fix(auth): avoid out-of-bounds read in auth_nvctr()
auth_nvctr() does not check that the buffer provided is long enough to
hold an ASN.1 INTEGER, or even that the buffer is non-empty. Since
auth_nvctr() will only ever read 6 bytes, it is possible to read up to
6 bytes past the end of the buffer.
This out-of-bounds read turns out to be harmless. The only caller of
auth_nvctr() always passes a pointer into an X.509 TBSCertificate, and
all in-tree chains of trust require that the certificate’s signature has
already been validated. This means that the signature algorithm
identifier is at least 4 bytes and the signature itself more than that.
Therefore, the data read will be from the certificate itself. Even if
the certificate signature has not been validated, an out-of-bounds read
is still not possible. Since there are at least two bytes (tag and
length) in both the signature algorithm ID and the signature itself, an
out-of-bounds read would require that the tag byte of the signature
algorithm ID would need to be either the tag or length byte of the
DER-encoded nonvolatile counter. However, this byte must be
(MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE) (0x30), which is
greater than 4 and not equal to MBEDTLS_ASN1_INTEGER (2). Therefore,
auth_nvctr() will error out before reading the integer itself,
preventing an out-of-bounds read.
Change-Id: Ibdf1af702fbeb98a94c0c96456ebddd3d392ad44
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
diff --git a/drivers/auth/auth_mod.c b/drivers/auth/auth_mod.c
index fa9509a..1bf03d4 100644
--- a/drivers/auth/auth_mod.c
+++ b/drivers/auth/auth_mod.c
@@ -243,7 +243,7 @@
unsigned int *cert_nv_ctr,
bool *need_nv_ctr_upgrade)
{
- char *p;
+ unsigned char *p;
void *data_ptr = NULL;
unsigned int data_len, len, i;
unsigned int plat_nv_ctr;
@@ -258,16 +258,24 @@
/* Parse the DER encoded integer */
assert(data_ptr);
- p = (char *)data_ptr;
- if (*p != ASN1_INTEGER) {
+ p = (unsigned char *)data_ptr;
+
+ /*
+ * Integers must be at least 3 bytes: 1 for tag, 1 for length, and 1
+ * for value. The first byte (tag) must be ASN1_INTEGER.
+ */
+ if ((data_len < 3) || (*p != ASN1_INTEGER)) {
/* Invalid ASN.1 integer */
return 1;
}
p++;
- /* NV-counters are unsigned integers up to 32-bit */
- len = (unsigned int)(*p & 0x7f);
- if ((*p & 0x80) || (len > 4)) {
+ /*
+ * NV-counters are unsigned integers up to 31 bits. Trailing
+ * padding is not allowed.
+ */
+ len = (unsigned int)*p;
+ if ((len > 4) || (data_len - 2 != len)) {
return 1;
}
p++;