Re: general protection fault in asn1_ber_decoder

[Eric Biggers]

On Mon, Nov 06, 2017 at 10:05:45PM +0000, David Howells wrote:
> diff --git a/lib/asn1_decoder.c b/lib/asn1_decoder.c
> index fef5d2e114be..048de2c20ae9 100644
> --- a/lib/asn1_decoder.c
> +++ b/lib/asn1_decoder.c
> @@ -201,6 +201,13 @@ int asn1_ber_decoder(const struct asn1_decoder *decoder,
>  	if (datalen > 65535)
>  		return -EMSGSIZE;
>  
> +	/* We don't currently support 0-length messages - the underrun checks
> +	 * will fail if datalen is 0 because we check against datalen - 1 with
> +	 * unsigned arithmetic.
> +	 */
> +	if (datalen == 0)
> +		return -EBADMSG;
> +

Hi David, you just beat me to it, but I don't think this is the best way to fix
the problem.  The length check just needs to be rewritten to not overflow.  Also
it seems there is another broken length check later in the function.  How about
this:

commit 8bbe85872c660c891eb978a037f590198319e3b2
Author: Eric Biggers <ebiggers@xxxxxxxxxx>
Date:   Mon Nov 6 10:06:32 2017 -0800

    KEYS: fix NULL pointer dereference during ASN.1 parsing
    
    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@xxxxxxxxxxxxxxxx>
    Cc: <stable@xxxxxxxxxxxxxxx> # v3.7+
    Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

diff --git a/lib/asn1_decoder.c b/lib/asn1_decoder.c
index fef5d2e114be..1ef0cec38d78 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--) {