5.10-stable review patch. If anyone has any objections, please let me know.
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From: Seth Forshee (DigitalOcean) sforshee@kernel.org
[ Upstream commit e1c5ae59c0f22f7fe5c07fb5513a29e4aad868c9 ]
Christian noticed that it is possible for a privileged user to mount most filesystems with a non-initial user namespace in sb->s_user_ns. When fsopen() is called in a non-init namespace the caller's namespace is recorded in fs_context->user_ns. If the returned file descriptor is then passed to a process priviliged in init_user_ns, that process can call fsconfig(fd_fs, FSCONFIG_CMD_CREATE), creating a new superblock with sb->s_user_ns set to the namespace of the process which called fsopen().
This is problematic. We cannot assume that any filesystem which does not set FS_USERNS_MOUNT has been written with a non-initial s_user_ns in mind, increasing the risk for bugs and security issues.
Prevent this by returning EPERM from sget_fc() when FS_USERNS_MOUNT is not set for the filesystem and a non-initial user namespace will be used. sget() does not need to be updated as it always uses the user namespace of the current context, or the initial user namespace if SB_SUBMOUNT is set.
Fixes: cb50b348c71f ("convenience helpers: vfs_get_super() and sget_fc()") Reported-by: Christian Brauner brauner@kernel.org Signed-off-by: Seth Forshee (DigitalOcean) sforshee@kernel.org Link: https://lore.kernel.org/r/20240724-s_user_ns-fix-v1-1-895d07c94701@kernel.or... Reviewed-by: Alexander Mikhalitsyn aleksandr.mikhalitsyn@canonical.com Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/super.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/fs/super.c b/fs/super.c index f9795e72e3bf8..282aa36901eb1 100644 --- a/fs/super.c +++ b/fs/super.c @@ -518,6 +518,17 @@ struct super_block *sget_fc(struct fs_context *fc, struct user_namespace *user_ns = fc->global ? &init_user_ns : fc->user_ns; int err;
+ /* + * Never allow s_user_ns != &init_user_ns when FS_USERNS_MOUNT is + * not set, as the filesystem is likely unprepared to handle it. + * This can happen when fsconfig() is called from init_user_ns with + * an fs_fd opened in another user namespace. + */ + if (user_ns != &init_user_ns && !(fc->fs_type->fs_flags & FS_USERNS_MOUNT)) { + errorfc(fc, "VFS: Mounting from non-initial user namespace is not allowed"); + return ERR_PTR(-EPERM); + } + retry: spin_lock(&sb_lock); if (test) {