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

[Mike Rapoport <rppt@kernel.org>]

On Tue, Oct 01, 2024 at 09:00:41AM +0100, Patrick Roy wrote:
> 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>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  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
> 

-- 
Sincerely yours,
Mike.