[PATCH] secretmem: disable memfd_secret() if arch cannot set direct map

[Patrick Roy <roypat@amazon.co.uk>]

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.

From going through the iterations of the original memfd_secret patch
series, it seems that disabling the syscall in these scenarios was the
intended behavior [1] (preferred over having
set_direct_map_invalid_noflush return an error as that would result in
SIGBUSes at page-fault time), however the check for it got dropped
between v16 [2] and v17 [3], when secretmem moved away from CMA
allocations.

[1]: https://lore.kernel.org/lkml/20201124164930.GK8537@kernel.org/
[2]: https://lore.kernel.org/lkml/20210121122723.3446-11-rppt@kernel.org/#t
[3]: https://lore.kernel.org/lkml/20201125092208.12544-10-rppt@kernel.org/

Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas")
Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
---
 mm/secretmem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index 3afb5ad701e14..399552814fd0f 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -238,7 +238,7 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
 	/* make sure local flags do not confict with global fcntl.h */
 	BUILD_BUG_ON(SECRETMEM_FLAGS_MASK & O_CLOEXEC);
 
-	if (!secretmem_enable)
+	if (!secretmem_enable || !can_set_direct_map())
 		return -ENOSYS;
 
 	if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC))
@@ -280,7 +280,7 @@ static struct file_system_type secretmem_fs = {
 
 static int __init secretmem_init(void)
 {
-	if (!secretmem_enable)
+	if (!secretmem_enable || !can_set_direct_map())
 		return 0;
 
 	secretmem_mnt = kern_mount(&secretmem_fs);

base-commit: abf2050f51fdca0fd146388f83cddd95a57a008d
-- 
2.46.2