| Message ID | 20161220110457.22254-1-asarai@suse.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 12/20, Aleksa Sarai wrote: > > @@ -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); Cosmetic, but perhaps this should go into flush_old_exec() then? But I am fine either way, up to you. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
>> @@ -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); > > Cosmetic, but perhaps this should go into flush_old_exec() then? Yeah, I was thinking the same thing. Also, I'll cc stable (v3.2+) to the patch.
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);
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: dev@opencontainers.org Reported-by: Michael Crosby <crosbymichael@gmail.com> Signed-off-by: Aleksa Sarai <asarai@suse.de> --- fs/exec.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)