linux-kernel - Re: [PATCH v3 5/6] ipc: Clamp msgmni and shmmni to the real IPCMNI limit

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Date:   Thu, 8 Mar 2018 18:14:04 +0000
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 5/6] ipc: Clamp msgmni and shmmni to the real IPCMNI
 limit

On Thu, Mar 01, 2018 at 12:43:39PM -0500, Waiman Long wrote:
> A user can write arbitrary integer values to msgmni and shmmni sysctl
> parameters without getting error, but the actual limit is really
> IPCMNI (32k). This can mislead users as they think they can get a
> value that is not real.
> 
> Enforcing the limit by failing the sysctl parameter write, however,
> can break existing user applications. Instead, the range clamping flag
> is set to enforce the limit without failing existing user code. Users
> can easily figure out if the sysctl parameter value is out of range
> by either reading back the parameter value or checking the kernel
> ring buffer for warning.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  ipc/ipc_sysctl.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c
> index 8ad93c2..8eb7268 100644
> --- a/ipc/ipc_sysctl.c
> +++ b/ipc/ipc_sysctl.c
> @@ -41,12 +41,21 @@ static int proc_ipc_dointvec(struct ctl_table *table, int write,
>  static int proc_ipc_dointvec_minmax(struct ctl_table *table, int write,
>  	void __user *buffer, size_t *lenp, loff_t *ppos)
>  {
> +	int ret;
>  	struct ctl_table ipc_table;
>  
>  	memcpy(&ipc_table, table, sizeof(ipc_table));
>  	ipc_table.data = get_ipc(table);
>  
> -	return proc_dointvec_minmax(&ipc_table, write, buffer, lenp, ppos);
> +	ret = proc_dointvec_minmax(&ipc_table, write, buffer, lenp, ppos);
> +
> +	/*
> +	 * Copy back the CTL_FLAGS_OOR_WARNED flag which may be set in
> +	 * the temporary ctl_table entry.
> +	 */
> +	table->flags |= (ipc_table.flags & CTL_FLAGS_OOR_WARNED);

Again, why is this needed? Cant' we do this for the developer somehow?
Seems fragile, and if we can do it why not?

  Luis