linux-kernel - [PATCH 4/7] exec: Move bprm_mm_init into alloc

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Message-ID: <87eepe6x7p.fsf@x220.int.ebiederm.org>
Date:   Tue, 14 Jul 2020 08:30:02 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Luis Chamberlain <mcgrof@...nel.org>,
        <linux-fsdevel@...r.kernel.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-security-module@...r.kernel.org,
        "Serge E. Hallyn" <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Casey Schaufler <casey@...aufler-ca.com>,
        John Johansen <john.johansen@...onical.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: [PATCH 4/7] exec: Move bprm_mm_init into alloc_bprm

Currently it is necessary for the usermode helper code and the code that
launches init to use set_fs so that pages coming from the kernel look like
they are coming from userspace.

To allow that usage of set_fs to be removed cleanly the argument copying
from userspace needs to happen earlier.  Move the allocation and
initialization of bprm->mm into alloc_bprm so that the bprm->mm is
available early to store the new user stack into.  This is a prerequisite
for copying argv and envp into the new user stack early before ther rest of
exec.

To keep the things consistent the cleanup of bprm->mm is moved into
free_bprm.  So that bprm->mm will be cleaned up whenever bprm->mm is
allocated and free_bprm are called.

Moving bprm_mm_init earlier is safe as it does not depend on any files,
current->in_execve, current->fs->in_exec, bprm->unsafe, or the if the file
table is shared. (AKA bprm_mm_init does not depend on any of the code that
happens between alloc_bprm and where it was previously called.)

This moves bprm->mm cleanup after current->fs->in_exec is set to 0.  This
is safe because current->fs->in_exec is only used to preventy taking an
additional reference on the fs_struct.

This moves bprm->mm cleanup after current->in_execve is set to 0.  This is
safe because current->in_execve is only used by the lsms (apparmor and
tomoyou) and always for LSM specific functions, never for anything to do
with the mm.

This adds bprm->mm cleanup into the successful return path.  This is safe
because being on the successful return path implies that begin_new_exec
succeeded and set brpm->mm to NULL.  As bprm->mm is NULL bprm cleanup I am
moving into free_bprm will do nothing.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/exec.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 7e8af27dd199..afb168bf5e23 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1543,6 +1543,10 @@ static int prepare_bprm_creds(struct linux_binprm *bprm)

 static void free_bprm(struct linux_binprm *bprm)
 {
+	if (bprm->mm) {
+		acct_arg_size(bprm, 0);
+		mmput(bprm->mm);
+	}
 	free_arg_pages(bprm);
 	if (bprm->cred) {
 		mutex_unlock(&current->signal->cred_guard_mutex);
@@ -1582,6 +1586,10 @@ static struct linux_binprm *alloc_bprm(int fd, struct filename *filename)
 		bprm->filename = bprm->fdpath;
 	}
 	bprm->interp = bprm->filename;
+
+	retval = bprm_mm_init(bprm);
+	if (retval)
+		goto out_free;
 	return bprm;

 out_free:
@@ -1911,10 +1919,6 @@ static int do_execveat_common(int fd, struct filename *filename,
 	    close_on_exec(fd, rcu_dereference_raw(current->files->fdt)))
 		bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;

-	retval = bprm_mm_init(bprm);
-	if (retval)
-		goto out_unmark;
-
 	retval = prepare_arg_pages(bprm, argv, envp);
 	if (retval < 0)
 		goto out;
@@ -1962,10 +1966,6 @@ static int do_execveat_common(int fd, struct filename *filename,
 	 */
 	if (bprm->point_of_no_return && !fatal_signal_pending(current))
 		force_sigsegv(SIGSEGV);
-	if (bprm->mm) {
-		acct_arg_size(bprm, 0);
-		mmput(bprm->mm);
-	}

 out_unmark:
 	current->fs->in_exec = 0;
-- 
2.25.0