From 9520628e8ceb69fa9a4aee6b57f22675d9e1b709 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Mon, 30 Jul 2012 14:39:15 -0700 Subject: fs: make dumpable=2 require fully qualified path When the suid_dumpable sysctl is set to "2", and there is no core dump pipe defined in the core_pattern sysctl, a local user can cause core files to be written to root-writable directories, potentially with user-controlled content. This means an admin can unknowningly reintroduce a variation of CVE-2006-2451, allowing local users to gain root privileges. $ cat /proc/sys/fs/suid_dumpable 2 $ cat /proc/sys/kernel/core_pattern core $ ulimit -c unlimited $ cd / $ ls -l core ls: cannot access core: No such file or directory $ touch core touch: cannot touch `core': Permission denied $ OHAI="evil-string-here" ping localhost >/dev/null 2>&1 & $ pid=$! $ sleep 1 $ kill -SEGV $pid $ ls -l core -rw------- 1 root kees 458752 Jun 21 11:35 core $ sudo strings core | grep evil OHAI=evil-string-here While cron has been fixed to abort reading a file when there is any parse error, there are still other sensitive directories that will read any file present and skip unparsable lines. Instead of introducing a suid_dumpable=3 mode and breaking all users of mode 2, this only disables the unsafe portion of mode 2 (writing to disk via relative path). Most users of mode 2 (e.g. Chrome OS) already use a core dump pipe handler, so this change will not break them. For the situations where a pipe handler is not defined but mode 2 is still active, crash dumps will only be written to fully qualified paths. If a relative path is defined (e.g. the default "core" pattern), dump attempts will trigger a printk yelling about the lack of a fully qualified path. Signed-off-by: Kees Cook Cc: Alexander Viro Cc: Alan Cox Cc: "Eric W. Biederman" Cc: Doug Ledford Cc: Serge Hallyn Cc: James Morris Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/exec.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) (limited to 'fs') diff --git a/fs/exec.c b/fs/exec.c index e95aeeddd25c..95aae3f9c036 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -2111,6 +2111,7 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs) int retval = 0; int flag = 0; int ispipe; + bool need_nonrelative = false; static atomic_t core_dump_count = ATOMIC_INIT(0); struct coredump_params cprm = { .signr = signr, @@ -2136,14 +2137,16 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs) if (!cred) goto fail; /* - * We cannot trust fsuid as being the "true" uid of the - * process nor do we know its entire history. We only know it - * was tainted so we dump it as root in mode 2. + * We cannot trust fsuid as being the "true" uid of the process + * nor do we know its entire history. We only know it was tainted + * so we dump it as root in mode 2, and only into a controlled + * environment (pipe handler or fully qualified path). */ if (__get_dumpable(cprm.mm_flags) == 2) { /* Setuid core dump mode */ flag = O_EXCL; /* Stop rewrite attacks */ cred->fsuid = GLOBAL_ROOT_UID; /* Dump root private */ + need_nonrelative = true; } retval = coredump_wait(exit_code, &core_state); @@ -2223,6 +2226,14 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs) if (cprm.limit < binfmt->min_coredump) goto fail_unlock; + if (need_nonrelative && cn.corename[0] != '/') { + printk(KERN_WARNING "Pid %d(%s) can only dump core "\ + "to fully qualified path!\n", + task_tgid_vnr(current), current->comm); + printk(KERN_WARNING "Skipping core dump\n"); + goto fail_unlock; + } + cprm.file = filp_open(cn.corename, O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE | flag, 0600); -- cgit v1.2.3