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126 lines
4.7 KiB
Diff
126 lines
4.7 KiB
Diff
From 0d62e9dd6da45bbf0f33a8617afc5fe774c8f45f Mon Sep 17 00:00:00 2001
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From: David Howells <dhowells@redhat.com>
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Date: Wed, 5 Aug 2015 12:54:46 +0100
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Subject: ASN.1: Fix non-match detection failure on data overrun
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If the ASN.1 decoder is asked to parse a sequence of objects, non-optional
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matches get skipped if there's no more data to be had rather than a
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data-overrun error being reported.
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This is due to the code segment that decides whether to skip optional
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matches (ie. matches that could get ignored because an element is marked
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OPTIONAL in the grammar) due to a lack of data also skips non-optional
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elements if the data pointer has reached the end of the buffer.
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This can be tested with the data decoder for the new RSA akcipher algorithm
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that takes three non-optional integers. Currently, it skips the last
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integer if there is insufficient data.
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Without the fix, #defining DEBUG in asn1_decoder.c will show something
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like:
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next_op: pc=0/13 dp=0/270 C=0 J=0
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- match? 30 30 00
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- TAG: 30 266 CONS
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next_op: pc=2/13 dp=4/270 C=1 J=0
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- match? 02 02 00
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- TAG: 02 257
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- LEAF: 257
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next_op: pc=5/13 dp=265/270 C=1 J=0
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- match? 02 02 00
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- TAG: 02 3
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- LEAF: 3
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next_op: pc=8/13 dp=270/270 C=1 J=0
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next_op: pc=11/13 dp=270/270 C=1 J=0
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- end cons t=4 dp=270 l=270/270
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The next_op line for pc=8/13 should be followed by a match line.
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This is not exploitable for X.509 certificates by means of shortening the
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message and fixing up the ASN.1 CONS tags because:
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(1) The relevant records being built up are cleared before use.
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(2) If the message is shortened sufficiently to remove the public key, the
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ASN.1 parse of the RSA key will fail quickly due to a lack of data.
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(3) Extracted signature data is either turned into MPIs (which cope with a
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0 length) or is simpler integers specifying algoritms and suchlike
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(which can validly be 0); and
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(4) The AKID and SKID extensions are optional and their removal is handled
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without risking passing a NULL to asymmetric_key_generate_id().
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(5) If the certificate is truncated sufficiently to remove the subject,
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issuer or serialNumber then the ASN.1 decoder will fail with a 'Cons
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stack underflow' return.
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This is not exploitable for PKCS#7 messages by means of removal of elements
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from such a message from the tail end of a sequence:
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(1) Any shortened X.509 certs embedded in the PKCS#7 message are survivable
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as detailed above.
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(2) The message digest content isn't used if it shows a NULL pointer,
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similarly, the authattrs aren't used if that shows a NULL pointer.
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(3) A missing signature results in a NULL MPI - which the MPI routines deal
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with.
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(4) If data is NULL, it is expected that the message has detached content and
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that is handled appropriately.
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(5) If the serialNumber is excised, the unconditional action associated
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with it will pick up the containing SEQUENCE instead, so no NULL
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pointer will be seen here.
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If both the issuer and the serialNumber are excised, the ASN.1 decode
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will fail with an 'Unexpected tag' return.
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In either case, there's no way to get to asymmetric_key_generate_id()
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with a NULL pointer.
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(6) Other fields are decoded to simple integers. Shortening the message
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to omit an algorithm ID field will cause checks on this to fail early
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in the verification process.
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This can also be tested by snipping objects off of the end of the ASN.1 stream
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such that mandatory tags are removed - or even from the end of internal
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SEQUENCEs. If any mandatory tag is missing, the error EBADMSG *should* be
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produced. Without this patch ERANGE or ENOPKG might be produced or the parse
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may apparently succeed, perhaps with ENOKEY or EKEYREJECTED being produced
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later, depending on what gets snipped.
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Just snipping off the final BIT_STRING or OCTET_STRING from either sample
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should be a start since both are mandatory and neither will cause an EBADMSG
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without the patches
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Reported-by: Marcel Holtmann <marcel@holtmann.org>
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Signed-off-by: David Howells <dhowells@redhat.com>
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Tested-by: Marcel Holtmann <marcel@holtmann.org>
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Reviewed-by: David Woodhouse <David.Woodhouse@intel.com>
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---
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lib/asn1_decoder.c | 5 ++---
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1 file changed, 2 insertions(+), 3 deletions(-)
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diff --git a/lib/asn1_decoder.c b/lib/asn1_decoder.c
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index 55980d7..3f74dd3 100644
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--- a/lib/asn1_decoder.c
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+++ b/lib/asn1_decoder.c
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@@ -210,9 +210,8 @@ next_op:
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unsigned char tmp;
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/* Skip conditional matches if possible */
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- if ((op & ASN1_OP_MATCH__COND &&
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- flags & FLAG_MATCHED) ||
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- dp == datalen) {
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+ if ((op & ASN1_OP_MATCH__COND && flags & FLAG_MATCHED) ||
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+ (op & ASN1_OP_MATCH__SKIP && dp == datalen)) {
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flags &= ~FLAG_LAST_MATCHED;
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pc += asn1_op_lengths[op];
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goto next_op;
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--
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cgit v1.1
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