Re: [PATCH] x86/mm/cpa: Prevent large page split when ftrace flips RW on kernel text

[Song Liu]

> On Aug 28, 2019, at 3:31 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> ftrace does not use text_poke() for enabling trace functionality. It uses
> its own mechanism and flips the whole kernel text to RW and back to RO.
> 
> The CPA rework removed a loop based check of 4k pages which tried to
> preserve a large page by checking each 4k page whether the change would
> actually cover all pages in the large page.
> 
> This resulted in endless loops for nothing as in testing it turned out that
> it actually never preserved anything. Of course testing missed to include
> ftrace, which is the one and only case which benefitted from the 4k loop.
> 
> As a consequence enabling function tracing or ftrace based kprobes results
> in a full 4k split of the kernel text, which affects iTLB performance.
> 
> The kernel RO protection is the only valid case where this can actually
> preserve large pages.
> 
> All other static protections (RO data, data NX, PCI, BIOS) are truly
> static.  So a conflict with those protections which results in a split
> should only ever happen when a change of memory next to a protected region
> is attempted. But these conflicts are rightfully splitting the large page
> to preserve the protected regions. In fact a change to the protected
> regions itself is a bug and is warned about.
> 
> Add an exception for the static protection check for kernel text RO when
> the to be changed region spawns a full large page which allows to preserve
> the large mappings. This also prevents the syslog to be spammed about CPA
> violations when ftrace is used.
> 
> The exception needs to be removed once ftrace switched over to text_poke()
> which avoids the whole issue.
> 
> Fixes: 585948f4f695 ("x86/mm/cpa: Avoid the 4k pages check completely")
> Reported-by: Song Liu <songliubraving@xxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

This looks great. Much cleaner than my workaround. 

Thanks!

Reviewed-and-tested-by: Song Liu <songliubraving@xxxxxx>

We need this for v4.20 to v5.3 (assuming Peter's patches will land in 5.4). 


> ---
> arch/x86/mm/pageattr.c |   26 ++++++++++++++++++--------
> 1 file changed, 18 insertions(+), 8 deletions(-)
> 
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -516,7 +516,7 @@ static inline void check_conflict(int wa
>  */
> static inline pgprot_t static_protections(pgprot_t prot, unsigned long start,
> 					  unsigned long pfn, unsigned long npg,
> -					  int warnlvl)
> +					  unsigned long lpsize, int warnlvl)
> {
> 	pgprotval_t forbidden, res;
> 	unsigned long end;
> @@ -535,9 +535,17 @@ static inline pgprot_t static_protection
> 	check_conflict(warnlvl, prot, res, start, end, pfn, "Text NX");
> 	forbidden = res;
> 
> -	res = protect_kernel_text_ro(start, end);
> -	check_conflict(warnlvl, prot, res, start, end, pfn, "Text RO");
> -	forbidden |= res;
> +	/*
> +	 * Special case to preserve a large page. If the change spawns the
> +	 * full large page mapping then there is no point to split it
> +	 * up. Happens with ftrace and is going to be removed once ftrace
> +	 * switched to text_poke().
> +	 */
> +	if (lpsize != (npg * PAGE_SIZE) || (start & (lpsize - 1))) {
> +		res = protect_kernel_text_ro(start, end);
> +		check_conflict(warnlvl, prot, res, start, end, pfn, "Text RO");
> +		forbidden |= res;
> +	}
> 
> 	/* Check the PFN directly */
> 	res = protect_pci_bios(pfn, pfn + npg - 1);
> @@ -819,7 +827,7 @@ static int __should_split_large_page(pte
> 	 * extra conditional required here.
> 	 */
> 	chk_prot = static_protections(old_prot, lpaddr, old_pfn, numpages,
> -				      CPA_CONFLICT);
> +				      psize, CPA_CONFLICT);
> 
> 	if (WARN_ON_ONCE(pgprot_val(chk_prot) != pgprot_val(old_prot))) {
> 		/*
> @@ -855,7 +863,7 @@ static int __should_split_large_page(pte
> 	 * protection requirement in the large page.
> 	 */
> 	new_prot = static_protections(req_prot, lpaddr, old_pfn, numpages,
> -				      CPA_DETECT);
> +				      psize, CPA_DETECT);
> 
> 	/*
> 	 * If there is a conflict, split the large page.
> @@ -906,7 +914,8 @@ static void split_set_pte(struct cpa_dat
> 	if (!cpa->force_static_prot)
> 		goto set;
> 
> -	prot = static_protections(ref_prot, address, pfn, npg, CPA_PROTECT);
> +	/* Hand in lpsize = 0 to enforce the protection mechanism */
> +	prot = static_protections(ref_prot, address, pfn, npg, 0, CPA_PROTECT);
> 
> 	if (pgprot_val(prot) == pgprot_val(ref_prot))
> 		goto set;
> @@ -1503,7 +1512,8 @@ static int __change_page_attr(struct cpa
> 		pgprot_val(new_prot) |= pgprot_val(cpa->mask_set);
> 
> 		cpa_inc_4k_install();
> -		new_prot = static_protections(new_prot, address, pfn, 1,
> +		/* Hand in lpsize = 0 to enforce the protection mechanism */
> +		new_prot = static_protections(new_prot, address, pfn, 1, 0,
> 					      CPA_PROTECT);
> 
> 		new_prot = pgprot_clear_protnone_bits(new_prot);