The patch titled
Subject: secretmem: disable memfd_secret() if arch cannot set direct map
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
secretmem-disable-memfd_secret-if-arch-cannot-set-direct-map.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/secretmem-disable-memfd_secret-if-arch-cannot-set-direct-map.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Patrick Roy <roypat@xxxxxxxxxxxx>
Subject: secretmem: disable memfd_secret() if arch cannot set direct map
Date: Tue, 1 Oct 2024 09:00:41 +0100
Return -ENOSYS from memfd_secret() syscall if !can_set_direct_map(). This
is the case for example on some arm64 configurations, where marking 4k
PTEs in the direct map not present can only be done if the direct map is
set up at 4k granularity in the first place (as ARM's break-before-make
semantics do not easily allow breaking apart large/gigantic pages).
More precisely, on arm64 systems with !can_set_direct_map(),
set_direct_map_invalid_noflush() is a no-op, however it returns success
(0) instead of an error. This means that memfd_secret will seemingly
"work" (e.g. syscall succeeds, you can mmap the fd and fault in pages),
but it does not actually achieve its goal of removing its memory from the
direct map.
Note that with this patch, memfd_secret() will start erroring on systems
where can_set_direct_map() returns false (arm64 with
CONFIG_RODATA_FULL_DEFAULT_ENABLED=n, CONFIG_DEBUG_PAGEALLOC=n and
CONFIG_KFENCE=n), but that still seems better than the current silent
failure. Since CONFIG_RODATA_FULL_DEFAULT_ENABLED defaults to 'y', most
arm64 systems actually have a working memfd_secret() and aren't be
affected.
