Re: [PATCH] tmpfs: verify {g,u}id mount options correctly

[Seth Forshee <sforshee@kernel.org>]

On Tue, Aug 01, 2023 at 06:17:04PM +0200, Christian Brauner wrote:
> A while ago we received the following report:
> 
> "The other outstanding issue I noticed comes from the fact that
> fsconfig syscalls may occur in a different userns than that which
> called fsopen. That means that resolving the uid/gid via
> current_user_ns() can save a kuid that isn't mapped in the associated
> namespace when the filesystem is finally mounted. This means that it
> is possible for an unprivileged user to create files owned by any
> group in a tmpfs mount (since we can set the SUID bit on the tmpfs
> directory), or a tmpfs that is owned by any user, including the root
> group/user."
> 
> The contract for {g,u}id mount options and {g,u}id values in general set
> from userspace has always been that they are translated according to the
> caller's idmapping. In so far, tmpfs has been doing the correct thing.
> But since tmpfs is mountable in unprivileged contexts it is also
> necessary to verify that the resulting {k,g}uid is representable in the
> namespace of the superblock to avoid such bugs as above.
> 
> The new mount api's cross-namespace delegation abilities are already
> widely used. After having talked to a bunch of userspace this is the
> most faithful solution with minimal regression risks. I know of one
> users - systemd - that makes use of the new mount api in this way and
> they don't set unresolable {g,u}ids. So the regression risk is minimal.
> 
> Link: https://lore.kernel.org/lkml/CALxfFW4BXhEwxR0Q5LSkg-8Vb4r2MONKCcUCVioehXQKr35eHg@mail.gmail.com
> Fixes: f32356261d44 ("vfs: Convert ramfs, shmem, tmpfs, devtmpfs, rootfs to use the new mount API")
> Reported-by: Seth Jenkins <sethjenkins@google.com>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> 
> ---
>  mm/shmem.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 2f2e0e618072..1c0b2dafafe5 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3636,6 +3636,8 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
>  	unsigned long long size;
>  	char *rest;
>  	int opt;
> +	kuid_t kuid;
> +	kgid_t kgid;
>  
>  	opt = fs_parse(fc, shmem_fs_parameters, param, &result);
>  	if (opt < 0)
> @@ -3671,14 +3673,32 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
>  		ctx->mode = result.uint_32 & 07777;
>  		break;
>  	case Opt_uid:
> -		ctx->uid = make_kuid(current_user_ns(), result.uint_32);
> -		if (!uid_valid(ctx->uid))
> +		kuid = make_kuid(current_user_ns(), result.uint_32);
> +		if (!uid_valid(kuid))
>  			goto bad_value;
> +
> +		/*
> +		 * The requested uid must be representable in the
> +		 * filesystem's idmapping.
> +		 */
> +		if (!kuid_has_mapping(fc->user_ns, kuid))
> +			goto bad_value;
> +
> +		ctx->uid = kuid;

This seems like the most sensible way to handle ids in mount options.
Wouldn't some other filesystems (e.g. fuse) benefit from the same sort
of handling though? Rather than having filesystems handle these checks
themselves, what about adding k{uid,gid}_t members to the
fs_parse_result union with fsparam_is_{uid,gid}() helpers which peform
these checks?

Seth

>  		break;
>  	case Opt_gid:
> -		ctx->gid = make_kgid(current_user_ns(), result.uint_32);
> -		if (!gid_valid(ctx->gid))
> +		kgid = make_kgid(current_user_ns(), result.uint_32);
> +		if (!gid_valid(kgid))
>  			goto bad_value;
> +
> +		/*
> +		 * The requested gid must be representable in the
> +		 * filesystem's idmapping.
> +		 */
> +		if (!kgid_has_mapping(fc->user_ns, kgid))
> +			goto bad_value;
> +
> +		ctx->gid = kgid;
>  		break;
>  	case Opt_huge:
>  		ctx->huge = result.uint_32;
> 
> ---
> base-commit: 06c2afb862f9da8dc5efa4b6076a0e48c3fbaaa5
> change-id: 20230801-vfs-fs_context-uidgid-7756c8dcb1c0
>