[PATCH 5.4 097/320] x86/mce/inject: Avoid out-of-bounds write when setting flags

Greg Kroah-Hartman
2022-01-24T19:32:51Z

From: Zhang Zixun <zhang133010@icloud.com>

[ 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@icloud.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/linux-edac/YcnePfF1OOqoQwrX@zn.tnic/
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 eb2d41c1816d6..e1fda5b19b6f6 100644
--- a/arch/x86/kernel/cpu/mce/inject.c
+++ b/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))
--
2.34.1