fs: fix acl translation

Commit Message

Christian Brauner April 19, 2022, 1:14 p.m. UTC

Last cycle we extended the idmapped mounts infrastructure to support idmapped
mounts of idmapped filesystems (No such filesystem yet exist.).
Since then, the meaning of an idmapped mount is a mount whose idmapping is
different from the filesystems idmapping.

While doing that work we missed to adapt the acl translation helpers. They
still assume that checking for the identity mapping is enough. But they need to
use the no_idmapping() helper instead.

Note, POSIX ACLs are always translated right at the userspace-kernel boundary
using the caller's current idmapping and the initial idmapping. The order
depends on whether we're coming from or going to userspace. The filesystem's
idmapping doesn't matter at the border.

Consequently, if a non-idmapped mount is passed we need to make sure to always
pass the initial idmapping as the mount's idmapping and not the filesystem
idmapping. Since it's irrelevant here it would yield invalid ids and prevent
setting acls for filesystems that are mountable in a userns and support posix
acls (tmpfs and fuse).

I verified the regression reported in [1] and verified that this patch fixes
it. A regression test will be added to xfstests in parallel.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=215849 [1]
Fixes: bd303368b776 ("fs: support mapped mounts of mapped filesystems")
Cc: Seth Forshee <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: <[e-mail]> # 5.17
Cc: <[e-mail]>
Signed-off-by: Christian Brauner (Microsoft) <[e-mail]>
---

Hey Linus,

Last cycle we introduced a regression that got reported to me right over Easter
on Bugzilla. It is only relevant for 5.17 and current mainline. Could you
please apply this regression fix?

Thanks!
Christian
---
 fs/posix_acl.c                  | 10 ++++++++++
 fs/xattr.c                      |  6 ++++--
 include/linux/posix_acl_xattr.h |  4 ++++
 3 files changed, 18 insertions(+), 2 deletions(-)


base-commit: b2d229d4ddb17db541098b83524d901257e93845

Patch

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 80acb6885cf9..962d32468eb4 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -759,9 +759,14 @@  static void posix_acl_fix_xattr_userns(
 }
 
 void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
+				   struct inode *inode,
 				   void *value, size_t size)
 {
 	struct user_namespace *user_ns = current_user_ns();
+
+	/* Leave ids untouched on non-idmapped mounts. */
+	if (no_idmapping(mnt_userns, i_user_ns(inode)))
+		mnt_userns = &init_user_ns;
 	if ((user_ns == &init_user_ns) && (mnt_userns == &init_user_ns))
 		return;
 	posix_acl_fix_xattr_userns(&init_user_ns, user_ns, mnt_userns, value,
@@ -769,9 +774,14 @@  void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
 }
 
 void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
+				 struct inode *inode,
 				 void *value, size_t size)
 {
 	struct user_namespace *user_ns = current_user_ns();
+
+	/* Leave ids untouched on non-idmapped mounts. */
+	if (no_idmapping(mnt_userns, i_user_ns(inode)))
+		mnt_userns = &init_user_ns;
 	if ((user_ns == &init_user_ns) && (mnt_userns == &init_user_ns))
 		return;
 	posix_acl_fix_xattr_userns(user_ns, &init_user_ns, mnt_userns, value,
diff --git a/fs/xattr.c b/fs/xattr.c
index 5c8c5175b385..998045165916 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -569,7 +569,8 @@  setxattr(struct user_namespace *mnt_userns, struct dentry *d,
 		}
 		if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
 		    (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
-			posix_acl_fix_xattr_from_user(mnt_userns, kvalue, size);
+			posix_acl_fix_xattr_from_user(mnt_userns, d_inode(d),
+						      kvalue, size);
 	}
 
 	error = vfs_setxattr(mnt_userns, d, kname, kvalue, size, flags);
@@ -667,7 +668,8 @@  getxattr(struct user_namespace *mnt_userns, struct dentry *d,
 	if (error > 0) {
 		if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
 		    (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
-			posix_acl_fix_xattr_to_user(mnt_userns, kvalue, error);
+			posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
+						    kvalue, error);
 		if (size && copy_to_user(value, kvalue, error))
 			error = -EFAULT;
 	} else if (error == -ERANGE && size >= XATTR_SIZE_MAX) {
diff --git a/include/linux/posix_acl_xattr.h b/include/linux/posix_acl_xattr.h
index 060e8d203181..1766e1de6956 100644
--- a/include/linux/posix_acl_xattr.h
+++ b/include/linux/posix_acl_xattr.h
@@ -34,15 +34,19 @@  posix_acl_xattr_count(size_t size)
 
 #ifdef CONFIG_FS_POSIX_ACL
 void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
+				   struct inode *inode,
 				   void *value, size_t size);
 void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
+				   struct inode *inode,
 				 void *value, size_t size);
 #else
 static inline void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
+						 struct inode *inode,
 						 void *value, size_t size)
 {
 }
 static inline void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
+					       struct inode *inode,
 					       void *value, size_t size)
 {
 }