Messages in this thread | | | Date | Fri, 26 Jul 2019 13:59:59 +0200 | From | Christian Brauner <> | Subject | Regression in 5.3 for some FS_USERNS_MOUNT (aka user-namespace-mountable) filesystems |
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Hey everyone,
We have another mount api regression. With current 5.3-rc1 it is not possible anymore to mount filesystems that have FS_USERNS_MOUNT set and their fs_context's global member set to true. At least sysfs is affected, likely also cgroup{2}fs.
The commit that introduced the regression is:
commit 0ce0cf12fc4c6a089717ff613d76457052cf4303 Author: Al Viro <viro@zeniv.linux.org.uk> Date: Sun May 12 15:42:48 2019 -0400
consolidate the capability checks in sget_{fc,userns}()
... into a common helper - mount_capable(type, userns)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
mount_capable() will select the user namespace in which to check for CAP_SYS_ADMIN based on the global property of the filesystem's fs_context.
Since sysfs has global set to true mount_capable() will check for CAP_SYS_ADMIN in init_user_ns and fail the mount with EPERM for any non-init userns root. The same check is present in sget_fc().
To me it looks like that global is overriding FS_USERNS_MOUNT which seems odd. Afaict, there are two ways to fix this: - remove global from sysfs - remove the global check from mount_capable() and possibly sget_fc()
The latter feels more correct but I'm not sure *why* that global thing got introduced. Seems there could be an additional flag on affected filesystems instead of this "global" thing. But not sure.
I can whip up a patch in case that does make sense. And it would probably be a good thing if we had some sort of test (if there isn't one already) so that this doesn't happen again. It could be as simple as:
unshare -U -m --map-root -n mkdir whatever mount -t sysfs sysfs ./whatever
Thanks! Christian
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