On Mon, 16 Jan 2023 19:58:10 +0800 Wupeng Ma <mawupeng1@xxxxxxxxxx> wrote:
> From: Ma Wupeng <mawupeng1@xxxxxxxxxx>
>
> 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.
> 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);
_
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?
