Hello.
Thank you for your email and interested in using memfd_noexec !
On Wed, Jun 28, 2023 at 4:43 AM Dominique Martinet asmadeus@codewreck.org wrote:
jeffxu@chromium.org wrote on Thu, Dec 15, 2022 at 12:12:03AM +0000:
From: Jeff Xu jeffxu@google.com
The new MFD_NOEXEC_SEAL and MFD_EXEC flags allows application to set executable bit at creation time (memfd_create).
When MFD_NOEXEC_SEAL is set, memfd is created without executable bit (mode:0666), and sealed with F_SEAL_EXEC, so it can't be chmod to be executable (mode: 0777) after creation.
when MFD_EXEC flag is set, memfd is created with executable bit (mode:0777), this is the same as the old behavior of memfd_create.
The new pid namespaced sysctl vm.memfd_noexec has 3 values: 0: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like MFD_EXEC was set. 1: memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like MFD_NOEXEC_SEAL was set. 2: memfd_create() without MFD_NOEXEC_SEAL will be rejected.
So, erm, I'm a bit late to the party but I was just looking at a way of blocking memfd_create+exec in a container and this sounded perfect: my reading is that this is a security feature meant to be set for container's namespaces that'd totally disable something like memfd_create followed by fexecve (because we don't want weird binaries coming from who knows where to be executed on a shiny secure system), but. . . is this actually supposed to work? (see below)
[...] --- a/mm/memfd.c +++ b/mm/memfd.c @@ -263,12 +264,14 @@ long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg) #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
-#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB) +#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC)
SYSCALL_DEFINE2(memfd_create, const char __user *, uname, unsigned int, flags) {
char comm[TASK_COMM_LEN];struct pid_namespace *ns; unsigned int *file_seals; struct file *file; int fd, error;@@ -285,6 +288,39 @@ SYSCALL_DEFINE2(memfd_create, return -EINVAL; }
/* Invalid if both EXEC and NOEXEC_SEAL are set.*/if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL))return -EINVAL;if (!(flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {[code that checks the sysctl]If flags already has either MFD_EXEC or MFD_NOEXEC_SEAL, you don't check the sysctl at all.
This can be verified easily:
$ cat > memfd_exec.c <<'EOF' #define _GNU_SOURCE
#include <errno.h> #include <stdio.h> #include <sys/mman.h> #include <sys/types.h> #include <sys/wait.h>
#ifndef MFD_EXEC #define MFD_EXEC 0x0010U #endif
int main() { int fd = memfd_create("script", MFD_EXEC); if (fd == -1)l perror("memfd");
char prog[] = "#!/bin/sh\necho Ran script\n"; if (write(fd, prog, sizeof(prog)-1) != sizeof(prog)-1) perror("write"); char *const argv[] = { "script", NULL }; char *const envp[] = { NULL }; fexecve(fd, argv, envp); perror("fexecve");} EOF $ gcc -o memfd_exec memfd_exec.c $ ./memfd_exec Ran script $ sysctl vm.memfd_noexec vm.memfd_noexec = 2
(as opposed to failing hard on memfd_create if flag unset on sysctl=2, and failing on fexecve with flag unset and sysctl=1)
What am I missing?
At one point, I was thinking of having a security hook to block executable memfd [1], so this sysctl only works for the application that doesn't set EXEC bit. Now I think it makes sense to use vm.memfd_noexec = 2 to block the MFD_EXEC also. Anyway the commit msg says: 2: memfd_create() without MFD_NOEXEC_SEAL will be rejected. Not doing that is a bug. I will send a fix for that.
[1] https://lore.kernel.org/lkml/20221206150233.1963717-7-jeffxu@google.com/
BTW I find the current behaviour rather hard to use: setting this to 2 should still set NOEXEC by default in my opinion, just refuse anything that explicitly requested EXEC.
At one point [2] (v2 of patch) there were two sysctls, one is doing overwrite, one is enforcing, later I decided with one sysctl, the rationale is the kernel will eventually get out of the business of overwriting user space code. Yes. It might take a long time to migrate all of the userspace.
In the meantime, to meet what you want, the solution is keep vm.memfd_noexec = 1 (for overwrite), and a new security policy (SELInux or Landlock) that uses security hook security_memfd_create, this can block one process from creating executable memfd. Indeed, security policy is better fit to cases like this than sysctl.
[2] https://lore.kernel.org/linux-mm/CABi2SkWGo9Jrd=i1e2PoDWYGenGhR=pG=yGsQP5VLm...
Sure there's a warn_once that memfd_create was used without seal, but right now on my system it's "used up" 5 seconds after boot by systemd: [ 5.854378] memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL, pid=1 'systemd'
And anyway, older kernels will barf up EINVAL when calling memfd_create with MFD_NOEXEC_SEAL, so even if userspace will want to adapt they'll need to try calling memfd_create with the flag once and retry on EINVAL, which let's face it is going to take a while to happen. (Also, the flag has been added to glibc, but not in any release yet)
Yes. Application will need to do some detection of the kernel. This is not avoidable.
Making calls default to noexec AND refuse exec does what you want (forbid use of exec in an app that wasn't in a namespace that allows exec) while allowing apps that require it to work; that sounds better than making all applications that haven't taken the pain of adding the new flag to me. Well, I guess an app that did require exec without setting the flag will fail in a weird place instead of failing at memfd_create and having a chance to fallback, so it's not like it doesn't make any sense; I don't have such strong feelings about this if the sysctl works, but for my use case I'm more likely to want to take a chance at memfd_create not needing exec than having the flag set. Perhaps a third value if I cared enough...
-- Dominique Martinet | Asmadeus
Thanks -Jeff