linux-kernel - [PATCH v6 4/8] ipc: Clamp msgmni and shmmni to the real IPCMNI limit

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Date:   Fri, 27 Apr 2018 17:00:34 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-doc@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v6 4/8] ipc: Clamp msgmni and shmmni to the real IPCMNI limit

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 in case they are writing a value
greater than 32k. 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..d71f949 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_SIGNED_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_SIGNED_RANGE,
 	},
 	{
 		.procname	= "auto_msgmni",
-- 
1.8.3.1