sched_setattr: switch to copy_struct_from_user()

Switch sched_setattr() syscall from it's own copying struct sched_attr
from userspace to the new dedicated copy_struct_from_user() helper.

The change is very straightforward, and helps unify the syscall
interface for struct-from-userspace syscalls. Ideally we could also
unify sched_getattr(2)-style syscalls as well, but unfortunately the
correct semantics for such syscalls are much less clear (see [1] for
more detail). In future we could come up with a more sane idea for how
the syscall interface should look.

[1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
     robustify sched_read_attr() ABI logic and code")

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: improve commit message]
Link: https://lore.kernel.org/r/20191001011055.19283-4-cyphar@cyphar.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7880f4f64d0eea19dab03a0408625a505b4454ed..dd05a378631a866c2f499c56986e5de7a703e766 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5106,9 +5106,6 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
        u32 size;
        int ret;
 
-       if (!access_ok(uattr, SCHED_ATTR_SIZE_VER0))
-               return -EFAULT;
-
        /* Zero the full structure, so that a short copy will be nice: */
        memset(attr, 0, sizeof(*attr));
 
@@ -5116,45 +5113,19 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
        if (ret)
                return ret;
 
-       /* Bail out on silly large: */
-       if (size > PAGE_SIZE)
-               goto err_size;
-
        /* ABI compatibility quirk: */
        if (!size)
                size = SCHED_ATTR_SIZE_VER0;
-
-       if (size < SCHED_ATTR_SIZE_VER0)
+       if (size < SCHED_ATTR_SIZE_VER0 || size > PAGE_SIZE)
                goto err_size;
 
-       /*
-        * If we're handed a bigger struct than we know of,
-        * ensure all the unknown bits are 0 - i.e. new
-        * user-space does not rely on any kernel feature
-        * extensions we dont know about yet.
-        */
-       if (size > sizeof(*attr)) {
-               unsigned char __user *addr;
-               unsigned char __user *end;
-               unsigned char val;
-
-               addr = (void __user *)uattr + sizeof(*attr);
-               end  = (void __user *)uattr + size;
-
-               for (; addr < end; addr++) {
-                       ret = get_user(val, addr);
-                       if (ret)
-                               return ret;
-                       if (val)
-                               goto err_size;
-               }
-               size = sizeof(*attr);
+       ret = copy_struct_from_user(attr, sizeof(*attr), uattr, size);
+       if (ret) {
+               if (ret == -E2BIG)
+                       goto err_size;
+               return ret;
        }
 
-       ret = copy_from_user(attr, uattr, size);
-       if (ret)
-               return -EFAULT;
-
        if ((attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) &&
            size < SCHED_ATTR_SIZE_VER1)
                return -EINVAL;
@@ -5354,7 +5325,7 @@ sched_attr_copy_to_user(struct sched_attr __user *uattr,
  * sys_sched_getattr - similar to sched_getparam, but with sched_attr
  * @pid: the pid in question.
  * @uattr: structure containing the extended parameters.
- * @usize: sizeof(attr) that user-space knows about, for forwards and backwards compatibility.
+ * @usize: sizeof(attr) for fwd/bwd comp.
  * @flags: for future extension.
  */
 SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,