From: Al Viro <viro@zeniv.linux.org.uk>
Date: Fri, 8 May 2020 00:16:31 -0400
Subject: bpf: make bpf_check_uarg_tail_zero() use check_zeroed_user()
Patch-mainline: v5.8-rc1
Git-commit: b7e4b65f3fe92abbf4a1f57987a54c820969aebd
References: bsc#1177028

... rather than open-coding it, and badly, at that.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Gary Lin <glin@suse.com>
---
 kernel/bpf/syscall.c |   25 ++++++-------------------
 1 file changed, 6 insertions(+), 19 deletions(-)

--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -73,32 +73,19 @@ int bpf_check_uarg_tail_zero(void __user
 			     size_t expected_size,
 			     size_t actual_size)
 {
-	unsigned char __user *addr;
-	unsigned char __user *end;
-	unsigned char val;
-	int err;
+	unsigned char __user *addr = uaddr + expected_size;
+	int res;
 
 	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;
-
-	for (; addr < end; addr++) {
-		err = get_user(val, addr);
-		if (err)
-			return err;
-		if (val)
-			return -E2BIG;
-	}
-
-	return 0;
+	res = check_zeroed_user(addr, actual_size - expected_size);
+	if (res < 0)
+		return res;
+	return res ? 0 : -E2BIG;
 }
 
 const struct bpf_map_ops bpf_map_offload_ops = {