[PATCH 5.16 0359/1039] x86/mce/inject: Avoid out-of-bounds write when setting flags

[Greg Kroah-Hartman]

From: Zhang Zixun <zhang133010@xxxxxxxxxx>

[ Upstream commit de768416b203ac84e02a757b782a32efb388476f ]

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@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lore.kernel.org/linux-edac/YcnePfF1OOqoQwrX@xxxxxxx/
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/mce/inject.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mce/inject.c b/arch/x86/kernel/cpu/mce/inject.c
index 0bfc14041bbb4..b63b548497c14 100644
--- a/arch/x86/kernel/cpu/mce/inject.c
+++ b/arch/x86/kernel/cpu/mce/inject.c
@@ -350,7 +350,7 @@ 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))
-- 
2.34.1