fs: take i_mutex during prepare_binprm for set[ug]id executables

This prevents a race between chown() and execve(), where chowning a
setuid-user binary to root would momentarily make the binary setuid
root.

This patch was mostly written by Linus Torvalds.

Signed-off-by: Jann Horn <jann@thejh.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/exec.c b/fs/exec.c
index 02bfd980a40c7df568bc1f9b847600fd76c2861d..49a1c61433b73722683cad25eef1fb92045e265a 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1275,6 +1275,53 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
        spin_unlock(&p->fs->lock);
 }
 
+static void bprm_fill_uid(struct linux_binprm *bprm)
+{
+       struct inode *inode;
+       unsigned int mode;
+       kuid_t uid;
+       kgid_t gid;
+
+       /* clear any previous set[ug]id data from a previous binary */
+       bprm->cred->euid = current_euid();
+       bprm->cred->egid = current_egid();
+
+       if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
+               return;
+
+       if (task_no_new_privs(current))
+               return;
+
+       inode = file_inode(bprm->file);
+       mode = READ_ONCE(inode->i_mode);
+       if (!(mode & (S_ISUID|S_ISGID)))
+               return;
+
+       /* Be careful if suid/sgid is set */
+       mutex_lock(&inode->i_mutex);
+
+       /* reload atomically mode/uid/gid now that lock held */
+       mode = inode->i_mode;
+       uid = inode->i_uid;
+       gid = inode->i_gid;
+       mutex_unlock(&inode->i_mutex);
+
+       /* We ignore suid/sgid if there are no mappings for them in the ns */
+       if (!kuid_has_mapping(bprm->cred->user_ns, uid) ||
+                !kgid_has_mapping(bprm->cred->user_ns, gid))
+               return;
+
+       if (mode & S_ISUID) {
+               bprm->per_clear |= PER_CLEAR_ON_SETID;
+               bprm->cred->euid = uid;
+       }
+
+       if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
+               bprm->per_clear |= PER_CLEAR_ON_SETID;
+               bprm->cred->egid = gid;
+       }
+}
+
 /*
  * Fill the binprm structure from the inode.
  * Check permissions, then read the first 128 (BINPRM_BUF_SIZE) bytes
@@ -1283,36 +1330,9 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
  */
 int prepare_binprm(struct linux_binprm *bprm)
 {
-       struct inode *inode = file_inode(bprm->file);
-       umode_t mode = inode->i_mode;
        int retval;
 
-
-       /* clear any previous set[ug]id data from a previous binary */
-       bprm->cred->euid = current_euid();
-       bprm->cred->egid = current_egid();
-
-       if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) &&
-           !task_no_new_privs(current) &&
-           kuid_has_mapping(bprm->cred->user_ns, inode->i_uid) &&
-           kgid_has_mapping(bprm->cred->user_ns, inode->i_gid)) {
-               /* Set-uid? */
-               if (mode & S_ISUID) {
-                       bprm->per_clear |= PER_CLEAR_ON_SETID;
-                       bprm->cred->euid = inode->i_uid;
-               }
-
-               /* Set-gid? */
-               /*
-                * If setgid is set but no group execute bit then this
-                * is a candidate for mandatory locking, not a setgid
-                * executable.
-                */
-               if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
-                       bprm->per_clear |= PER_CLEAR_ON_SETID;
-                       bprm->cred->egid = inode->i_gid;
-               }
-       }
+       bprm_fill_uid(bprm);
 
        /* fill in binprm security blob */
        retval = security_bprm_set_creds(bprm);