🇰🇷 한글 설명
리눅스 커널의 exec 과정에서 권한 확인과 set-uid/gid 적용 사이에 발생하는 ToCToU(Time-of-Check to Time-of-Use) 취약점입니다. 파일 메타데이터가 권한 확인 직후 변경될 경우, 의도하지 않은 권한 상승이 발생할 수 있습니다.
💡 쉬운 설명
파일의 권한을 확인하는 순간과 실제로 실행하는 순간 사이에 파일의 성격이 바뀌어 발생하는 보안 허점입니다. 공격자가 이 틈을 이용하면 원래는 실행할 수 없는 권한을 얻어 관리자 권한을 탈취할 수 있습니다.
🇺🇸 원문 (English)
In the Linux kernel, the following vulnerability has been resolved:
exec: Fix ToCToU between perm check and set-uid/gid usage
When opening a file for exec via do_filp_open(), permission checking is
done against the file's metadata at that moment, and on success, a file
pointer is passed back. Much later in the execve() code path, the file
metadata (specifically mode, uid, and gid) is used to determine if/how
to set the uid and gid. However, those values may have changed since the
permissions check, meaning the execution may gain unintended privileges.
For example, if a file could change permissions from executable and not
set-id:
---------x 1 root root 16048 Aug 7 13:16 target
to set-id and non-executable:
---S------ 1 root root 16048 Aug 7 13:16 target
it is possible to gain root privileges when execution should have been
disallowed.
While this race condition is rare in real-world scenarios, it has been
observed (and proven exploitable) when package managers are updating
the setuid bits of installed programs. Such files start with being
world-executable but then are adjusted to be group-exec with a set-uid
bit. For example, "chmod o-x,u+s target" makes "target" executable only
by uid "root" and gid "cdrom", while also becoming setuid-root:
-rwxr-xr-x 1 root cdrom 16048 Aug 7 13:16 target
becomes:
-rwsr-xr-- 1 root cdrom 16048 Aug 7 13:16 target
But racing the chmod means users without group "cdrom" membership can
get the permission to execute "target" just before the chmod, and when
the chmod finishes, the exec reaches brpm_fill_uid(), and performs the
setuid to root, violating the expressed authorization of "only cdrom
group members can setuid to root".
Re-check that we still have execute permissions in case the metadata
has changed. It would be better to keep a copy from the perm-check time,
but until we can do that refactoring, the least-bad option is to do a
full inode_permission() call (under inode lock). It is understood that
this is safe against dead-locks, but hardly optimal.
📍 영향받는 시스템
Linux Kernel (패키지 관리자가 setuid 비트를 업데이트하는 환경 포함)
⚔️ 공격 벡터
파일 권한이 변경되는 찰나의 타이밍을 이용한 레이스 컨디션(Race Condition) 공격
🛡️ 대응 방법
리눅스 커널을 최신 보안 패치가 적용된 버전으로 업데이트하십시오.
🔧 검증 코드
출력된 커널 버전이 해당 취약점이 해결된 패치 버전 이후인지 확인하십시오.