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* [PATCH v2] fs: Validate umount flags before looking up path in ksys_umount
@ 2021-01-04 21:54 Sargun Dhillon
  2021-01-05  8:06 ` Christoph Hellwig
  0 siblings, 1 reply; 2+ messages in thread
From: Sargun Dhillon @ 2021-01-04 21:54 UTC (permalink / raw)
  To: Christoph Hellwig, Al Viro
  Cc: Sargun Dhillon, Kyle Anderson, Manas Alekar, linux-fsdevel,
	linux-api, Rob Gulewich, Zoran Simic, stable

ksys_umount was refactored to into split into another function
(path_umount) to enable sharing code. This changed the order that flags and
permissions are validated in, and made it so that user_path_at was called
before validating flags.

Unfortunately, libfuse2[1] and libmount[2] rely on the old flag validation
behaviour to determine whether or not the kernel supports UMOUNT_NOFOLLOW.
The other path that this validation is being checked on is
init_umount->path_umount->can_umount. That's all internal to the kernel. We
can safely move flag checking to ksys_umount, and let other users of
path_umount know they need to perform their own validation.

[1]: https://github.com/libfuse/libfuse/blob/9bfbeb576c5901b62a171d35510f0d1a922020b7/util/fusermount.c#L403
[2]: https://github.com/karelzak/util-linux/blob/7ed579523b556b1270f28dbdb7ee07dee310f157/libmount/src/context_umount.c#L813

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Cc: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Fixes: 41525f56e256 ("fs: refactor ksys_umount")
---
 fs/namespace.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index cebaa3e81794..752f82121dd4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1710,8 +1710,6 @@ static int can_umount(const struct path *path, int flags)
 {
 	struct mount *mnt = real_mount(path->mnt);
 
-	if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
-		return -EINVAL;
 	if (!may_mount())
 		return -EPERM;
 	if (path->dentry != path->mnt->mnt_root)
@@ -1725,6 +1723,13 @@ static int can_umount(const struct path *path, int flags)
 	return 0;
 }
 
+
+/*
+ * path_umount - unmount by path
+ *
+ * path_umount does not check the validity of flags. It is up to the caller
+ * to ensure that it only contains valid umount options.
+ */
 int path_umount(struct path *path, int flags)
 {
 	struct mount *mnt = real_mount(path->mnt);
@@ -1746,6 +1751,10 @@ static int ksys_umount(char __user *name, int flags)
 	struct path path;
 	int ret;
 
+	/* Check flag validity first to allow probing of supported flags */
+	if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
+		return -EINVAL;
+
 	if (!(flags & UMOUNT_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
 	ret = user_path_at(AT_FDCWD, name, lookup_flags, &path);
-- 
2.25.1


^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [PATCH v2] fs: Validate umount flags before looking up path in ksys_umount
  2021-01-04 21:54 [PATCH v2] fs: Validate umount flags before looking up path in ksys_umount Sargun Dhillon
@ 2021-01-05  8:06 ` Christoph Hellwig
  0 siblings, 0 replies; 2+ messages in thread
From: Christoph Hellwig @ 2021-01-05  8:06 UTC (permalink / raw)
  To: Sargun Dhillon
  Cc: Christoph Hellwig, Al Viro, Kyle Anderson, Manas Alekar,
	linux-fsdevel, linux-api, Rob Gulewich, Zoran Simic, stable

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

^ permalink raw reply	[flat|nested] 2+ messages in thread

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2021-01-04 21:54 [PATCH v2] fs: Validate umount flags before looking up path in ksys_umount Sargun Dhillon
2021-01-05  8:06 ` Christoph Hellwig

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