Re: [PATCH 7/9] Simplify bound checks in capabilities for copy_from_user

[Dave Airlie]

On Sun, Sep 27, 2009 at 4:53 AM, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
> From: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> Subject: [PATCH 7/9] Simplify bound checks in capabilities for copy_from_user
> CC: James Morris <jmorris@xxxxxxxxx>
>
> The capabilities syscall has a copy_from_user() call where gcc currently
> cannot prove to itself that the copy is always within bounds.
>
> This patch adds a very explicity bound check to prove to gcc that
> this copy_from_user cannot overflow its destination buffer.
>
> Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
>
> diff --git a/kernel/capability.c b/kernel/capability.c
> index 4e17041..204f11f 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -238,7 +241,7 @@ SYSCALL_DEFINE2(capget, cap_user_header_t, header, cap_user_data_t, dataptr)
>  SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
>  {
>        struct __user_cap_data_struct kdata[_KERNEL_CAPABILITY_U32S];
> -       unsigned i, tocopy;
> +       unsigned i, tocopy, copybytes;
>        kernel_cap_t inheritable, permitted, effective;
>        struct cred *new;
>        int ret;
> @@ -255,8 +258,11 @@ SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
>        if (pid != 0 && pid != task_pid_vnr(current))
>                return -EPERM;
>
> -       if (copy_from_user(&kdata, data,
> -                          tocopy * sizeof(struct __user_cap_data_struct)))
> +       copybytes = tocopy * sizeof(struct __user_cap_data_struct);
> +       if (copybytes > _KERNEL_CAPABILITY_U32S)
> +               return -EFAULT;

This is broken, it breaks dbus at least for me. you compare bytes
to u32s wrongly.

Dave.

> +
> +       if (copy_from_user(&kdata, data, copybytes))
>                return -EFAULT;
>
>        for (i = 0; i < tocopy; i++) {
>
>
>
> --
> Arjan van de Ven        Intel Open Source Technology Centre
> For development, discussion and tips for power savings,
> visit http://www.lesswatts.org
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