Only allow local login if password is non-empty

[zeripath]

Prohibit local login if password is empty. This protects oauth2 users created by
an admin with an empty password and with must_change_password set to false.

With thanks to @petercolberg

[zeripath]

@techknowlogick Does this look like the correct approach?

[codecov-io]

Codecov Report

Merging #5906 into master will not change coverage.
The diff coverage is 50%.

@@          Coverage Diff           @@
##           master   #5906   +/-   ##
======================================
  Coverage      38%     38%           
======================================
  Files         328     328           
  Lines       48343   48343           
======================================
  Hits        18375   18375           
+ Misses      27329   27328    -1     
- Partials     2639    2640    +1

Impacted Files	Coverage Δ
modules/lfs/server.go	44.27% <0%> (ø)	⬆️
models/login_source.go	26.3% <100%> (ø)	⬆️
models/repo_list.go	63.29% <0%> (-1.27%)	⬇️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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[lafriks]

@zeripath please backport

[petercolberg]

Thank you for addressing this issue!

I was pondering a bit about the difference between your and my patch.

The committed solution introduces a subtle timing issue, where a third party may query whether a user has an unset password (IsPasswordSet() == false yields one call to ConstantTimeCompare()) or a set password (IsPasswordSet() == true and subsequent, second ValidatePassword() yield two calls to ConstantTimeCompare()). Whereas my patch rejects invalid input that is provided by the attacker. This does not seem a problem per se, but it would be nice to address this subtle issue in a future version.

From dbcc3df481ed1af490fe6243c8aabb60dd1b8caa Mon Sep 17 00:00:00 2001
From: Peter Colberg <peter@colberg.org>
Date: Thu, 24 Jan 2019 23:29:39 -0500
Subject: [PATCH] Reject empty password on validation

Prohibit unauthorized API access through an oauth2 user that was created by
an admin with an empty password and with must_change_password set to false.

Signed-off-by: Peter Colberg <peter@colberg.org>
---
 models/user.go      | 13 ++++++++++---
 models/user_test.go | 17 +++++++++++++++++
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/models/user.go b/models/user.go
index 4ab78ec04..10e034da9 100644
--- a/models/user.go
+++ b/models/user.go
@@ -433,15 +433,22 @@ func (u *User) HashPassword(passwd string) {
        u.Passwd = hashPassword(passwd, u.Salt)
 }

-// ValidatePassword checks if given password matches the one belongs to the user.
-func (u *User) ValidatePassword(passwd string) bool {
+func validatePassword(u *User, passwd string) bool {
        tempHash := hashPassword(passwd, u.Salt)
        return subtle.ConstantTimeCompare([]byte(u.Passwd), []byte(tempHash)) == 1
 }

+// ValidatePassword checks if given password matches the one belongs to the user.
+func (u *User) ValidatePassword(passwd string) bool {
+       if (passwd == "") {
+               return false
+       }
+       return validatePassword(u, passwd);
+}
+
 // IsPasswordSet checks if the password is set or left empty
 func (u *User) IsPasswordSet() bool {
-       return !u.ValidatePassword("")
+       return !validatePassword(u, "")
 }
  
 // UploadAvatar saves custom avatar for user.
diff --git a/models/user_test.go b/models/user_test.go
index 9d011f32a..c44bbe48a 100644
--- a/models/user_test.go
+++ b/models/user_test.go
@@ -161,6 +161,23 @@ func BenchmarkHashPassword(b *testing.B) {
        }
 }

+func TestValidatePassword(t *testing.T) {
+       bs := make([]byte, 16)
+       rand.Read(bs)
+       u := &User{Salt: string(bs)}
+
+       u.HashPassword("")
+       assert.False(t, u.IsPasswordSet())
+       assert.False(t, u.ValidatePassword(""))
+       assert.False(t, u.ValidatePassword("password1337"))
+
+       u.HashPassword("rvpwt4wdrbaxdknm")
+       assert.True(t, u.IsPasswordSet())
+       assert.False(t, u.ValidatePassword(""))
+       assert.False(t, u.ValidatePassword("password1337"))
+       assert.True(t, u.ValidatePassword("rvpwt4wdrbaxdknm"))
+}
+
 func TestGetOrgRepositoryIDs(t *testing.T) {
        assert.NoError(t, PrepareTestDatabase())
        user2 := AssertExistsAndLoadBean(t, &User{ID: 2}).(*User)
--
2.20.1