Messages in this thread | | | Date | Sun, 1 Nov 2020 12:09:35 +0100 | From | Hagen Paul Pfeifer <> | Subject | Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create "secret" memory areas |
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* Mike Rapoport | 2020-09-24 16:28:58 [+0300]:
>This is an implementation of "secret" mappings backed by a file descriptor. >I've dropped the boot time reservation patch for now as it is not strictly >required for the basic usage and can be easily added later either with or >without CMA.
Isn't memfd_secret currently *unnecessarily* designed to be a "one task feature"? memfd_secret fulfills exactly two (generic) features:
- address space isolation from kernel (aka SECRET_EXCLUSIVE, not in kernel's direct map) - hide from kernel, great - disabling processor's memory caches against speculative-execution vulnerabilities (spectre and friends, aka SECRET_UNCACHED), also great
But, what about the following use-case: implementing a hardened IPC mechanism where even the kernel is not aware of any data and optionally via SECRET_UNCACHED even the hardware caches are bypassed! With the patches we are so close to achieving this.
How? Shared, SECRET_EXCLUSIVE and SECRET_UNCACHED mmaped pages for IPC involved tasks required to know this mapping (and memfd_secret fd). After IPC is done, tasks can copy sensitive data from IPC pages into memfd_secret() pages, un-sensitive data can be used/copied everywhere.
One missing piece is still the secure zeroization of the page(s) if the mapping is closed by last process to guarantee a secure cleanup. This can probably done as an general mmap feature, not coupled to memfd_secret() and can be done independently ("reverse" MAP_UNINITIALIZED feature).
PS: thank you Mike for your effort!
See the following pseudo-code as an example:
// simple assume file-descriptor and mapping is inherited // by child for simplicity, ptr is int fd = memfd_secret(SECRETMEM_UNCACHED); ftruncate(fd, PAGE_SIZE); uint32_t *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
pid_t pid_other;
void signal_handler(int sig) { // update IPC data on shared, uncachaed, exclusive mapped page *ptr += 1; // inform other sleep(1); kill(pid_other, SIGUSR1); }
void ipc_loop(void) { signal(SIGUSR1, signal_handler); while (1) { sleep(1); } }
int main(void) { pid_t child_pid;
switch (child_pid = fork()) { case 0: pid_other = getppid(); break; default: pid_other = child_pid break; } ipc_loop(); }
Hagen
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