From d2890c3778b164fde587bc16583f3a1c87233ec5 Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Sun, 26 Nov 2017 23:16:49 -0800
Subject: [PATCH] crypto: rsa - fix buffer overread when stripping leading zeroes
Git-commit: d2890c3778b164fde587bc16583f3a1c87233ec5
Patch-mainline: v4.15-rc4
References: bsc#1051510

In rsa_get_n(), if the buffer contained all 0's and "FIPS mode" is
enabled, we would read one byte past the end of the buffer while
scanning the leading zeroes.  Fix it by checking 'n_sz' before '!*ptr'.

This bug was reachable by adding a specially crafted key of type
"asymmetric" (requires CONFIG_RSA and CONFIG_X509_CERTIFICATE_PARSER).

KASAN report:

    BUG: KASAN: slab-out-of-bounds in rsa_get_n+0x19e/0x1d0 crypto/rsa_helper.c:33
    Read of size 1 at addr ffff88003501a708 by task keyctl/196

    CPU: 1 PID: 196 Comm: keyctl Not tainted 4.14.0-09238-g1d3b78bbc6e9 #26
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-20171110_100015-anatol 04/01/2014
    Call Trace:
     rsa_get_n+0x19e/0x1d0 crypto/rsa_helper.c:33
     asn1_ber_decoder+0x82a/0x1fd0 lib/asn1_decoder.c:328
     rsa_set_pub_key+0xd3/0x320 crypto/rsa.c:278
     crypto_akcipher_set_pub_key ./include/crypto/akcipher.h:364 [inline]
     pkcs1pad_set_pub_key+0xae/0x200 crypto/rsa-pkcs1pad.c:117
     crypto_akcipher_set_pub_key ./include/crypto/akcipher.h:364 [inline]
     public_key_verify_signature+0x270/0x9d0 crypto/asymmetric_keys/public_key.c:106
     x509_check_for_self_signed+0x2ea/0x480 crypto/asymmetric_keys/x509_public_key.c:141
     x509_cert_parse+0x46a/0x620 crypto/asymmetric_keys/x509_cert_parser.c:129
     x509_key_preparse+0x61/0x750 crypto/asymmetric_keys/x509_public_key.c:174
     asymmetric_key_preparse+0xa4/0x150 crypto/asymmetric_keys/asymmetric_type.c:388
     key_create_or_update+0x4d4/0x10a0 security/keys/key.c:850
     SYSC_add_key security/keys/keyctl.c:122 [inline]
     SyS_add_key+0xe8/0x290 security/keys/keyctl.c:62
     entry_SYSCALL_64_fastpath+0x1f/0x96

    Allocated by task 196:
     __do_kmalloc mm/slab.c:3711 [inline]
     __kmalloc_track_caller+0x118/0x2e0 mm/slab.c:3726
     kmemdup+0x17/0x40 mm/util.c:118
     kmemdup ./include/linux/string.h:414 [inline]
     x509_cert_parse+0x2cb/0x620 crypto/asymmetric_keys/x509_cert_parser.c:106
     x509_key_preparse+0x61/0x750 crypto/asymmetric_keys/x509_public_key.c:174
     asymmetric_key_preparse+0xa4/0x150 crypto/asymmetric_keys/asymmetric_type.c:388
     key_create_or_update+0x4d4/0x10a0 security/keys/key.c:850
     SYSC_add_key security/keys/keyctl.c:122 [inline]
     SyS_add_key+0xe8/0x290 security/keys/keyctl.c:62
     entry_SYSCALL_64_fastpath+0x1f/0x96

Fixes: 5a7de97309f5 ("crypto: rsa - return raw integers for the ASN.1 parser")
Cc: <stable@vger.kernel.org> # v4.8+
Cc: Tudor Ambarus <tudor-dan.ambarus@nxp.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 crypto/rsa_helper.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/crypto/rsa_helper.c
+++ b/crypto/rsa_helper.c
@@ -30,7 +30,7 @@ int rsa_get_n(void *context, size_t hdrl
 		return -EINVAL;
 
 	if (fips_enabled) {
-		while (!*ptr && n_sz) {
+		while (n_sz && !*ptr) {
 			ptr++;
 			n_sz--;
 		}