On Wed, 14 Dec 2022 13:40:12 -0600
Michael Roth <michael.roth@xxxxxxx> wrote:
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>
> When SEV-SNP is enabled globally, a write from the host goes through the
> RMP check. When the host writes to pages, hardware checks the following
> conditions at the end of page walk:
>
> 1. Assigned bit in the RMP table is zero (i.e page is shared).
> 2. 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.
> 3. Immutable bit in the RMP table is not zero.
>
Just being curious. AMD APM table 15-37 "RMP Page Assignment Settings" shows
Immuable bit is "don't care" when a page is owned by the hypervisor. The
table 15-39 "RMP Memory Access Checks" shows the hardware will do
"Hypervisor-owned" check for host data write and page table access. I suppose
"Hypervisor-owned" check means HW will check if the RMP entry is configured
according to the table 15-37 (Assign bit = 0, ASID = 0, Immutable = X)
None of them mentions that Immutable bit in the related RMP-entry should
be 1 for hypervisor-owned page.
I can understand 1) 2). Can you explain more about 3)?
> The hardware will raise page fault if one of the above conditions is not
> met. Try resolving the fault instead of taking fault again and again. If
> the host attempts to write to the guest private memory then send the
> SIGBUS signal to kill the process. If the page level between the host and
> RMP entry does not match, then split the address to keep the RMP and host
> page levels in sync.
>
> Co-developed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxx>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxx>
> Co-developed-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
> arch/x86/mm/fault.c | 97 ++++++++++++++++++++++++++++++++++++++++
> include/linux/mm.h | 3 +-
> include/linux/mm_types.h | 3 ++
> mm/memory.c | 10 +++++
> 4 files changed, 112 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index f8193b99e9c8..d611051dcf1e 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -33,6 +33,7 @@
> #include <asm/kvm_para.h> /* kvm_handle_async_pf */
> #include <asm/vdso.h> /* fixup_vdso_exception() */
> #include <asm/irq_stack.h>
> +#include <asm/sev.h> /* snp_lookup_rmpentry() */
>
> #define CREATE_TRACE_POINTS
> #include <asm/trace/exceptions.h>
> @@ -414,6 +415,7 @@ static void dump_pagetable(unsigned long address)
> pr_cont("PTE %lx", pte_val(*pte));
> out:
> pr_cont("\n");
> +
> return;
> bad:
> pr_info("BAD\n");
> @@ -1240,6 +1242,90 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
> }
> NOKPROBE_SYMBOL(do_kern_addr_fault);
>
> +enum rmp_pf_ret {
> + RMP_PF_SPLIT = 0,
> + RMP_PF_RETRY = 1,
> + RMP_PF_UNMAP = 2,
> +};
> +
> +/*
> + * The goal of RMP faulting routine is really to check whether the
> + * page that faulted should be accessible. That can be determined
> + * simply by looking at the RMP entry for the 4k address being accessed.
> + * If that entry has Assigned=1 then it's a bad address. It could be
> + * because the 2MB region was assigned as a large page, or it could be
> + * because the region is all 4k pages and that 4k was assigned.
> + * In either case, it's a bad access.
> + * There are basically two main possibilities:
> + * 1. The 2M entry has Assigned=1 and Page_Size=1. Then all 511 middle
> + * entries also have Assigned=1. This entire 2M region is a guest page.
> + * 2. The 2M entry has Assigned=0 and Page_Size=0. Then the 511 middle
> + * entries can be anything, this region consists of individual 4k assignments.
> + */
> +static int handle_user_rmp_page_fault(struct pt_regs *regs, unsigned long error_code,
> + unsigned long address)
> +{
> + int rmp_level, level;
> + pgd_t *pgd;
> + pte_t *pte;
> + u64 pfn;
> +
> + pgd = __va(read_cr3_pa());
> + pgd += pgd_index(address);
> +
> + pte = lookup_address_in_pgd(pgd, address, &level);
> +
> + /*
> + * It can happen if there was a race between an unmap event and
> + * the RMP fault delivery.
> + */
> + if (!pte || !pte_present(*pte))
> + return RMP_PF_UNMAP;
> +
> + /*
> + * RMP page fault handler follows this algorithm:
> + * 1. Compute the pfn for the 4kb page being accessed
> + * 2. Read that RMP entry -- If it is assigned then kill the process
> + * 3. Otherwise, check the level from the host page table
> + * If level=PG_LEVEL_4K then the page is already smashed
> + * so just retry the instruction
> + * 4. If level=PG_LEVEL_2M/1G, then the host page needs to be split
> + */
> +
> + pfn = pte_pfn(*pte);
> +
> + /* If its large page then calculte the fault pfn */
> + if (level > PG_LEVEL_4K)
> + pfn = pfn | PFN_DOWN(address & (page_level_size(level) - 1));
> +
> + /*
> + * If its a guest private page, then the fault cannot be resolved.
