The integrity guarantee of SEV-SNP is enforced through the RMP table. The RMP is used in conjuntion with standard x86 and IOMMU page tables to enforce memory restrictions and page access rights. The RMP is indexed by system physical address, and is checked at the end of CPU and IOMMU table walks. The RMP check is enforced as soon as SEV-SNP is enabled globally in the system. Not every memory access requires an RMP check. In particular, the read accesses from the hypervisor do not require RMP checks because the data confidentiality is already protected via memory encryption. When hardware encounters an RMP checks failure, it raise a page-fault exception. The RMP bit in fault error code can be used to determine if the fault was due to an RMP checks failure.
A write from the hypervisor goes through the RMP checks. When the hypervisor writes to pages, hardware checks to ensures that the assigned bit in the RMP is zero (i.e page is shared). If the page table entry that gives the sPA indicates that the target page size is a large page, then all RMP entries for the 4KB constituting pages of the target must have the assigned bit 0. If one of entry does not have assigned bit 0 then hardware will raise an RMP violation. To resolve it, we must split the page table entry leading to target page into 4K. This poses a challenge in the Linux memory model. The Linux kernel creates a direct mapping of all the physical memory -- referred to as the physmap. The physmap may contain a valid mapping of guest owned pages. During the page table walk, we may get into the situation where one of the pages within the large page is owned by the guest (i.e assigned bit is set in RMP). A write to a non-guest within the large page will raise an RMP violation. To workaround it, we call set_memory_4k() to split the physmap before adding the page in the RMP table. This ensures that the pages added in the RMP table are used as 4K in the physmap. The spliting of the physmap is a temporary solution until we work to improve the kernel page fault handler to split the pages on demand. One of the disadvtange of splitting is that eventually, we will end up breaking down the entire physmap unless we combine the split pages back to a large page. I am open to the suggestation on various approaches we could take to address this problem. Cc: Thomas Gleixner <t...@linutronix.de> Cc: Ingo Molnar <mi...@redhat.com> Cc: Borislav Petkov <b...@alien8.de> Cc: Joerg Roedel <jroe...@suse.de> Cc: "H. Peter Anvin" <h...@zytor.com> Cc: Tony Luck <tony.l...@intel.com> Cc: Dave Hansen <dave.han...@intel.com> Cc: "Peter Zijlstra (Intel)" <pet...@infradead.org> Cc: Paolo Bonzini <pbonz...@redhat.com> Cc: Tom Lendacky <thomas.lenda...@amd.com> Cc: David Rientjes <rient...@google.com> Cc: Sean Christopherson <sea...@google.com> Cc: x...@kernel.org Cc: k...@vger.kernel.org Signed-off-by: Brijesh Singh <brijesh.si...@amd.com> --- arch/x86/mm/mem_encrypt.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c index 7a0138cb3e17..4047acb37c30 100644 --- a/arch/x86/mm/mem_encrypt.c +++ b/arch/x86/mm/mem_encrypt.c @@ -674,6 +674,12 @@ int rmptable_rmpupdate(struct page *page, struct rmpupdate *val) if (!static_branch_unlikely(&snp_enable_key)) return -ENXIO; + ret = set_memory_4k((unsigned long)page_to_virt(page), 1); + if (ret) { + pr_err("SEV-SNP: failed to split physical address 0x%lx (%d)\n", spa, ret); + return ret; + } + /* Retry if another processor is modifying the RMP entry. */ do { asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE" -- 2.17.1