From: Mike Rapoport <rppt@xxxxxxxxxxxxx> Subject: userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK A while ago Andy noticed (http://lkml.kernel.org/r/CALCETrWY+5ynDct7eU_nDUqx=okQvjm=Y5wJvA4ahBja=CQXGw@xxxxxxxxxxxxxx) that UFFD_FEATURE_EVENT_FORK used by an unprivileged user may have security implications. As the first step of the solution the following patch limits the availably of UFFD_FEATURE_EVENT_FORK only for those having CAP_SYS_PTRACE. The usage of CAP_SYS_PTRACE ensures compatibility with CRIU. Yet, if there are other users of non-cooperative userfaultfd that run without CAP_SYS_PTRACE, they would be broken :( Current implementation of UFFD_FEATURE_EVENT_FORK modifies the file descriptor table from the read() implementation of uffd, which may have security implications for unprivileged use of the userfaultfd. Limit availability of UFFD_FEATURE_EVENT_FORK only for callers that have CAP_SYS_PTRACE. Link: http://lkml.kernel.org/r/1572967777-8812-2-git-send-email-rppt@xxxxxxxxxxxxx Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Daniel Colascione <dancol@xxxxxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx> Cc: Nick Kralevich <nnk@xxxxxxxxxx> Cc: Nosh Minwalla <nosh@xxxxxxxxxx> Cc: Pavel Emelyanov <ovzxemul@xxxxxxxxx> Cc: Tim Murray <timmurray@xxxxxxxxxx> Cc: Aleksa Sarai <cyphar@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/userfaultfd.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) --- a/fs/userfaultfd.c~userfaultfd-require-cap_sys_ptrace-for-uffd_feature_event_fork +++ a/fs/userfaultfd.c @@ -1835,13 +1835,12 @@ static int userfaultfd_api(struct userfa if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api))) goto out; features = uffdio_api.features; - if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) { - memset(&uffdio_api, 0, sizeof(uffdio_api)); - if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api))) - goto out; - ret = -EINVAL; - goto out; - } + ret = -EINVAL; + if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) + goto err_out; + ret = -EPERM; + if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE)) + goto err_out; /* report all available features and ioctls to userland */ uffdio_api.features = UFFD_API_FEATURES; uffdio_api.ioctls = UFFD_API_IOCTLS; @@ -1854,6 +1853,11 @@ static int userfaultfd_api(struct userfa ret = 0; out: return ret; +err_out: + memset(&uffdio_api, 0, sizeof(uffdio_api)); + if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api))) + ret = -EFAULT; + goto out; } static long userfaultfd_ioctl(struct file *file, unsigned cmd, _
