binfmt_elf: switch to new creds when switching to new mm

We used to delay switching to the new credentials until after we had
mapped the executable (and possible elf interpreter).  That was kind of
odd to begin with, since the new executable will actually then _run_
with the new creds, but whatever.

The bigger problem was that we also want to make sure that we turn off
prof events and tracing before we start mapping the new executable
state.  So while this is a cleanup, it's also a fix for a possible
information leak.

Reported-by: Robert Święcki <[email protected]>
Tested-by: Peter Zijlstra <[email protected]>
Acked-by: David Howells <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Acked-by: Eric W. Biederman <[email protected]>
Cc: Willy Tarreau <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: torvalds

fs/binfmt_elf.c

@@ -853,6 +853,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		current->flags |= PF_RANDOMIZE;
 
 	setup_new_exec(bprm);
+	install_exec_creds(bprm);
 
 	/* Do this so that we can load the interpreter, if need be.  We will
 	   change some of these later */
@@ -1044,7 +1045,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		goto out;
 #endif /* ARCH_HAS_SETUP_ADDITIONAL_PAGES */
 
-	install_exec_creds(bprm);
 	retval = create_elf_tables(bprm, &loc->elf_ex,
 			  load_addr, interp_load_addr);
 	if (retval < 0)