[PATCH 3.16 103/366] NFSv4: Fix possible 1-byte stack overflow in nfs_idmap_read_and_verify_message

Ben Hutchings
2018-11-11T19:49:05Z

From: Dave Wysochanski <dwysocha@redhat.com>

commit d68894800ec5712d7ddf042356f11e36f87d7f78 upstream.

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.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Reported-by: Stephen Johnston <sjohnsto@redhat.com>
Fixes: cf4ab538f1516 ("NFSv4: Fix the string length returned by the idmapper")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/nfs/idmap.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/fs/nfs/idmap.c
+++ b/fs/nfs/idmap.c
@@ -339,7 +339,7 @@ static ssize_t nfs_idmap_lookup_name(__u
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;
@@ -636,7 +636,8 @@ static int nfs_idmap_read_and_verify_mes
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;