linux-kernel - Re: [PATCH v4 4/6] ipc: Clamp msgmni and shmmni to the real IPCMNI limit

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Date:   Tue, 13 Mar 2018 13:17:24 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
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 v4 4/6] ipc: Clamp msgmni and shmmni to the real IPCMNI limit

Waiman Long <longman@...hat.com> writes:

> 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.

Which applications examples please.

I am seeing this patchset late but it looks like a whole lot of changes
to avoid a theoretical possibility.

Changes that have an impact on more than just the ipc code you are
patching.

That makes me feel very uncomfortable with these changes.

Eric


> 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 | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c
> index 8ad93c2..1955dd4 100644
> --- a/ipc/ipc_sysctl.c
> +++ b/ipc/ipc_sysctl.c
> @@ -99,6 +99,7 @@ static int proc_ipc_auto_msgmni(struct ctl_table *table, int write,
>  static int zero;
>  static int one = 1;
>  static int int_max = INT_MAX;
> +static int ipc_mni = IPCMNI;
>  
>  static struct ctl_table ipc_kern_table[] = {
>  	{
> @@ -120,7 +121,10 @@ static int proc_ipc_auto_msgmni(struct ctl_table *table, int write,
>  		.data		= &init_ipc_ns.shm_ctlmni,
>  		.maxlen		= sizeof(init_ipc_ns.shm_ctlmni),
>  		.mode		= 0644,
> -		.proc_handler	= proc_ipc_dointvec,
> +		.proc_handler	= proc_ipc_dointvec_minmax,
> +		.extra1		= &zero,
> +		.extra2		= &ipc_mni,
> +		.flags		= CTL_FLAGS_CLAMP_RANGE,
>  	},
>  	{
>  		.procname	= "shm_rmid_forced",
> @@ -147,7 +151,8 @@ static int proc_ipc_auto_msgmni(struct ctl_table *table, int write,
>  		.mode		= 0644,
>  		.proc_handler	= proc_ipc_dointvec_minmax,
>  		.extra1		= &zero,
> -		.extra2		= &int_max,
> +		.extra2		= &ipc_mni,
> +		.flags		= CTL_FLAGS_CLAMP_RANGE,
>  	},
>  	{
>  		.procname	= "auto_msgmni",