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authorAleksa Sarai <asarai@suse.de>2016-12-21 16:26:24 +1100
committerAl Viro <viro@zeniv.linux.org.uk>2016-12-22 23:03:41 -0500
commit613cc2b6f272c1a8ad33aefa21cad77af23139f7 (patch)
tree145d05796d97e0d2f5919880fe32597cbf0d72e0 /fs/exec.c
parente522751d605d99a81508e58390a8f51ee96fb662 (diff)
fs: exec: apply CLOEXEC before changing dumpable task flags
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 Cc: <stable@vger.kernel.org> # v3.2+ Reported-by: Michael Crosby <crosbymichael@gmail.com> Signed-off-by: Aleksa Sarai <asarai@suse.de> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/exec.c')
-rw-r--r--fs/exec.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/fs/exec.c b/fs/exec.c
index 8112eacf10f3..eadbf5069c38 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>
@@ -1268,6 +1268,13 @@ int flush_old_exec(struct linux_binprm * bprm)
flush_thread();
current->personality &= ~bprm->per_clear;
+ /*
+ * We have to apply CLOEXEC before we change whether the process is
+ * dumpable (in setup_new_exec) 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);
return 0;
out:
@@ -1330,7 +1337,6 @@ 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);