> + * Send a SIGBUS to terminate the process.
> + *
> + * As documented in APM vol3 pseudo-code for RMPUPDATE, when the 2M range
> + * is covered by a valid (Assigned=1) 2M entry, the middle 511 4k entries
> + * also have Assigned=1. This means that if there is an access to a page
> + * which happens to lie within an Assigned 2M entry, the 4k RMP entry
> + * will also have Assigned=1. Therefore, the kernel should see that
> + * the page is not a valid page and the fault cannot be resolved.
> + */
> + if (snp_lookup_rmpentry(pfn, &rmp_level)) {
> + pr_info("Fatal RMP page fault, terminating process, entry assigned for pfn 0x%llx\n",
> + pfn);
> + do_sigbus(regs, error_code, address, VM_FAULT_SIGBUS);
> + return RMP_PF_RETRY;
> + }
> +
> + /*
> + * The backing page level is higher than the RMP page level, request
> + * to split the page.
> + */
> + if (level > rmp_level)
> + return RMP_PF_SPLIT;
> +
> + return RMP_PF_RETRY;
> +}
> +
> /*
> * Handle faults in the user portion of the address space. Nothing in here
> * should check X86_PF_USER without a specific justification: for almost
> @@ -1337,6 +1423,17 @@ void do_user_addr_fault(struct pt_regs *regs,
> if (error_code & X86_PF_INSTR)
> flags |= FAULT_FLAG_INSTRUCTION;
>
> + /*
> + * If its an RMP violation, try resolving it.
> + */
> + if (error_code & X86_PF_RMP) {
> + if (handle_user_rmp_page_fault(regs, error_code, address))
> + return;
> +
> + /* Ask to split the page */
> + flags |= FAULT_FLAG_PAGE_SPLIT;
> + }
> +
> #ifdef CONFIG_X86_64
> /*
> * Faults in the vsyscall page might need emulation. The
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 3c84f4e48cd7..2fd8e16d149c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -466,7 +466,8 @@ static inline bool fault_flag_allow_retry_first(enum fault_flag flags)
> { FAULT_FLAG_USER, "USER" }, \
> { FAULT_FLAG_REMOTE, "REMOTE" }, \
> { FAULT_FLAG_INSTRUCTION, "INSTRUCTION" }, \
> - { FAULT_FLAG_INTERRUPTIBLE, "INTERRUPTIBLE" }
> + { FAULT_FLAG_INTERRUPTIBLE, "INTERRUPTIBLE" }, \
> + { FAULT_FLAG_PAGE_SPLIT, "PAGESPLIT" }
>
> /*
> * vm_fault is filled by the pagefault handler and passed to the vma's
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 500e536796ca..06ba34d51638 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -962,6 +962,8 @@ typedef struct {
> * mapped R/O.
> * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
> * We should only access orig_pte if this flag set.
> + * @FAULT_FLAG_PAGE_SPLIT: The fault was due page size mismatch, split the
> + * region to smaller page size and retry.
> *
> * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
> * whether we would allow page faults to retry by specifying these two
> @@ -999,6 +1001,7 @@ enum fault_flag {
> FAULT_FLAG_INTERRUPTIBLE = 1 << 9,
> FAULT_FLAG_UNSHARE = 1 << 10,
> FAULT_FLAG_ORIG_PTE_VALID = 1 << 11,
> + FAULT_FLAG_PAGE_SPLIT = 1 << 12,
> };
>
> typedef unsigned int __bitwise zap_flags_t;
> diff --git a/mm/memory.c b/mm/memory.c
> index f88c351aecd4..e68da7e403c6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4996,6 +4996,12 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
> return 0;
> }
>
> +static int handle_split_page_fault(struct vm_fault *vmf)
> +{
> + __split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
> + return 0;
> +}
> +
> /*
> * By the time we get here, we already hold the mm semaphore
> *
> @@ -5078,6 +5084,10 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
> pmd_migration_entry_wait(mm, vmf.pmd);
> return 0;
> }
> +
> + if (flags & FAULT_FLAG_PAGE_SPLIT)
> + return handle_split_page_fault(&vmf);
> +
> if (pmd_trans_huge(vmf.orig_pmd) || pmd_devmap(vmf.orig_pmd)) {
> if (pmd_protnone(vmf.orig_pmd) && vma_is_accessible(vma))
> return do_huge_pmd_numa_page(&vmf);
