Patch "afs: Fix dynamic root lookup DNS check" has been added to the 5.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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

    afs: Fix dynamic root lookup DNS check

to the 5.4-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:
     afs-fix-dynamic-root-lookup-dns-check.patch
and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit e5a49695056523106d7e1102341df83d45fea335
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Mon Dec 11 15:15:02 2023 +0000

    afs: Fix dynamic root lookup DNS check
    
    [ Upstream commit 74cef6872ceaefb5b6c5c60641371ea28702d358 ]
    
    In the afs dynamic root directory, the ->lookup() function does a DNS check
    on the cell being asked for and if the DNS upcall reports an error it will
    report an error back to userspace (typically ENOENT).
    
    However, if a failed DNS upcall returns a new-style result, it will return
    a valid result, with the status field set appropriately to indicate the
    type of failure - and in that case, dns_query() doesn't return an error and
    we let stat() complete with no error - which can cause confusion in
    userspace as subsequent calls that trigger d_automount then fail with
    ENOENT.
    
    Fix this by checking the status result from a valid dns_query() and
    returning an error if it indicates a failure.
    
    Fixes: bbb4c4323a4d ("dns: Allow the dns resolver to retrieve a server set")
    Reported-by: Markus Suvanto <markus.suvanto@xxxxxxxxx>
    Closes: https://bugzilla.kernel.org/show_bug.cgi?id=216637
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
    Tested-by: Markus Suvanto <markus.suvanto@xxxxxxxxx>
    cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
    cc: linux-afs@xxxxxxxxxxxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index f4f2ab6d877f3..d06994990fc35 100644
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -20,6 +20,7 @@ static int afs_probe_cell_name(struct dentry *dentry)
 	struct afs_net *net = afs_d2net(dentry);
 	const char *name = dentry->d_name.name;
 	size_t len = dentry->d_name.len;
+	char *result = NULL;
 	int ret;
 
 	/* Names prefixed with a dot are R/W mounts. */
@@ -37,9 +38,22 @@ static int afs_probe_cell_name(struct dentry *dentry)
 	}
 
 	ret = dns_query(net->net, "afsdb", name, len, "srv=1",
-			NULL, NULL, false);
-	if (ret == -ENODATA || ret == -ENOKEY)
+			&result, NULL, false);
+	if (ret == -ENODATA || ret == -ENOKEY || ret == 0)
 		ret = -ENOENT;
+	if (ret > 0 && ret >= sizeof(struct dns_server_list_v1_header)) {
+		struct dns_server_list_v1_header *v1 = (void *)result;
+
+		if (v1->hdr.zero == 0 &&
+		    v1->hdr.content == DNS_PAYLOAD_IS_SERVER_LIST &&
+		    v1->hdr.version == 1 &&
+		    (v1->status != DNS_LOOKUP_GOOD &&
+		     v1->status != DNS_LOOKUP_GOOD_WITH_BAD))
+			return -ENOENT;
+
+	}
+
+	kfree(result);
 	return ret;
 }