From: Jordy Zomer <jordy@xxxxxxxxxxxxxxxxxxxx> Subject: mm/secretmem: use refcount_t instead of atomic_t When a secret memory region is active, memfd_secret disables hibernation. One of the goals is to keep the secret data from being written to persistent-storage. It accomplishes this by maintaining a reference count to `secretmem_users`. Once this reference is held your system can not be hibernated due to the check in `hibernation_available()`. However, because `secretmem_users` is of type `atomic_t`, reference counter overflows are possible. As you can see there's an `atomic_inc` for each `memfd` that is opened in the `memfd_secret` syscall. If a local attacker succeeds to open 2^32 memfd's, the counter will wrap around to 0. This implies that you may hibernate again, even though there are still regions of this secret memory, thereby bypassing the security check. In an attempt to fix this I have used `refcount_t` instead of `atomic_t` which prevents reference counter overflows. Link: https://lkml.kernel.org/r/20210820043339.2151352-1-jordy@pwning.systems Signed-off-by: Jordy Zomer <jordy@pwning.systems> Cc: Kees Cook <keescook@xxxxxxxxxxxx>, Cc: Jordy Zomer <jordy@xxxxxxxxxxxxxxxxxxxx> Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> Cc: Mike Rapoport <rppt@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/secretmem.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) --- a/mm/secretmem.c~mm-secretmem-use-refcount_t-instead-of-atomic_t +++ a/mm/secretmem.c @@ -18,6 +18,7 @@ #include <linux/secretmem.h> #include <linux/set_memory.h> #include <linux/sched/signal.h> +#include <linux/refcount.h> #include <uapi/linux/magic.h> @@ -40,11 +41,11 @@ module_param_named(enable, secretmem_ena MODULE_PARM_DESC(secretmem_enable, "Enable secretmem and memfd_secret(2) system call"); -static atomic_t secretmem_users; +static refcount_t secretmem_users; bool secretmem_active(void) { - return !!atomic_read(&secretmem_users); + return !!refcount_read(&secretmem_users); } static vm_fault_t secretmem_fault(struct vm_fault *vmf) @@ -103,7 +104,7 @@ static const struct vm_operations_struct static int secretmem_release(struct inode *inode, struct file *file) { - atomic_dec(&secretmem_users); + refcount_dec(&secretmem_users); return 0; } @@ -217,7 +218,7 @@ SYSCALL_DEFINE1(memfd_secret, unsigned i file->f_flags |= O_LARGEFILE; fd_install(fd, file); - atomic_inc(&secretmem_users); + refcount_inc(&secretmem_users); return fd; err_put_fd: _
