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SubjectRe: [PATCH v6 2/5] fuse: Fail all requests with invalid uids or gids
On Wed, Feb 21, 2018 at 9:29 PM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> Upon a cursory examinination the uid and gid of a fuse request are
> necessary for correct operation. Failing a fuse request where those
> values are not reliable seems a straight forward and reliable means of
> ensuring that fuse requests with bad data are not sent or processed.
>
> In most cases the vfs will avoid actions it suspects will cause
> an inode write back of an inode with an invalid uid or gid. But that does
> not map precisely to what fuse is doing, so test for this and solve
> this at the fuse level as well.
>
> Performing this work in fuse_req_init_context is cheap as the code is
> already performing the translation here and only needs to check the
> result of the translation to see if things are not representable in
> a form the fuse server can handle.
>
> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
> ---
> fs/fuse/dev.c | 20 +++++++++++++-------
> 1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 0fb58f364fa6..216db3f51a31 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -112,11 +112,13 @@ static void __fuse_put_request(struct fuse_req *req)
> refcount_dec(&req->count);
> }
>
> -static void fuse_req_init_context(struct fuse_conn *fc, struct fuse_req *req)
> +static bool fuse_req_init_context(struct fuse_conn *fc, struct fuse_req *req)
> {
> - req->in.h.uid = from_kuid_munged(&init_user_ns, current_fsuid());
> - req->in.h.gid = from_kgid_munged(&init_user_ns, current_fsgid());
> + req->in.h.uid = from_kuid(&init_user_ns, current_fsuid());
> + req->in.h.gid = from_kgid(&init_user_ns, current_fsgid());
> req->in.h.pid = pid_nr_ns(task_pid(current), fc->pid_ns);
> +
> + return (req->in.h.uid != ((uid_t)-1)) && (req->in.h.gid != ((gid_t)-1));
> }
>
> void fuse_set_initialized(struct fuse_conn *fc)
> @@ -162,12 +164,13 @@ static struct fuse_req *__fuse_get_req(struct fuse_conn *fc, unsigned npages,
> wake_up(&fc->blocked_waitq);
> goto out;
> }
> -
> - fuse_req_init_context(fc, req);
> __set_bit(FR_WAITING, &req->flags);
> if (for_background)
> __set_bit(FR_BACKGROUND, &req->flags);
> -
> + if (unlikely(!fuse_req_init_context(fc, req))) {
> + fuse_put_request(fc, req);
> + return ERR_PTR(-EOVERFLOW);
> + }
> return req;
>
> out:
> @@ -256,9 +259,12 @@ struct fuse_req *fuse_get_req_nofail_nopages(struct fuse_conn *fc,
> if (!req)
> req = get_reserved_req(fc, file);
>
> - fuse_req_init_context(fc, req);
> __set_bit(FR_WAITING, &req->flags);
> __clear_bit(FR_BACKGROUND, &req->flags);
> + if (unlikely(!fuse_req_init_context(fc, req))) {
> + fuse_put_request(fc, req);
> + return ERR_PTR(-EOVERFLOW);
> + }

I think failing the "_nofail" variant is the wrong thing to do. This
is called to allocate a FLUSH request on close() and in readdirplus to
allocate a FORGET request. Failing the latter results in refcount
leak in userspace. Failing the former results in missing unlock on
close() of posix locks.

Thanks,
Miklos

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