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>

1 files changed, 7 insertions, 36 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7880f4f64d0e..dd05a378631a 100644
--- 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,