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Message-ID: <202308301222.3BD87A6@keescook>
Date: Wed, 30 Aug 2023 12:27:04 -0700
From: Kees Cook <keescook@...omium.org>
To: Azeem Shaikh <azeemshaikh38@...il.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jirislaby@...nel.org>,
linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-serial@...r.kernel.org,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] vt: Fix potential read overflow of kernel memory
On Wed, Aug 30, 2023 at 04:04:10PM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.
Perhaps add:
"... and returns the size of the source string, not the destination
string, which can be accidentally misused."
> This read may exceed the destination size limit if
> a source string is not NUL-terminated [1].
>
> The copy_to_user() call uses @len returned from strlcpy() directly
> without checking its value. This could potentially lead to read
> overflow.
Since the code as written today is "accidentally correct", it's worth
clarifying this: there is no existing bug, but as written it is very
fragile and specifically uses a strlcpy() result without sanity checking
and using it to copy to userspace. (This is the exact anti-pattern for
strlcpy(), and only because the source strings are known good is this
safe.)
> In an effort to remove strlcpy() completely [2], replace
> strlcpy() here with strscpy().
>
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> [2] https://github.com/KSPP/linux/issues/89
>
> Signed-off-by: Azeem Shaikh <azeemshaikh38@...il.com>
> ---
> drivers/tty/vt/keyboard.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index 358f216c6cd6..15359c328a23 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -2079,12 +2079,15 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
> return -ENOMEM;
>
> spin_lock_irqsave(&func_buf_lock, flags);
> - len = strlcpy(kbs, func_table[kb_func] ? : "", len);
> + len = strscpy(kbs, func_table[kb_func] ? : "", len);
> spin_unlock_irqrestore(&func_buf_lock, flags);
>
> + if (len < 0) {
> + ret = -EFAULT;
I think this should be -ENOSPC as EFAULT implies an actual memory fault.
> + break;
> + }
> ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
> -EFAULT : 0;
> -
> break;
> }
> case KDSKBSENT:
> --
> 2.42.0.rc2.253.gd59a3bf2b4-goog
>
>
Thanks for sticking with these refactorings; we're almost free from
strlcpy. :)
-Kees
--
Kees Cook
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