Re: [PATCH Part2 v6 10/49] x86/fault: Add support to dump RMP entry on fault

[Borislav Petkov]

On Mon, Jun 20, 2022 at 11:03:58PM +0000, Ashish Kalra wrote:
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
> 
> When SEV-SNP is enabled globally, a write from the host goes through the
> RMP check. If the hardware encounters the check failure, then it raises
> the #PF (with RMP set). Dump the RMP entry at the faulting pfn to help
> the debug.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/include/asm/sev.h |  7 +++++++
>  arch/x86/kernel/sev.c      | 43 ++++++++++++++++++++++++++++++++++++++
>  arch/x86/mm/fault.c        | 17 +++++++++++----
>  include/linux/sev.h        |  2 ++
>  4 files changed, 65 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 6ab872311544..c0c4df817159 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -113,6 +113,11 @@ struct __packed rmpentry {
>  
>  #define rmpentry_assigned(x)	((x)->info.assigned)
>  #define rmpentry_pagesize(x)	((x)->info.pagesize)
> +#define rmpentry_vmsa(x)	((x)->info.vmsa)
> +#define rmpentry_asid(x)	((x)->info.asid)
> +#define rmpentry_validated(x)	((x)->info.validated)
> +#define rmpentry_gpa(x)		((unsigned long)(x)->info.gpa)
> +#define rmpentry_immutable(x)	((x)->info.immutable)

If you're going to do that, use inline functions pls so that it checks
the argument at least.

Also, add such functions only when they're called multiple times - no
need to add one for every field if you're going to access that field
only once in the whole kernel.

>  
>  #define RMPADJUST_VMSA_PAGE_BIT		BIT(16)
>  
> @@ -205,6 +210,7 @@ void snp_set_wakeup_secondary_cpu(void);
>  bool snp_init(struct boot_params *bp);
>  void snp_abort(void);
>  int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, unsigned long *fw_err);
> +void dump_rmpentry(u64 pfn);
>  #else
>  static inline void sev_es_ist_enter(struct pt_regs *regs) { }
>  static inline void sev_es_ist_exit(void) { }
> @@ -229,6 +235,7 @@ static inline int snp_issue_guest_request(u64 exit_code, struct snp_req_data *in
>  {
>  	return -ENOTTY;
>  }
> +static inline void dump_rmpentry(u64 pfn) {}
>  #endif
>  
>  #endif
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 734cddd837f5..6640a639fffc 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2414,6 +2414,49 @@ static struct rmpentry *__snp_lookup_rmpentry(u64 pfn, int *level)
>  	return entry;
>  }
>  
> +void dump_rmpentry(u64 pfn)

External function - it better belong to a namespace:

sev_dump_rmpentry()

> +{
> +	unsigned long pfn_end;
> +	struct rmpentry *e;
> +	int level;
> +
> +	e = __snp_lookup_rmpentry(pfn, &level);
> +	if (!e) {
> +		pr_alert("failed to read RMP entry pfn 0x%llx\n", pfn);

Why alert?

Dumping stuff is either pr_debug or pr_info...

> +		return;
> +	}
> +
> +	if (rmpentry_assigned(e)) {
> +		pr_alert("RMPEntry paddr 0x%llx [assigned=%d immutable=%d pagesize=%d gpa=0x%lx"
> +			" asid=%d vmsa=%d validated=%d]\n", pfn << PAGE_SHIFT,
> +			rmpentry_assigned(e), rmpentry_immutable(e), rmpentry_pagesize(e),
> +			rmpentry_gpa(e), rmpentry_asid(e), rmpentry_vmsa(e),
> +			rmpentry_validated(e));
> +		return;
> +	}
> +
> +	/*
> +	 * If the RMP entry at the faulting pfn was not assigned, then we do not

Who's "we"?

> +	 * know what caused the RMP violation. To get some useful debug information,
> +	 * let iterate through the entire 2MB region, and dump the RMP entries if
> +	 * one of the bit in the RMP entry is set.
> +	 */
> +	pfn = pfn & ~(PTRS_PER_PMD - 1);
> +	pfn_end = pfn + PTRS_PER_PMD;
> +
> +	while (pfn < pfn_end) {
> +		e = __snp_lookup_rmpentry(pfn, &level);
> +		if (!e)
> +			return;
> +
> +		if (e->low || e->high)

This is going to confuse people because they're going to miss a zero
entry. Just dump the whole thing.

...

> @@ -579,7 +588,7 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
>  		show_ldttss(&gdt, "TR", tr);
>  	}
>  
> -	dump_pagetable(address);
> +	dump_pagetable(address, error_code & X86_PF_RMP);

Eww.

I'd prefer to see

	pfn = dump_pagetable(address);

	if (error_code & X86_PF_RMP)
		sev_dump_rmpentry(pfn);

instead of passing around this SEV-specific arg in generic x86 fault code.

The change to return the pfn from dump_pagetable() should be a pre-patch ofc.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette