linux-kernel - Re: [RFC Part2 PATCH 06/30] x86/fault: dump the RMP entry on #PF

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Date:   Wed, 24 Mar 2021 10:47:30 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC Part2 PATCH 06/30] x86/fault: dump the RMP entry on #PF

On Wed, Mar 24, 2021 at 10:04 AM Brijesh Singh <brijesh.singh@....com> wrote:
>
> If hardware detects an RMP violation, it will raise a page-fault exception
> with the RMP bit set. To help the debug, dump the RMP entry of the faulting
> address.
>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Joerg Roedel <jroedel@...e.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: x86@...nel.org
> Cc: kvm@...r.kernel.org
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/mm/fault.c | 75 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 75 insertions(+)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index f39b551f89a6..7605e06a6dd9 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -31,6 +31,7 @@
>  #include <asm/pgtable_areas.h>         /* VMALLOC_START, ...           */
>  #include <asm/kvm_para.h>              /* kvm_handle_async_pf          */
>  #include <asm/vdso.h>                  /* fixup_vdso_exception()       */
> +#include <asm/sev-snp.h>               /* lookup_rmpentry ...          */
>
>  #define CREATE_TRACE_POINTS
>  #include <asm/trace/exceptions.h>
> @@ -147,6 +148,76 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
>  DEFINE_SPINLOCK(pgd_lock);
>  LIST_HEAD(pgd_list);
>
> +static void dump_rmpentry(struct page *page, rmpentry_t *e)
> +{
> +       unsigned long paddr = page_to_pfn(page) << PAGE_SHIFT;
> +
> +       pr_alert("RMPEntry paddr 0x%lx [assigned=%d immutable=%d pagesize=%d gpa=0x%lx asid=%d "
> +               "vmsa=%d validated=%d]\n", paddr, rmpentry_assigned(e), rmpentry_immutable(e),
> +               rmpentry_pagesize(e), rmpentry_gpa(e), rmpentry_asid(e), rmpentry_vmsa(e),
> +               rmpentry_validated(e));
> +       pr_alert("RMPEntry paddr 0x%lx %016llx %016llx\n", paddr, e->high, e->low);
> +}
> +
> +static void show_rmpentry(unsigned long address)
> +{
> +       struct page *page = virt_to_page(address);

This is an error path, and I don't think you have any particular
guarantee that virt_to_page(address) is valid.  Please add appropriate
validation or use one of the slow lookup helpers.