Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers

On 2019-09-05, Christian Brauner [off-list ref] wrote:

On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:

quoted

A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases). This is done
in both directions -- hence two helpers -- though it's more common to
have to copy user space structs into kernel space.

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[1]). A future
patch replaces all of the common uses of this pattern to use the new
copy_struct_{to,from}_user() helpers.

[1]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
     similar checks to copy_struct_from_user() while rt_sigprocmask(2)
     always rejects differently-sized struct arguments.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Aleksa Sarai <redacted>

[...]

quoted

+	if (unlikely(!access_ok(src, usize)))
+		return -EFAULT;
+
+	/* Deal with trailing bytes. */
+	if (usize < ksize)
+		memset(dst + size, 0, rest);

[...]

That's a change in behavior for clone3() and sched at least, no? Unless
- which I guess you might have done - you have moved the "error out when
the struct is too small" part before the call to copy_struct_from_user()
for them.

Yes, I've put the minimum size check to the callers in all of the
cases (in the case of clone3() I've #define'd a CLONE_ARGS_SIZE_VER0 to
match the others -- see patch 2 of the series).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>