4.0-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jann Horn <jann@xxxxxxxxx>
commit 8b01fc86b9f425899f8a3a8fc1c47d73c2c20543 upstream.
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@xxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/exec.c | 76 +++++++++++++++++++++++++++++++++++++++-----------------------
1 file changed, 48 insertions(+), 28 deletions(-)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1265,6 +1265,53 @@ static void check_unsafe_exec(struct lin
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
@@ -1273,36 +1320,9 @@ static void check_unsafe_exec(struct lin
*/
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);
--
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