[PATCH v2 1/3] bpf: use check_zeroed_user() in bpf_check_uarg_tail_zero()

[Christian Brauner <christian.brauner@ubuntu.com>]

In v5.4-rc2 we added a new helper (cf. [1]) check_zeroed_user() which
does what bpf_check_uarg_tail_zero() is doing generically. We're slowly
switching such codepaths over to use check_zeroed_user() instead of
using their own hand-rolled version.

[1]: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: bpf@vger.kernel.org
Acked-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v1 */
Link: https://lore.kernel.org/r/20191009160907.10981-2-christian.brauner@ubuntu.com

/* v2 */
- Alexei Starovoitov <ast@kernel.org>:
  - Add a comment in bpf_check_uarg_tail_zero() to clarify that
    copy_struct_from_user() should be used whenever possible instead.
---
 kernel/bpf/syscall.c | 26 +++++++++++---------------
 1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 82eabd4e38ad..b289ae747cae 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -58,35 +58,31 @@ static const struct bpf_map_ops * const bpf_map_types[] = {
  * There is a ToCToU between this function call and the following
  * copy_from_user() call. However, this is not a concern since this function is
  * meant to be a future-proofing of bits.
+ *
+ * Note, instead of using bpf_check_uarg_tail_zero() followed by
+ * copy_from_user() use the dedicated copy_struct_from_user() helper which
+ * performs both tasks whenever possible.
  */
 int bpf_check_uarg_tail_zero(void __user *uaddr,
 			     size_t expected_size,
 			     size_t actual_size)
 {
-	unsigned char __user *addr;
-	unsigned char __user *end;
-	unsigned char val;
+	size_t size = min(expected_size, actual_size);
+	size_t rest = max(expected_size, actual_size) - size;
 	int err;
 
 	if (unlikely(actual_size > PAGE_SIZE))	/* silly large */
 		return -E2BIG;
 
-	if (unlikely(!access_ok(uaddr, actual_size)))
-		return -EFAULT;
-
 	if (actual_size <= expected_size)
 		return 0;
 
-	addr = uaddr + expected_size;
-	end  = uaddr + actual_size;
+	err = check_zeroed_user(uaddr + expected_size, rest);
+	if (err < 0)
+		return err;
 
-	for (; addr < end; addr++) {
-		err = get_user(val, addr);
-		if (err)
-			return err;
-		if (val)
-			return -E2BIG;
-	}
+	if (err)
+		return -E2BIG;
 
 	return 0;
 }
-- 
2.23.0