NFSv4: Fix possible 1-byte stack overflow in nfs_idmap_read_and_verify_message

In nfs_idmap_read_and_verify_message there is an incorrect sprintf '%d' that converts the __u32 'im_id' from struct idmap_msg to 'id_str', which is a stack char array variable of length NFS_UINT_MAXLEN == 11. If a uid or gid value is > 2147483647 = 0x7fffffff, the conversion overflows into a negative value, for example:
crash> p (unsigned) (0x80000000)
$1 = 2147483648
crash> p (signed) (0x80000000)
$2 = -2147483648
The '-' sign is written to the buffer and this causes a 1 byte overflow when the NULL byte is written, which corrupts kernel stack memory. If CONFIG_CC_STACKPROTECTOR_STRONG is set we see a stack-protector panic:

[11558053.616565] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: ffffffffa05b8a8c
[11558053.639063] CPU: 6 PID: 9423 Comm: rpc.idmapd Tainted: G        W      ------------ T 3.10.0-514.el7.x86_64 #1
[11558053.641990] Hardware name: Red Hat OpenStack Compute, BIOS 1.10.2-3.el7_4.1 04/01/2014
[11558053.644462]  ffffffff818c7bc0 00000000b1f3aec1 ffff880de0f9bd48 ffffffff81685eac
[11558053.646430]  ffff880de0f9bdc8 ffffffff8167f2b3 ffffffff00000010 ffff880de0f9bdd8
[11558053.648313]  ffff880de0f9bd78 00000000b1f3aec1 ffffffff811dcb03 ffffffffa05b8a8c
[11558053.650107] Call Trace:
[11558053.651347]  [<ffffffff81685eac>] dump_stack+0x19/0x1b
[11558053.653013]  [<ffffffff8167f2b3>] panic+0xe3/0x1f2
[11558053.666240]  [<ffffffff811dcb03>] ? kfree+0x103/0x140
[11558053.682589]  [<ffffffffa05b8a8c>] ? idmap_pipe_downcall+0x1cc/0x1e0 [nfsv4]
[11558053.689710]  [<ffffffff810855db>] __stack_chk_fail+0x1b/0x30
[11558053.691619]  [<ffffffffa05b8a8c>] idmap_pipe_downcall+0x1cc/0x1e0 [nfsv4]
[11558053.693867]  [<ffffffffa00209d6>] rpc_pipe_write+0x56/0x70 [sunrpc]
[11558053.695763]  [<ffffffff811fe12d>] vfs_write+0xbd/0x1e0
[11558053.702236]  [<ffffffff810acccc>] ? task_work_run+0xac/0xe0
[11558053.704215]  [<ffffffff811fec4f>] SyS_write+0x7f/0xe0
[11558053.709674]  [<ffffffff816964c9>] system_call_fastpath+0x16/0x1b

Fix this by calling the internally defined nfs_map_numeric_to_string() function which properly uses '%u' to convert this __u32. For consistency, also replace the one other place where snprintf is called.

---
 fs/nfs/nfs4idmap.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

index 22dc30a679a0..b6f9d84ba19b 100644
@@ -343,7 +343,7 @@ static ssize_t nfs_idmap_lookup_name(__u32 id, const char *type, char *buf,
 	int id_len;
 	ssize_t ret;
 
-	id_len = snprintf(id_str, sizeof(id_str), "%u", id);
+	id_len = nfs_map_numeric_to_string(id, id_str, sizeof(id_str));
 	ret = nfs_idmap_get_key(id_str, id_len, type, buf, buflen, idmap);
 	if (ret < 0)
 		return -EINVAL;

@@ -627,7 +627,8 @@ static int nfs_idmap_read_and_verify_message(struct idmap_msg *im,
 		if (strcmp(upcall->im_name, im->im_name) != 0)
 			break;
 		/* Note: here we store the NUL terminator too */
-		len = sprintf(id_str, "%d", im->im_id) + 1;
+		len = 1 + nfs_map_numeric_to_string(im->im_id, id_str,
+						    sizeof(id_str));
 		ret = nfs_idmap_instantiate(key, authkey, id_str, len);
 		break;
 	case IDMAP_CONV_IDTONAME:

2.14.3