x86/mce/inject: Avoid out-of-bounds write when setting flags

stable inclusion
from stable-v5.10.94
commit 595e1ec55b307d232f8672ccbe6c84089b277b43
bugzilla: https://gitee.com/openeuler/kernel/issues/I531X9

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=595e1ec55b307d232f8672ccbe6c84089b277b43

--------------------------------

[ Upstream commit de768416 ]

A contrived zero-length write, for example, by using write(2):

  ...
  ret = write(fd, str, 0);
  ...

to the "flags" file causes:

  BUG: KASAN: stack-out-of-bounds in flags_write
  Write of size 1 at addr ffff888019be7ddf by task writefile/3787

  CPU: 4 PID: 3787 Comm: writefile Not tainted 5.16.0-rc7+ #12
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014

due to accessing buf one char before its start.

Prevent such out-of-bounds access.

  [ bp: Productize into a proper patch. Link below is the next best
    thing because the original mail didn't get archived on lore. ]

Fixes: 0451d14d ("EDAC, mce_amd_inj: Modify flags attribute to use string arguments")
Signed-off-by: NZhang Zixun <zhang133010@icloud.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/linux-edac/YcnePfF1OOqoQwrX@zn.tnic/Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>

arch/x86/kernel/cpu/mce/inject.c

@@ -347,7 +347,7 @@ static ssize_t flags_write(struct file *filp, const char __user *ubuf,
 	char buf[MAX_FLAG_OPT_SIZE], *__buf;
 	int err;
 
-	if (cnt > MAX_FLAG_OPT_SIZE)
+	if (!cnt || cnt > MAX_FLAG_OPT_SIZE)
 		return -EINVAL;
 
 	if (copy_from_user(&buf, ubuf, cnt))