[PATCH 3.10 302/319] KEYS: Fix short sprintf buffer in /proc/keys show function

Willy Tarreau
2017-02-05T20:06:14Z

From: David Howells <dhowells@redhat.com>

commit 03dab869b7b239c4e013ec82aea22e181e441cfc upstream.

This fixes CVE-2016-7042.

Fix a short sprintf buffer in proc_keys_show(). If the gcc stack protector
is turned on, this can cause a panic due to stack corruption.

The problem is that xbuf[] is not big enough to hold a 64-bit timeout
rendered as weeks:

(gdb) p 0xffffffffffffffffULL/(60*60*24*7)
$2 = 30500568904943

That's 14 chars plus NUL, not 11 chars plus NUL.

Expand the buffer to 16 chars.

I think the unpatched code apparently works if the stack-protector is not
enabled because on a 32-bit machine the buffer won't be overflowed and on a
64-bit machine there's a 64-bit aligned pointer at one side and an int that
isn't checked again on the other side.

The panic incurred looks something like:

Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: ffffffff81352ebe
CPU: 0 PID: 1692 Comm: reproducer Not tainted 4.7.2-201.fc24.x86_64 #1
Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
0000000000000086 00000000fbbd2679 ffff8800a044bc00 ffffffff813d941f
ffffffff81a28d58 ffff8800a044bc98 ffff8800a044bc88 ffffffff811b2cb6
ffff880000000010 ffff8800a044bc98 ffff8800a044bc30 00000000fbbd2679
Call Trace:
[<ffffffff813d941f>] dump_stack+0x63/0x84
[<ffffffff811b2cb6>] panic+0xde/0x22a
[<ffffffff81352ebe>] ? proc_keys_show+0x3ce/0x3d0
[<ffffffff8109f7f9>] __stack_chk_fail+0x19/0x30
[<ffffffff81352ebe>] proc_keys_show+0x3ce/0x3d0
[<ffffffff81350410>] ? key_validate+0x50/0x50
[<ffffffff8134db30>] ? key_default_cmp+0x20/0x20
[<ffffffff8126b31c>] seq_read+0x2cc/0x390
[<ffffffff812b6b12>] proc_reg_read+0x42/0x70
[<ffffffff81244fc7>] __vfs_read+0x37/0x150
[<ffffffff81357020>] ? security_file_permission+0xa0/0xc0
[<ffffffff81246156>] vfs_read+0x96/0x130
[<ffffffff81247635>] SyS_read+0x55/0xc0
[<ffffffff817eb872>] entry_SYSCALL_64_fastpath+0x1a/0xa4

Reported-by: Ondrej Kozina <okozina@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Ondrej Kozina <okozina@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
security/keys/proc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/keys/proc.c b/security/keys/proc.c
index 217b685..374c330 100644
--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -188,7 +188,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
struct timespec now;
unsigned long timo;
key_ref_t key_ref, skey_ref;
- char xbuf[12];
+ char xbuf[16];
int rc;

key_ref = make_key_ref(key, 0);
--
2.8.0.rc2.1.gbe9624a