>> -- 
>> 2.31.1
>>
> 


-- 
Thanks,

David / dhildenb