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SubjectRe: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address
On 6/21/22 01:03, Ashish Kalra 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.
>
> 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.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/mm/fault.c | 66 ++++++++++++++++++++++++++++++++++++++++
> include/linux/mm.h | 3 +-
> include/linux/mm_types.h | 3 ++
> mm/memory.c | 13 ++++++++
> 4 files changed, 84 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index a4c270e99f7f..f5de9673093a 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -19,6 +19,7 @@
> #include <linux/uaccess.h> /* faulthandler_disabled() */
> #include <linux/efi.h> /* efi_crash_gracefully_on_page_fault()*/
> #include <linux/mm_types.h>
> +#include <linux/sev.h> /* snp_lookup_rmpentry() */
>
> #include <asm/cpufeature.h> /* boot_cpu_has, ... */
> #include <asm/traps.h> /* dotraplinkage, ... */
> @@ -1209,6 +1210,60 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
> }
> NOKPROBE_SYMBOL(do_kern_addr_fault);
>
> +static inline size_t pages_per_hpage(int level)
> +{
> + return page_level_size(level) / PAGE_SIZE;
> +}
> +
> +/*
> + * Return 1 if the caller need to retry, 0 if it the address need to be split
> + * in order to resolve the fault.
> + */
> +static int handle_user_rmp_page_fault(struct pt_regs *regs, unsigned long error_code,
> + unsigned long address)
> +{
> + int rmp_level, level;
> + pte_t *pte;
> + u64 pfn;
> +
> + pte = lookup_address_in_mm(current->mm, 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 1;
> +
> + pfn = pte_pfn(*pte);
> +
> + /* If its large page then calculte the fault pfn */
> + if (level > PG_LEVEL_4K) {
> + unsigned long mask;
> +
> + mask = pages_per_hpage(level) - pages_per_hpage(level - 1);
> + pfn |= (address >> PAGE_SHIFT) & mask;
> + }
> +
> + /*
> + * If its a guest private page, then the fault cannot be resolved.
> + * Send a SIGBUS to terminate the process.
> + */
> + if (snp_lookup_rmpentry(pfn, &rmp_level)) {
> + do_sigbus(regs, error_code, address, VM_FAULT_SIGBUS);
> + return 1;
> + }
> +
> + /*
> + * The backing page level is higher than the RMP page level, request
> + * to split the page.
> + */
> + if (level > rmp_level)
> + return 0;

I don't see any checks that make sure this is in fact a THP, and not e.g.
hugetlb (which is disallowed only later in patch 25/49), or even something
else unexpected. Calling blindly __split_huge_pmd() in
handle_split_page_fault() on anything that's not a THP will just make it
return without splitting anything, and then this will result in a page fault
loop? Some kind of warning and a SIGBUS would be more safe I think.

> +
> + return 1;
> +}
> +
> /*
> * Handle faults in the user portion of the address space. Nothing in here
> * should check X86_PF_USER without a specific justification: for almost
> @@ -1306,6 +1361,17 @@ 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 de32c0383387..2ccc562d166f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -463,7 +463,8 @@ 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 6dfaf271ebf8..aa2d8d48ce3e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -818,6 +818,8 @@ 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
> @@ -855,6 +857,7 @@ 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 7274f2b52bca..c2187ffcbb8e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4945,6 +4945,15 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
> return 0;
> }
>
> +static int handle_split_page_fault(struct vm_fault *vmf)
> +{
> + if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
> + return VM_FAULT_SIGBUS;
> +
> + __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
> *
> @@ -5024,6 +5033,10 @@ 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);

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