Kees Cook <[email protected]> writes: > In commit 221af7f87b97 ("Split 'flush_old_exec' into two functions"), > the comment about the point of no return should have stayed in > flush_old_exec() since it refers to "bprm->mm = NULL;" line, but prior > changes in commits c89681ed7d0e ("remove steal_locks()"), and > fd8328be874f ("sanitize handling of shared descriptor tables in failing > execve()") made it look like it meant the current->sas_ss_sp line instead. > > The comment was referring to the fact that once bprm->mm is NULL, all > failures from a binfmt load_binary hook (e.g. load_elf_binary), will > get SEGV raised against current. Move this comment and expand the > explanation a bit, putting it above the assignment this time, and add > details about the true nature of "point of no return" being the call > to flush_old_exec() itself. > > This also removes an erroneous commet about when credentials are being > installed. That has its own dedicated function, install_exec_creds(), > which carries a similar (and correct) comment, so remove the bogus comment > where installation is not actually happening.
Acked-by: "Eric W. Biederman" <[email protected]> > > Cc: David Howells <[email protected]> > Cc: Eric W. Biederman <[email protected]> > Signed-off-by: Kees Cook <[email protected]> > --- > fs/exec.c | 16 +++++++++++++--- > 1 file changed, 13 insertions(+), 3 deletions(-) > > diff --git a/fs/exec.c b/fs/exec.c > index 53ffa739fb58..f9480d3e0b82 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1258,6 +1258,12 @@ void __set_task_comm(struct task_struct *tsk, const > char *buf, bool exec) > perf_event_comm(tsk, exec); > } > > +/* > + * Calling this is the point of no return. None of the failures will be > + * seen by userspace since either the process is already taking a fatal > + * signal (via de_thread() or coredump), or will have SEGV raised > + * (after exec_mmap()) by search_binary_handlers (see below). > + */ > int flush_old_exec(struct linux_binprm * bprm) > { > int retval; > @@ -1285,7 +1291,13 @@ int flush_old_exec(struct linux_binprm * bprm) > if (retval) > goto out; > > - bprm->mm = NULL; /* We're using it now */ > + /* > + * After clearing bprm->mm (to mark that current is using the > + * prepared mm now), we have nothing left of the original > + * process. If anything from here on returns an error, the check > + * in search_binary_handler() will SEGV current. > + */ > + bprm->mm = NULL; > > set_fs(USER_DS); > current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD | > @@ -1339,7 +1351,6 @@ void setup_new_exec(struct linux_binprm * bprm) > > arch_pick_mmap_layout(current->mm); > > - /* This is the point of no return */ > current->sas_ss_sp = current->sas_ss_size = 0; > > if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), > current_gid())) > @@ -1357,7 +1368,6 @@ void setup_new_exec(struct linux_binprm * bprm) > */ > current->mm->task_size = TASK_SIZE; > > - /* install the new credentials */ > if (!uid_eq(bprm->cred->uid, current_euid()) || > !gid_eq(bprm->cred->gid, current_egid())) { > current->pdeath_signal = 0;
