On Thu, Feb 22, 2024 at 11:09:40PM -0800, Christoph Hellwig wrote: > On Thu, Feb 22, 2024 at 03:16:42PM -0800, Elliot Berman wrote: > > Firmware and hypervisor drivers can donate system heap memory to their > > respective firmware/hypervisor entities. Those drivers should unmap the > > pages from the kernel's logical map before doing so. > > > > Export can_set_direct_map, set_direct_map_invalid_noflush, and > > set_direct_map_default_noflush. > > Err, not they should not. And not using such super low-level interfaces > from modular code. Hi Cristoph, We've observed a few times that Linux can unintentionally access a page we've unmapped from host's stage 2 page table via an unaligned load from an adjacent page. The stage 2 is managed by Gunyah. There are few scenarios where even though we allocate and own a page from buddy, someone else could try to access the page without going through the hypervisor driver. One such instance we know about is load_unaligned_zeropad() via pathlookup_at() [1]. load_unaligned_zeropad() could be called near the end of a page. If the next page isn't mapped by the kernel in the stage one page tables, then the access from to the unmapped page from load_unaligned_zeropad() will land in __do_kernel_fault(), call fixup_exception(), and fill the remainder of the load with zeroes. If the page in question is mapped in stage 1 but was unmapped from stage 2, then the access lands back in Linux in do_sea(), leading to a panic(). Our preference would be to add fixup_exception() to S2 PTW errors for two reasons: 1. It's cheaper to do performance wise: we've already manipulated S2 page table and prevent intentional access to the page because pKVM/Gunyah drivers know that access to the page has been lost. 2. Page-granular S1 mappings only happen on arm64 with rodata=full. In an off-list discussion with the Android pkvm folks, their preference was to have the pages unmapped from stage 1. I've gone with that approach to get started but welcome discussion on the best approach. The Android (downstream) implementation of arm64 pkvm is currently implementing a hack where s2 ptw faults are given back to the host as s1 ptw faults (i.e. __do_kernel_fault() gets called and not do_sea()) -- allowing the kernel to fixup the exception. arm64 pKVM will also face this issue when implementing guest_memfd or when donating more memory to the hyp for s2 page tables, etc. As far as I can tell, this isn't an issue for arm64 pKVM today because memory isn't being dynamically donated to the hypervisor. Thanks, Elliot [1]: path_lookupat+0x340/0x3228 filename_lookup+0xbc/0x1c0 __arm64_sys_newfstatat+0xb0/0x4a0 invoke_syscall+0x58/0x118
