Re: [PATCH] KEYS: fix NULL pointer dereference during ASN.1 parsing

[Eric Biggers]

On Tue, Nov 07, 2017 at 03:19:27PM +0000, David Howells wrote:
> From: Eric Biggers <ebiggers3@xxxxxxxxx>
> 
> 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>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>

If it's not too late can you fix the From: line to have my @google.com address
to match the Signed-off-by?

	From: Eric Biggers <ebiggers@xxxxxxxxxx>

Also feel free to trim down the register dump like you asked.

Thanks!

Eric