lib/oid_registry.c: X.509: fix the buffer overflow in the utility function for OID...

The sprint_oid() utility function doesn't properly check the buffer size
that it causes that the warning in vsnprintf() be triggered.  For
example on v4.1 kernel:

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 2357 at lib/vsprintf.c:1867 vsnprintf+0x5a7/0x5c0()
  ...

We can trigger this issue by injecting maliciously crafted x509 cert in
DER format.  Just using hex editor to change the length of OID to over
the length of the SEQUENCE container.  For example:

    0:d=0  hl=4 l= 980 cons: SEQUENCE
    4:d=1  hl=4 l= 700 cons:  SEQUENCE
    8:d=2  hl=2 l=   3 cons:   cont [ 0 ]
   10:d=3  hl=2 l=   1 prim:    INTEGER           :02
   13:d=2  hl=2 l=   9 prim:   INTEGER           :9B47FAF791E7D1E3
   24:d=2  hl=2 l=  13 cons:   SEQUENCE
   26:d=3  hl=2 l=   9 prim:    OBJECT            :sha256WithRSAEncryption
   37:d=3  hl=2 l=   0 prim:    NULL
   39:d=2  hl=2 l= 121 cons:   SEQUENCE
   41:d=3  hl=2 l=  22 cons:    SET
   43:d=4  hl=2 l=  20 cons:     SEQUENCE      <=== the SEQ length is 20
   45:d=5  hl=2 l=   3 prim:      OBJECT            :organizationName
	<=== the original length is 3, change the length of OID to over the length of SEQUENCE

Pawel Wieczorkiewicz reported this problem and Takashi Iwai provided
patch to fix it by checking the bufsize in sprint_oid().

Link: http://lkml.kernel.org/r/20170903021646.2080-1-jlee@suse.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: "Lee, Chun-Yi" <jlee@suse.com>
Reported-by: Pawel Wieczorkiewicz <pwieczorkiewicz@suse.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Pawel Wieczorkiewicz <pwieczorkiewicz@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/oid_registry.c b/lib/oid_registry.c
index 318f382a010d62f0f6b0c536ee038f41100e57cf..41b9e50711a72af31d7548d3aea5be1177a45a9d 100644 (file)
--- a/lib/oid_registry.c
+++ b/lib/oid_registry.c
@@ -142,9 +142,9 @@ int sprint_oid(const void *data, size_t datasize, char *buffer, size_t bufsize)
                }
                ret += count = snprintf(buffer, bufsize, ".%lu", num);
                buffer += count;
-               bufsize -= count;
-               if (bufsize == 0)
+               if (bufsize <= count)
                        return -ENOBUFS;
+               bufsize -= count;
        }
 
        return ret;