| Message ID | 20221214194056.161492-21-michael.roth@amd.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support | expand |
On Wed, 14 Dec 2022 13:40:12 -0600 Michael Roth <michael.roth@amd.com> wrote: > From: Brijesh Singh <brijesh.singh@amd.com> > > 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@profian.com> > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@profian.com> > Co-developed-by: Ashish Kalra <ashish.kalra@amd.com> > Signed-off-by: Ashish Kalra <ashish.kalra@amd.com> > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com> > Signed-off-by: Michael Roth <michael.roth@amd.com> > --- > 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);
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);