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Cleanup ANDROID_DNS_MODE and BIONIC_DNSCACHE

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For security reasons, when a binary is executed which causes a security
transition (eg, a setuid binary, setgid binary, filesystem capabilities,
or SELinux domain transition), the AT_SECURE flag is set. This causes
certain blacklisted environment variables to be stripped before the
process is executed. The list of blacklisted environment variables is
stored in UNSAFE_VARIABLE_NAMES. Generally speaking, most environment
variables used internally by libc show up in this list.

Add ANDROID_DNS_MODE to the list of unsafe variables.
Similar to RESOLV_HOST_CONF and RES_OPTIONS (which are already
blacklisted), this variable controls how name resolution requests are
handled. Allowing ANDROID_DNS_MODE to be set across a security
boundary could induce resolution failures or otherwise impact
name resolution.

Remove BIONIC_DNSCACHE. This does not appear to be used, and setting
this variable across a security boundary could cause name resolution
problems.

Test: Android compiles and runs with no obvious problems.
Change-Id: I835a7b42d6afbc9c67866594c7951cfd9b355d81
This commit is contained in:
Nick Kralevich 2017-04-01 09:54:19 -07:00
parent 5d8b8310aa
commit 62c03a4ff4
2 changed files with 1 additions and 13 deletions
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1
libc/bionic/libc_init_common.cpp
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@ -244,6 +244,7 @@ static bool __is_unsafe_environment_variable(const char* name) {
// of executing a setuid program or the result of an SELinux
// security transition.
static constexpr const char* UNSAFE_VARIABLE_NAMES[] = {
"ANDROID_DNS_MODE",
"GCONV_PATH",
"GETCONF_DIR",
"HOSTALIASES",

13
libc/dns/resolv/res_cache.c
View file

@ -98,12 +98,6 @@
* is too short to accomodate the cached result.
*/
/* the name of an environment variable that will be checked the first time
* this code is called if its value is "0", then the resolver cache is
* disabled.
*/
#define CONFIG_ENV "BIONIC_DNSCACHE"
/* default number of entries kept in the cache. This value has been
* determined by browsing through various sites and counting the number
* of corresponding requests. Keep in mind that our framework is currently
@ -1812,13 +1806,6 @@ static void _res_cache_clear_stats_locked(struct resolv_cache_info* cache_info);
static void
_res_cache_init(void)
{
const char* env = getenv(CONFIG_ENV);
if (env && atoi(env) == 0) {
/* the cache is disabled */
return;
}
memset(&_res_cache_list, 0, sizeof(_res_cache_list));
pthread_mutex_init(&_res_cache_list_lock, NULL);
}