linux-kernel - [PATCH 4.2 004/124] netlink: fix locking around NETLINK_LIST

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Message-Id: <20151207144919.893441818@linuxfoundation.org>
Date:	Mon,  7 Dec 2015 09:54:54 -0500
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
	David Herrmann <dh.herrmann@...il.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.2 004/124] netlink: fix locking around NETLINK_LIST_MEMBERSHIPS

4.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Herrmann <dh.herrmann@...il.com>

[ Upstream commit 47191d65b647af5eb5c82ede70ed4c24b1e93ef4 ]

Currently, NETLINK_LIST_MEMBERSHIPS grabs the netlink table while copying
the membership state to user-space. However, grabing the netlink table is
effectively a write_lock_irq(), and as such we should not be triggering
page-faults in the critical section.

This can be easily reproduced by the following snippet:
    int s = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    void *p = mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
    int r = getsockopt(s, 0x10e, 9, p, (void*)((char*)p + 4092));

This should work just fine, but currently triggers EFAULT and a possible
WARN_ON below handle_mm_fault().

Fix this by reducing locking of NETLINK_LIST_MEMBERSHIPS to a read-side
lock. The write-lock was overkill in the first place, and the read-lock
allows page-faults just fine.

Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: David Herrmann <dh.herrmann@...il.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/netlink/af_netlink.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2346,7 +2346,7 @@ static int netlink_getsockopt(struct soc
 		int pos, idx, shift;
 
 		err = 0;
-		netlink_table_grab();
+		netlink_lock_table();
 		for (pos = 0; pos * 8 < nlk->ngroups; pos += sizeof(u32)) {
 			if (len - pos < sizeof(u32))
 				break;
@@ -2361,7 +2361,7 @@ static int netlink_getsockopt(struct soc
 		}
 		if (put_user(ALIGN(nlk->ngroups / 8, sizeof(u32)), optlen))
 			err = -EFAULT;
-		netlink_table_ungrab();
+		netlink_unlock_table();
 		break;
 	}
 	default:


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