[REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file

[Eric W. Biederman]

It is the reasonable expectation that if an executable file is not
readable there will be no way for a user without special privileges to
read the file.  This is enforced in ptrace_attach but if we are
already attached there is no enforcement if a readonly executable
is exec'd.

Therefore do the simple thing and if there is a non-dumpable
executable that we are tracing without privilege fail to exec it.

Fixes: v1.0
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/exec.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index fdec760bfac3..de107f74e055 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1230,6 +1230,11 @@ int flush_old_exec(struct linux_binprm * bprm)
 {
 	int retval;
 
+	/* Fail if the tracer can't read the executable */
+	if ((bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP) &&
+	    !ptracer_capable(current, bprm->mm->user_ns))
+		return -EPERM;
+
 	/*
 	 * Make sure we have a private signal table and that
 	 * we are unassociated from the previous thread group.
@@ -1301,7 +1306,6 @@ void setup_new_exec(struct linux_binprm * bprm)
 	    !gid_eq(bprm->cred->gid, current_egid())) {
 		current->pdeath_signal = 0;
 	} else {
-		would_dump(bprm, bprm->file);
 		if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP)
 			set_dumpable(current->mm, suid_dumpable);
 	}
@@ -1736,6 +1740,8 @@ static int do_execveat_common(int fd, struct filename *filename,
 	if (retval < 0)
 		goto out;
 
+	would_dump(bprm, bprm->file);
+
 	retval = exec_binprm(bprm);
 	if (retval < 0)
 		goto out;
-- 
2.10.1