Patch "netlink: fix locking around NETLINK_LIST_MEMBERSHIPS" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

netlink: fix locking around NETLINK_LIST_MEMBERSHIPS

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
netlink-fix-locking-around-netlink_list_memberships.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Herrmann <[email protected]>
Date: Wed, 21 Oct 2015 11:47:43 +0200
Subject: netlink: fix locking around NETLINK_LIST_MEMBERSHIPS

From: David Herrmann <[email protected]>

[ 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.

---
 net/netlink/af_netlink.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -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:

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/netlink-fix-locking-around-netlink_list_memberships.patch