Commit 275498a9 authored by Kees Cook's avatar Kees Cook
Browse files

exec: Add comments on check_unsafe_exec() fs counting 

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: default avatarKees Cook <keescook@chromium.org>
Acked-by: default avatarChristian Brauner (Microsoft) <brauner@kernel.org>
Link: https://lore.kernel.org/r/20221018071537.never.662-kees@kernel.org
parent 23a7aea5

fs/exec.c

+12 −0
Original line number Diff line number Diff line
@@ -1573,6 +1573,12 @@ 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);
@@ -1753,6 +1759,7 @@ 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;
@@ -1811,6 +1818,11 @@ 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;