> On Feb 22, 2019, at 9:43 AM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> On Fri, Feb 22, 2019 at 12:35 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>>
>> Or, can we do this?
>>
>> long __probe_user_read(void *dst, const void *src, size_t size)
>> {
>
> Add a
>
> if (!access_ok(src, size))
> ret = -EFAULT;
> else {
> .. do the pagefault_disable() etc ..
> }
>
> to after the "set_fs()", and it looks good to me. Make it clear that
> yes, this works _only_ for user reads.
>
> Adn that makes all the games with "kernel_uaccess_faults_ok"
> pointless, so you can just remove them.
>
> (note that the "access_ok()" has to come after we've done "set_fs()",
> because it takes the address limit from that).
>
> Also, since normally we'd expect that we already have USER_DS, it
> might be worthwhile to do this with a wrapper, something along the
> lines of
>
> mm_segment_t old_fs = get_fs();
>
> if (segment_eq(old_fs, USER_DS))
> return __normal_probe_user_read();
> set_fs(USER_DS);
> ret = __normal_probe_user_read();
> set_fs(old_fs);
> return ret;
>
> and have that __normal_probe_user_read() just do
>
> if (!access_ok(src, size))
> return -EFAULT;
> pagefault_disable();
> ret = __copy_from_user_inatomic(dst, ...);
> pagefault_enable();
> return ret ? -EFAULT : 0;
>
> which looks more obvious.
>
> Also, I would suggest that you just make the argument type be "const
> void __user *", since the whole point is that this takes a user
> pointer, and nothing else.
>
> Then we should still probably fix up "__probe_kernel_read()" to not
> allow user accesses. The easiest way to do that is actually likely to
> use the "unsafe_get_user()" functions *without* doing a
> uaccess_begin(), which will mean that modern CPU's will simply fault
> on a kernel access to user space.
>
> The nice thing about that is that usually developers will have access
> to exactly those modern boxes, so the people who notice that it
> doesn't work are the right people.
We use probe_kernel_read() from oops code. I’d rather it return -EFAULT than oops harder and kill the first oops.
