Linux Kernels

PKCS#7: pkcs7_validate_trust(): initialize the _trusted output argument

This change “PKCS#7: pkcs7_validate_trust(): initialize the _trusted output argument” (commit e543589) in Linux kernel is authored by Nicolai Stange <nicstange [at] gmail.com> on Sun Mar 20 23:23:46 2016 +0100.

Description of "PKCS#7: pkcs7_validate_trust(): initialize the _trusted output argument"

The change “PKCS#7: pkcs7_validate_trust(): initialize the _trusted output argument” introduces changes as follows.

PKCS#7: pkcs7_validate_trust(): initialize the _trusted output argument

Despite what the DocBook comment to pkcs7_validate_trust() says, the
*_trusted argument is never set to false.

pkcs7_validate_trust() only positively sets *_trusted upon encountering
a trusted PKCS#7 SignedInfo block.

This is quite unfortunate since its callers, system_verify_data() for
example, depend on pkcs7_validate_trust() clearing *_trusted on non-trust.

Indeed, UBSAN splats when attempting to load the uninitialized local
variable 'trusted' from system_verify_data() in pkcs7_validate_trust():

  UBSAN: Undefined behaviour in crypto/asymmetric_keys/pkcs7_trust.c:194:14
  load of value 82 is not a valid value for type '_Bool'
  [...]
  Call Trace:
    [<ffffffff818c4d35>] dump_stack+0xbc/0x117
    [<ffffffff818c4c79>] ? _atomic_dec_and_lock+0x169/0x169
    [<ffffffff8194113b>] ubsan_epilogue+0xd/0x4e
    [<ffffffff819419fa>] __ubsan_handle_load_invalid_value+0x111/0x158
    [<ffffffff819418e9>] ? val_to_string.constprop.12+0xcf/0xcf
    [<ffffffff818334a4>] ? x509_request_asymmetric_key+0x114/0x370
    [<ffffffff814b83f0>] ? kfree+0x220/0x370
    [<ffffffff818312c2>] ? public_key_verify_signature_2+0x32/0x50
    [<ffffffff81835e04>] pkcs7_validate_trust+0x524/0x5f0
    [<ffffffff813c391a>] system_verify_data+0xca/0x170
    [<ffffffff813c3850>] ? top_trace_array+0x9b/0x9b
    [<ffffffff81510b29>] ? __vfs_read+0x279/0x3d0
    [<ffffffff8129372f>] mod_verify_sig+0x1ff/0x290
    [...]

The implication is that pkcs7_validate_trust() effectively grants trust
when it really shouldn't have.

Fix this by explicitly setting *_trusted to false at the very beginning
of pkcs7_validate_trust().

Cc: <stable@vger.kernel.org>
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Linux kernel releases containing commit e543589

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes from "PKCS#7: pkcs7_validate_trust(): initialize the _trusted output argument"

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 crypto/asymmetric_keys/pkcs7_trust.c | 2 ++
 1 file changed, 2 insertions(+)
 
diff --git a/crypto/asymmetric_keys/pkcs7_trust.c b/crypto/asymmetric_keys/pkcs7_trust.c
index 90d6d47965b0..ecdb5a2ce085 100644
--- a/crypto/asymmetric_keys/pkcs7_trust.c
+++ b/crypto/asymmetric_keys/pkcs7_trust.c
@@ -178,6 +178,8 @@ int pkcs7_validate_trust(struct pkcs7_message *pkcs7,
 	int cached_ret = -ENOKEY;
 	int ret;
 
+	*_trusted = false;
+
 	for (p = pkcs7->certs; p; p = p->next)
 		p->seen = false;
 

The commit for this change in Linux stable tree is e543589 (patch).

Last modified: 2020/01/11 06:53