On Tue, Nov 1, 2022 at 7:45 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Tue, Nov 01, 2022 at 04:14:39PM -0700, Jeff Xu wrote: > > Sorry for the long overdue reply. > > No worries! I am a fan of thread necromancy. :) > > > [...] > > 1> memfd_create: > > Add two flags: > > #define MFD_EXEC 0x0008 > > #define MFD_NOEXEC_SEAL 0x0010 > > This lets application to set executable bit explicitly. > > (If application set both, it will be rejected) > > So no MFD_NOEXEC without seal? (I'm fine with that.) > no MFD_NOEXEC because memfd can be chmod to add x after creation, it is not secure. no MFD_EXEC_SEAL because it is better to apply both w and x seal within the same function call, and w seal can't be applied at creation time. > > 2> For old application that doesn't set executable bit: > > Add a pid name-spaced sysctl.kernel.pid_mfd_noexec, with: > > bikeshed: vm.memfd_noexec > (doesn't belong in "kernel", and seems better suited to "vm" than "fs") > SG, will use vm.memfd_noexec > > value = 0: Default_EXEC > > Honor MFD_EXEC and MFD_NOEXEC_SEAL > > When none is set, will fall back to original behavior (EXEC) > > Yeah. Rephrasing for myself to understand more clearly: > > "memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL act like MFD_EXEC > was set." > > > value = 1: Default_NOEXEC_SEAL > > Honor MFD_EXEC and MFD_NOEXEC_SEAL > > When none is set, will default to MFD_NOEXEC_SEAL > > "memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL act like > MFD_NOEXEC_SEAL was set." > Copy, this is clearer. Thanks. > Also, I think there should be a pr_warn_ratelimited() when > memfd_create() is used without either bit, so that there is some > pressure to please adjust their API calls to explicitly set a bit. > Sure > > 3> Add a pid name-spaced sysctl kernel.pid_mfd_noexec_enforced: with: > > value = 0: default, not enforced. > > value = 1: enforce NOEXEC_SEAL (overwrite everything) > > How about making this just mode "value 2" for the first sysctl? > "memfd_create() without MFD_NOEXEC_SEAL will be rejected." > Good point. Kernel overwriting might not be a good practice. I will add to vm.mfd_noexec. value = 2: "memfd_create() without MFD_NOEXEC_SEAL will be rejected." Thanks! Jeff > -Kees > > -- > Kees Cook
