exec: Add comments on check_unsafe_exec() fs counting
Commit Message
Add some comments about what the fs counting is doing in
check_unsafe_exec() and how it relates to the call graph.
Specifically, we can't force an unshare of the fs because
of at least Chrome:
https://lore.kernel.org/lkml/86CE201B-5632-4BB7-BCF6-7CB2C2895409@chromium.org/
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
fs/exec.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
Comments
On Tue, Oct 18, 2022 at 12:17:24AM -0700, Kees Cook wrote:
> Add some comments about what the fs counting is doing in
> check_unsafe_exec() and how it relates to the call graph.
> Specifically, we can't force an unshare of the fs because
> of at least Chrome:
> https://lore.kernel.org/lkml/86CE201B-5632-4BB7-BCF6-7CB2C2895409@chromium.org/
>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: linux-fsdevel@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
@@ -1571,6 +1571,12 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
if (task_no_new_privs(current))
bprm->unsafe |= LSM_UNSAFE_NO_NEW_PRIVS;
+ /*
+ * If another task is sharing our fs, we cannot safely
+ * suid exec because the differently privileged task
+ * will be able to manipulate the current directory, etc.
+ * It would be nice to force an unshare instead...
+ */
t = p;
n_fs = 1;
spin_lock(&p->fs->lock);
@@ -1752,6 +1758,7 @@ static int search_binary_handler(struct linux_binprm *bprm)
return retval;
}
+/* binfmt handlers will call back into begin_new_exec() on success. */
static int exec_binprm(struct linux_binprm *bprm)
{
pid_t old_pid, old_vpid;
@@ -1810,6 +1817,11 @@ static int bprm_execve(struct linux_binprm *bprm,
if (retval)
return retval;
+ /*
+ * Check for unsafe execution states before exec_binprm(), which
+ * will call back into begin_new_exec(), into bprm_creds_from_file(),
+ * where setuid-ness is evaluated.
+ */
check_unsafe_exec(bprm);
current->in_execve = 1;