5.4-stable review patch. If anyone has any objections, please let me know.
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From: Kees Cook kees@kernel.org
commit f50733b45d865f91db90919f8311e2127ce5a0cb upstream.
When opening a file for exec via do_filp_open(), permission checking is done against the file's metadata at that moment, and on success, a file pointer is passed back. Much later in the execve() code path, the file metadata (specifically mode, uid, and gid) is used to determine if/how to set the uid and gid. However, those values may have changed since the permissions check, meaning the execution may gain unintended privileges.
For example, if a file could change permissions from executable and not set-id:
---------x 1 root root 16048 Aug 7 13:16 target
to set-id and non-executable:
---S------ 1 root root 16048 Aug 7 13:16 target
it is possible to gain root privileges when execution should have been disallowed.
While this race condition is rare in real-world scenarios, it has been observed (and proven exploitable) when package managers are updating the setuid bits of installed programs. Such files start with being world-executable but then are adjusted to be group-exec with a set-uid bit. For example, "chmod o-x,u+s target" makes "target" executable only by uid "root" and gid "cdrom", while also becoming setuid-root:
-rwxr-xr-x 1 root cdrom 16048 Aug 7 13:16 target
becomes:
-rwsr-xr-- 1 root cdrom 16048 Aug 7 13:16 target
But racing the chmod means users without group "cdrom" membership can get the permission to execute "target" just before the chmod, and when the chmod finishes, the exec reaches brpm_fill_uid(), and performs the setuid to root, violating the expressed authorization of "only cdrom group members can setuid to root".
Re-check that we still have execute permissions in case the metadata has changed. It would be better to keep a copy from the perm-check time, but until we can do that refactoring, the least-bad option is to do a full inode_permission() call (under inode lock). It is understood that this is safe against dead-locks, but hardly optimal.
Reported-by: Marco Vanotti mvanotti@google.com Tested-by: Marco Vanotti mvanotti@google.com Suggested-by: Linus Torvalds torvalds@linux-foundation.org Cc: stable@vger.kernel.org Cc: Eric Biederman ebiederm@xmission.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Christian Brauner brauner@kernel.org Signed-off-by: Kees Cook kees@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/exec.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
--- a/fs/exec.c +++ b/fs/exec.c @@ -1558,6 +1558,7 @@ static void bprm_fill_uid(struct linux_b unsigned int mode; kuid_t uid; kgid_t gid; + int err;
/* * Since this can be called multiple times (via prepare_binprm), @@ -1582,12 +1583,17 @@ static void bprm_fill_uid(struct linux_b /* Be careful if suid/sgid is set */ inode_lock(inode);
- /* reload atomically mode/uid/gid now that lock held */ + /* Atomically reload and check mode/uid/gid now that lock held. */ mode = inode->i_mode; uid = inode->i_uid; gid = inode->i_gid; + err = inode_permission(inode, MAY_EXEC); inode_unlock(inode);
+ /* Did the exec bit vanish out from under us? Give up. */ + if (err) + return; + /* 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))