Fix potential double free in cert writing code

In case an entry with the given OID already exists in the list passed to
mbedtls_asn1_store_named_data() and there is not enough memory to allocate
room for the new value, the existing entry will be freed but the preceding
entry in the list will sill hold a pointer to it. (And the following entries
in the list are no longer reachable.) This results in memory leak or a double

The issue is we want to leave the list in a consistent state on allocation
failure. (We could add a warning that the list is left in inconsistent state
when the function returns NULL, but behaviour changes that require more care
from the user are undesirable, especially in a stable branch.)

The chosen solution is a bit inefficient in that there is a time where both
blocks are allocated, but at least it's safe and this should trump efficiency
here: this code is only used for generating certificates, which is unlikely to
be done on very constrained devices, or to be in the critical loop of
anything. Also, the sizes involved should be fairly small anyway.

fixes #367
parent 1630888a
mbed TLS ChangeLog (Sorted per branch, date)
= mbed TLS 2.2.1 released 2015-12-xx
* Fix potential double free when mbedtls_asn1_store_named_data() fails to
allocate memory. Only used for certificate generation, not triggerable
remotely in SSL/TLS. Found by Rafał Przywara. #367
= mbed TLS 2.2.0 released 2015-11-04
......@@ -339,19 +339,18 @@ mbedtls_asn1_named_data *mbedtls_asn1_store_named_data( mbedtls_asn1_named_data
else if( cur->val.len < val_len )
// Enlarge existing value buffer if needed
mbedtls_free( cur->val.p );
cur->val.p = NULL;
* Enlarge existing value buffer if needed
* Preserve old data until the allocation succeeded, to leave list in
* a consistent state in case allocation fails.
void *p = mbedtls_calloc( 1, val_len );
if( p == NULL )
return( NULL );
mbedtls_free( cur->val.p );
cur->val.p = p;
cur->val.len = val_len;
cur->val.p = mbedtls_calloc( 1, val_len );
if( cur->val.p == NULL )
mbedtls_free( cur->oid.p );
mbedtls_free( cur );
return( NULL );
if( val != NULL )
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