From: Aleksa Sarai <firstname.lastname@example.org> To: email@example.com, firstname.lastname@example.org, email@example.com Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, Aleksa Sarai <email@example.com>, firstname.lastname@example.org Subject: [PATCH] fs: exec: apply CLOEXEC before changing dumpable task flags Date: Tue, 20 Dec 2016 22:04:57 +1100 [thread overview] Message-ID: <email@example.com> (raw) If you have a process that has set itself to be non-dumpable, and it then undergoes exec(2), any CLOEXEC file descriptors it has open are "exposed" during a race window between the dumpable flags of the process being reset for exec(2) and CLOEXEC being applied to the file descriptors. This can be exploited by a process by attempting to access /proc/<pid>/fd/... during this window, without requiring CAP_SYS_PTRACE. The race in question is after set_dumpable has been (for get_link, though the trace is basically the same for readlink): [vfs] -> proc_pid_link_inode_operations.get_link -> proc_pid_get_link -> proc_fd_access_allowed -> ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS); Which will return 0, during the race window and CLOEXEC file descriptors will still be open during this window because do_close_on_exec has not been called yet. As a result, the ordering of these calls should be reversed to avoid this race window. This is of particular concern to container runtimes, where joining a PID namespace with file descriptors referring to the host filesystem can result in security issues (since PRCTL_SET_DUMPABLE doesn't protect against access of CLOEXEC file descriptors -- file descriptors which may reference filesystem objects the container shouldn't have access to). Cc: firstname.lastname@example.org Reported-by: Michael Crosby <email@example.com> Signed-off-by: Aleksa Sarai <firstname.lastname@example.org> --- fs/exec.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index 4e497b9ee71e..3ad50c379413 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -19,7 +19,7 @@ * current->executable is only used by the procfs. This allows a dispatch * table to check for several different types of binary formats. We keep * trying until we recognize the file or we run out of supported binary - * formats. + * formats. */ #include <linux/slab.h> @@ -1287,6 +1287,13 @@ void setup_new_exec(struct linux_binprm * bprm) /* This is the point of no return */ current->sas_ss_sp = current->sas_ss_size = 0; + /* + * We have to apply CLOEXEC before we change whether the process is + * dumpable to avoid a race with a process in userspace trying to access + * the should-be-closed file descriptors of a process undergoing exec(2). + */ + do_close_on_exec(current->files); + if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid())) set_dumpable(current->mm, SUID_DUMP_USER); else @@ -1315,7 +1322,6 @@ void setup_new_exec(struct linux_binprm * bprm) group */ current->self_exec_id++; flush_signal_handlers(current, 0); - do_close_on_exec(current->files); } EXPORT_SYMBOL(setup_new_exec); -- 2.11.0
next reply other threads:[~2016-12-20 11:05 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-12-20 11:04 Aleksa Sarai [this message] 2016-12-20 17:29 ` Oleg Nesterov 2016-12-21 5:04 ` Aleksa Sarai
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