elf: Ignore GLIBC_TUNABLES for setuid/setgid binaries

The tunable privilege levels were a retrofit to try and keep the malloc
tunable environment variables' behavior unchanged across security
boundaries.  However, CVE-2023-4911 shows how tricky can be
tunable parsing in a security-sensitive environment.

Not only parsing, but the malloc tunable essentially changes some
semantics on setuid/setgid processes.  Although it is not a direct
security issue, allowing users to change setuid/setgid semantics is not
a good security practice, and requires extra code and analysis to check
if each tunable is safe to use on all security boundaries.

It also means that security opt-in features, like aarch64 MTE, would
need to be explicit enabled by an administrator with a wrapper script
or with a possible future system-wide tunable setting.

Co-authored-by: Siddhesh Poyarekar  <siddhesh@sourceware.org>
Reviewed-by: DJ Delorie <dj@redhat.com>

1 files changed, 0 insertions, 9 deletions

diff --git a/manual/README.tunables b/manual/README.tunables
index 605ddd78cd..72ae00dc02 100644
--- a/manual/README.tunables
+++ b/manual/README.tunables
@@ -59,15 +59,6 @@ The list of allowed attributes are:
 
 - env_alias:		An alias environment variable
 
-- security_level:	Specify security level of the tunable for AT_SECURE
-			binaries.  Valid values are:
-
-			SXID_ERASE: (default) Do not read and do not pass on to
-			child processes.
-			SXID_IGNORE: Do not read, but retain for non-AT_SECURE
-			child processes.
-			NONE: Read all the time.
-
 2. Use TUNABLE_GET/TUNABLE_SET/TUNABLE_SET_WITH_BOUNDS to get and set tunables.
 
 3. OPTIONAL: If tunables in a namespace are being used multiple times within a