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Date: Tue, 1 Aug 2017 12:16:25 -0700 From: Kees Cook <keescook@...omium.org> To: linux-kernel@...r.kernel.org Cc: Kees Cook <keescook@...omium.org>, David Howells <dhowells@...hat.com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, James Morris <james.l.morris@...cle.com>, "Serge E. Hallyn" <serge@...lyn.com>, Andy Lutomirski <luto@...nel.org>, John Johansen <john.johansen@...onical.com>, Paul Moore <paul@...l-moore.com>, Casey Schaufler <casey@...aufler-ca.com>, Stephen Smalley <sds@...ho.nsa.gov>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, linux-fsdevel@...r.kernel.org, linux-security-module@...r.kernel.org Subject: [PATCH v5 02/15] exec: Correct comments about "point of no return" 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. Cc: David Howells <dhowells@...hat.com> Cc: Eric W. Biederman <ebiederm@...ssion.com> Signed-off-by: Kees Cook <keescook@...omium.org> Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com> Acked-by: Serge Hallyn <serge@...lyn.com> --- fs/exec.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index a0fff86269e4..26b98072be50 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1259,6 +1259,12 @@ 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; @@ -1286,7 +1292,13 @@ 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 | @@ -1333,7 +1345,6 @@ 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())) @@ -1351,7 +1362,6 @@ 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; -- 2.7.4
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