This is a note to let you know that I've just added the patch titled userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: userfaultfd-require-cap_sys_ptrace-for-uffd_feature_.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 9e18c69239dcfdfe37ecb89c477edc3f6b74a7b1 Author: Mike Rapoport <rppt@xxxxxxxxxxxxx> Date: Sat Nov 30 17:58:01 2019 -0800 userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK [ Upstream commit 3c1c24d91ffd536de0a64688a9df7f49e58fadbc ] 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> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 9c2955f67f70..d269d1139f7f 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1833,13 +1833,12 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx, 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; @@ -1852,6 +1851,11 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx, 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,