[PATCH] exec: Add comments on check_unsafe_exec() fs counting

[Kees Cook]

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@xxxxxxxxxxxx/

Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/exec.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 902bce45b116..01659c2ac7d8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -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;
 
-- 
2.34.1