Re: [PATCH v2 1/4] mm/mlock: return EINVAL if len overflows for mlock/munlock

[mawupeng <mawupeng1@huawei.com>]

On 2023/1/17 4:51, Andrew Morton wrote:
> On Mon, 16 Jan 2023 19:58:10 +0800 Wupeng Ma <mawupeng1@huawei.com> wrote:
> 
>> From: Ma Wupeng <mawupeng1@huawei.com>
>>
>> While testing mlock, we have a problem if the len of mlock is ULONG_MAX.
>> The return value of mlock is zero. But nothing will be locked since the
>> len in do_mlock overflows to zero due to the following code in mlock:
>>
>>   len = PAGE_ALIGN(len + (offset_in_page(start)));
>>
>> The same problem happens in munlock.
>>
>> Add new check and return -EINVAL to fix this overflowing scenarios since
>> they are absolutely wrong.
>>
>> ...
>>
>> --- a/mm/mlock.c
>> +++ b/mm/mlock.c
>> @@ -569,6 +569,7 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
>>  	unsigned long locked;
>>  	unsigned long lock_limit;
>>  	int error = -ENOMEM;
>> +	size_t old_len = len;
> 
> I'm not sure that "old_len" is a good identifier.  It reads to me like
> "the length of the old mlocked region" or something.
> 
> I really don't like it when functions modify the values of the incoming
> argument like this.  It would be better to leave `len' alone and create
> a new_len or something.

Thanks for your reviewing.

You do have a point in saying that.

> 
>>  	start = untagged_addr(start);
>>  
>> @@ -578,6 +579,9 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
>>  	len = PAGE_ALIGN(len + (offset_in_page(start)));
>>  	start &= PAGE_MASK;
>>  
>> +	if (old_len != 0 && len == 0)
>> +		return -EINVAL;
> 
> It would be clearer to do this immediately after calculating the new
> value of `len'.  Before going on to play with `start'.
> 
> Can we do something like this?
> 
> --- a/mm/mlock.c~a
> +++ a/mm/mlock.c
> @@ -575,7 +575,12 @@ static __must_check int do_mlock(unsigne
>  	if (!can_do_mlock())
>  		return -EPERM;
>  
> -	len = PAGE_ALIGN(len + (offset_in_page(start)));
> +	if (len) {
> +		len = PAGE_ALIGN(len + (offset_in_page(start)));
> +		if (len == 0)	/* overflow */
> +			return -EINVAL;
> +	}
> +
>  	start &= PAGE_MASK;
>  
>  	lock_limit = rlimit(RLIMIT_MEMLOCK);
> _

It's really more appropriate to check like this, I will use this in the next patchset.

> 
> That depends on how we handle len==0.  afaict, mlock(len==0) will
> presently burn a bunch of cpu cycles (not that we want to optimize this
> case), do nothing then return 0?

We can add and a new check in if len == 0, since the similar check appears in
mbind, set_mempolicy_home_node, msync.

The origin len == 0 check for mlock/munlock can be found in apply_vma_lock_flags,
We can move this check to here to avoid burn a bunch of cpu cycles.

do_mlock
  apply_vma_lock_flags
	end = start + len;
	if (end == start)
	  return 0;

Can we do something like this?

diff --git a/mm/mlock.c b/mm/mlock.c
index 7032f6dd0ce1..50a33abc1a2e 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -478,8 +478,6 @@ static int apply_vma_lock_flags(unsigned long start, size_t len,
        end = start + len;
        if (end < start)
                return -EINVAL;
-       if (end == start)
-               return 0;
        vma = mas_walk(&mas);
        if (!vma)
                return -ENOMEM;
@@ -575,7 +573,12 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
        if (!can_do_mlock())
                return -EPERM;
 
+       if (!len)
+               return 0;
        len = PAGE_ALIGN(len + (offset_in_page(start)));
+       if (len == 0)
+               return -EINVAL;
+
        start &= PAGE_MASK;
 
        lock_limit = rlimit(RLIMIT_MEMLOCK);
@@ -632,10 +635,14 @@ SYSCALL_DEFINE3(mlock2, unsigned long, start, size_t, len, int, flags)
 SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
 {
        int ret;
-
        start = untagged_addr(start);
 
+       if (!len)
+               return 0;
        len = PAGE_ALIGN(len + (offset_in_page(start)));
+       if (len == 0)
+               return -EINVAL;
+
        start &= PAGE_MASK;
 
        if (mmap_write_lock_killable(current->mm))

>