Re: Re: [PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs
From: Mickaël Salaün <mic@digikod.net>
Date: 2024-01-25 16:39:08
Also in:
linux-fsdevel, linux-hardening, linux-mm, lkml
On Wed, Jan 24, 2024 at 01:32:02PM -0800, Kees Cook wrote:
On Wed, Jan 24, 2024 at 12:47:34PM -0800, Linus Torvalds wrote:quoted
On Wed, 24 Jan 2024 at 12:15, Kees Cook [off-list ref] wrote:quoted
Hmpf, and frustratingly Ubuntu (and Debian) still builds with CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago.For completeness, Fedora hasn't had CONFIG_USELIB for a while now.quoted
Well, we could just remove the __FMODE_EXEC from uselib. It's kind of wrong anyway.Yeah.quoted
So I think just removing __FMODE_EXEC would just do the RightThing(tm), and changes nothing for any sane situation.Agreed about these: - fs/fcntl.c is just doing a bitfield sanity check. - nfs_open_permission_mask(), as you say, is only checking for unreadable case. - fsnotify would also see uselib() as a read, but afaict, that's what it would see for an mmap(), so this should be functionally safe. This one, though, I need some more time to examine: - AppArmor, TOMOYO, and LandLock will see uselib() as an open-for-read, so that might still be a problem? As you say, it's more of a mmap() call, but that would mean adding something a call like security_mmap_file() into uselib()...
If user space can emulate uselib() without opening a file with __FMODE_EXEC, then there is no security reason to keep __FMODE_EXEC for uselib(). Removing __FMODE_EXEC from uselib() looks OK for Landlock. We use __FMODE_EXEC to infer if a file is being open for execution i.e., by execve(2). If __FMODE_EXEC is removed from uselib(), I think it should also be backported to all stable kernels for consistency though.
quoted hunk ↗ jump to hunk
The issue isn't an insane "support uselib() under AppArmor" case, but rather "Can uselib() be used to bypass exec/mmap checks?" This totally untested patch might give appropriate coverage:diff --git a/fs/exec.c b/fs/exec.c index d179abb78a1c..0c9265312c8d 100644 --- a/fs/exec.c +++ b/fs/exec.c@@ -143,6 +143,10 @@ SYSCALL_DEFINE1(uselib, const char __user *, library) if (IS_ERR(file)) goto out; + error = security_mmap_file(file, PROT_READ | PROT_EXEC, MAP_FIXED | MAP_SHARED); + if (error) + goto exit; + /* * may_open() has already checked for this, so it should be * impossible to trip now. But we need to be extra cautiousquoted
Of course, as you say, not having CONFIG_USELIB enabled at all is the _truly_ sane thing, but the only thing that used the FMODE_EXEC bit were landlock and some special-case nfs stuff.Do we want to attempt deprecation again? This was suggested last time: https://lore.kernel.org/lkml/20200518130251.zih2s32q2rxhxg6f@wittgenstein/ (local) -Kees -- Kees Cook