[GIT PULL] keys: fix NULL pointer dereference during ASN.1 parsing

[James Morris <james.l.morris@oracle.com>]

Hi Linus,

Please pull this fix by Eric Biggers for the keys subsystem.

---

The following changes since commit fbc3edf7d7731d7a22c483c679700589bab936a3:

  drivers/ide-cd: Handle missing driver data during status check gracefully (2017-11-07 09:12:04 -0800)

are available in the git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git fixes-v4.14-rc8

for you to fetch changes up to 624f5ab8720b3371367327a822c267699c1823b8:

  KEYS: fix NULL pointer dereference during ASN.1 parsing [ver #2] (2017-11-09 00:38:21 +1100)

----------------------------------------------------------------
Eric Biggers (1):
      KEYS: fix NULL pointer dereference during ASN.1 parsing [ver #2]

 lib/asn1_decoder.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

---

commit 624f5ab8720b3371367327a822c267699c1823b8
Author: Eric Biggers <ebiggers@google.com>
Date:   Tue Nov 7 22:29:02 2017 +0000

    KEYS: fix NULL pointer dereference during ASN.1 parsing [ver #2]
    
    syzkaller reported a NULL pointer dereference in asn1_ber_decoder().  It
    can be reproduced by the following command, assuming
    CONFIG_PKCS7_TEST_KEY=y:
    
            keyctl add pkcs7_test desc '' @s
    
    The bug is that if the data buffer is empty, an integer underflow occurs
    in the following check:
    
            if (unlikely(dp >= datalen - 1))
                    goto data_overrun_error;
    
    This results in the NULL data pointer being dereferenced.
    
    Fix it by checking for 'datalen - dp < 2' instead.
    
    Also fix the similar check for 'dp >= datalen - n' later in the same
    function.  That one possibly could result in a buffer overread.
    
    The NULL pointer dereference was reproducible using the "pkcs7_test" key
    type but not the "asymmetric" key type because the "asymmetric" key type
    checks for a 0-length payload before calling into the ASN.1 decoder but
    the "pkcs7_test" key type does not.
    
    The bug report was:
    
        BUG: unable to handle kernel NULL pointer dereference at           (null)
        IP: asn1_ber_decoder+0x17f/0xe60 lib/asn1_decoder.c:233
        PGD 7b708067 P4D 7b708067 PUD 7b6ee067 PMD 0
        Oops: 0000 [#1] SMP
        Modules linked in:
        CPU: 0 PID: 522 Comm: syz-executor1 Not tainted 4.14.0-rc8 #7
        Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.3-20171021_125229-anatol 04/01/2014
        task: ffff9b6b3798c040 task.stack: ffff9b6b37970000
        RIP: 0010:asn1_ber_decoder+0x17f/0xe60 lib/asn1_decoder.c:233
        RSP: 0018:ffff9b6b37973c78 EFLAGS: 00010216
        RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000000000021c
        RDX: ffffffff814a04ed RSI: ffffb1524066e000 RDI: ffffffff910759e0
        RBP: ffff9b6b37973d60 R08: 0000000000000001 R09: ffff9b6b3caa4180
        R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000002
        R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
        FS:  00007f10ed1f2700(0000) GS:ffff9b6b3ea00000(0000) knlGS:0000000000000000
        CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
        CR2: 0000000000000000 CR3: 000000007b6f3000 CR4: 00000000000006f0
        Call Trace:
         pkcs7_parse_message+0xee/0x240 crypto/asymmetric_keys/pkcs7_parser.c:139
         verify_pkcs7_signature+0x33/0x180 certs/system_keyring.c:216
         pkcs7_preparse+0x41/0x70 crypto/asymmetric_keys/pkcs7_key_type.c:63
         key_create_or_update+0x180/0x530 security/keys/key.c:855
         SYSC_add_key security/keys/keyctl.c:122 [inline]
         SyS_add_key+0xbf/0x250 security/keys/keyctl.c:62
         entry_SYSCALL_64_fastpath+0x1f/0xbe
        RIP: 0033:0x4585c9
        RSP: 002b:00007f10ed1f1bd8 EFLAGS: 00000216 ORIG_RAX: 00000000000000f8
        RAX: ffffffffffffffda RBX: 00007f10ed1f2700 RCX: 00000000004585c9
        RDX: 0000000020000000 RSI: 0000000020008ffb RDI: 0000000020008000
        RBP: 0000000000000000 R08: ffffffffffffffff R09: 0000000000000000
        R10: 0000000000000000 R11: 0000000000000216 R12: 00007fff1b2260ae
        R13: 00007fff1b2260af R14: 00007f10ed1f2700 R15: 0000000000000000
        Code: dd ca ff 48 8b 45 88 48 83 e8 01 4c 39 f0 0f 86 a8 07 00 00 e8 53 dd ca ff 49 8d 46 01 48 89 85 58 ff ff ff 48 8b 85 60 ff ff ff <42> 0f b6 0c 30 89 c8 88 8d 75 ff ff ff 83 e0 1f 89 8d 28 ff ff
        RIP: asn1_ber_decoder+0x17f/0xe60 lib/asn1_decoder.c:233 RSP: ffff9b6b37973c78
        CR2: 0000000000000000
    
    Fixes: 42d5ec27f873 ("X.509: Add an ASN.1 decoder")
    Reported-by: syzbot <syzkaller@googlegroups.com>
    Cc: <stable@vger.kernel.org> # v3.7+
    Signed-off-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: David Howells <dhowells@redhat.com>
    Signed-off-by: James Morris <james.l.morris@oracle.com>

diff --git a/lib/asn1_decoder.c b/lib/asn1_decoder.c
index fef5d2e..1ef0cec 100644
--- a/lib/asn1_decoder.c
+++ b/lib/asn1_decoder.c
@@ -228,7 +228,7 @@ int asn1_ber_decoder(const struct asn1_decoder *decoder,
 		hdr = 2;
 
 		/* Extract a tag from the data */
-		if (unlikely(dp >= datalen - 1))
+		if (unlikely(datalen - dp < 2))
 			goto data_overrun_error;
 		tag = data[dp++];
 		if (unlikely((tag & 0x1f) == ASN1_LONG_TAG))
@@ -274,7 +274,7 @@ int asn1_ber_decoder(const struct asn1_decoder *decoder,
 				int n = len - 0x80;
 				if (unlikely(n > 2))
 					goto length_too_long;
-				if (unlikely(dp >= datalen - n))
+				if (unlikely(n > datalen - dp))
 					goto data_overrun_error;
 				hdr += n;
 				for (len = 0; n > 0; n--) {