On Mon, Oct 14, 2024 at 05:55:22PM +0200, Greg KH wrote:
On Mon, Oct 14, 2024 at 06:21:03PM +0300, Mike Rapoport wrote:
From: 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.
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 Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
mm/secretmem.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
What is the git id of this change in Linus's tree?
532b53cebe58f34ce1c0f34d866f5c0e335c53c6