On Mon, Oct 16, 2023 at 4:18 PM Matthew Wilcox willy@infradead.org wrote:
On Mon, Oct 16, 2023 at 02:38:19PM +0000, jeffxu@chromium.org wrote:
Modern CPUs support memory permissions such as RW and NX bits. Linux has supported NX since the release of kernel version 2.6.8 in August 2004 [1].
This seems like a confusing way to introduce the subject. Here, you're talking about page permissions, whereas (as far as I can tell), mseal() is about making _virtual_ addresses immutable, for some value of immutable.
Memory sealing additionally protects the mapping itself against modifications. This is useful to mitigate memory corruption issues where a corrupted pointer is passed to a memory management syscall. For example, such an attacker primitive can break control-flow integrity guarantees since read-only memory that is supposed to be trusted can become writable or .text pages can get remapped. Memory sealing can automatically be applied by the runtime loader to seal .text and .rodata pages and applications can additionally seal security critical data at runtime. A similar feature already exists in the XNU kernel with the VM_FLAGS_PERMANENT [3] flag and on OpenBSD with the mimmutable syscall [4]. Also, Chrome wants to adopt this feature for their CFI work [2] and this patchset has been designed to be compatible with the Chrome use case.
This [2] seems very generic and wide-ranging, not helpful. [5] was more useful to understand what you're trying to do.
The new mseal() is an architecture independent syscall, and with following signature:
mseal(void addr, size_t len, unsigned int types, unsigned int flags)
addr/len: memory range. Must be continuous/allocated memory, or else mseal() will fail and no VMA is updated. For details on acceptable arguments, please refer to comments in mseal.c. Those are also fully covered by the selftest.
Mmm. So when you say "continuous/allocated" what you really mean is "Must have contiguous VMAs" rather than "All pages in this range must be populated", yes?
types: bit mask to specify which syscall to seal, currently they are: MM_SEAL_MSEAL 0x1 MM_SEAL_MPROTECT 0x2 MM_SEAL_MUNMAP 0x4 MM_SEAL_MMAP 0x8 MM_SEAL_MREMAP 0x10
I don't understand why we want this level of granularity. The OpenBSD and XNU examples just say "This must be immutable*". For values of immutable that allow downgrading access (eg RW to RO or RX to RO), but not upgrading access (RW->RX, RO->*, RX->RW).
Each bit represents sealing for one specific syscall type, e.g. MM_SEAL_MPROTECT will deny mprotect syscall. The consideration of bitmask is that the API is extendable, i.e. when needed, the sealing can be extended to madvise, mlock, etc. Backward compatibility is also easy.
Honestly, it feels too flexible. Why not just two flags to mprotect() -- PROT_IMMUTABLE and PROT_DOWNGRADABLE. I can see a use for that -- maybe for some things we want to be able to downgrade and for other things, we don't.
I think it's worth pointing out that this suggestion (with PROT_*) could easily integrate with mmap() and as such allow for one-shot mmap() + mseal(). If we consider the common case as 'addr = mmap(...); mseal(addr);', it definitely sounds like a performance win as we halve the number of syscalls for a sealed mapping. And if we trivially look at e.g OpenBSD ld.so code, mmap() + mimmutable() and mprotect() + mimmutable() seem like common patterns.