From: Zhang Zixun <zhang133010@icloud.com>
Date: Mon, 27 Dec 2021 22:02:49 +0100
Subject: x86/mce/inject: Avoid out-of-bounds write when setting flags
Git-commit: de768416b203ac84e02a757b782a32efb388476f
Patch-mainline: v5.17-rc1
References: git-fixes

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: 0451d14d0561 ("EDAC, mce_amd_inj: Modify flags attribute to use string arguments")
Signed-off-by: Zhang Zixun <zhang133010@icloud.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/linux-edac/YcnePfF1OOqoQwrX@zn.tnic/

Acked-by: Nikolay Borisov <nik.borisov@suse.com>
---
 arch/x86/ras/mce_amd_inj.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/ras/mce_amd_inj.c
+++ b/arch/x86/ras/mce_amd_inj.c
@@ -151,7 +151,7 @@ static ssize_t flags_write(struct file *
 	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))