5.15-stable review patch. If anyone has any objections, please let me know.
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From: Patrick Roy roypat@amazon.co.uk
commit 532b53cebe58f34ce1c0f34d866f5c0e335c53c6 upstream.
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/
Link: https://lkml.kernel.org/r/20241001080056.784735-1-roypat@amazon.co.uk Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas") Signed-off-by: Patrick Roy roypat@amazon.co.uk Reviewed-by: Mike Rapoport (Microsoft) rppt@kernel.org Cc: Alexander Graf graf@amazon.com Cc: David Hildenbrand david@redhat.com Cc: James Gowans jgowans@amazon.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/secretmem.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/secretmem.c +++ b/mm/secretmem.c @@ -234,7 +234,7 @@ SYSCALL_DEFINE1(memfd_secret, unsigned i /* 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)) @@ -278,7 +278,7 @@ static int secretmem_init(void) { int ret = 0;
- if (!secretmem_enable) + if (!secretmem_enable || !can_set_direct_map()) return ret;
secretmem_mnt = kern_mount(&secretmem_fs);