Re: [PATCH 4/4] MIPS: Sync icache & dcache in set_pte_at

[Paul Burton]

On Fri, Mar 04, 2016 at 07:02:24PM +0000, Lars Persson wrote:
> Hi
> 
> Some further thoughts on the matter. You have so far not showed a
> valid example of a race condition. The two examples you give in the
> commit message are for a _single_ thread existing in the address space
> (fork and execve).

Hi Lars,

Neither fork nor exec are limited to a single thread existing in the
address space - I'm not sure what you're saying? fork by its very
definition results in 2.

Thanks,
    Paul

> BR,
>  Lars
> 
> > 1 mars 2016 kl. 03:39 skrev Paul Burton <paul.burton@xxxxxxxxxx>:
> > 
> > It's possible for pages to become visible prior to update_mmu_cache
> > running if a thread within the same address space preempts the current
> > thread or runs simultaneously on another CPU. That is, the following
> > scenario is possible:
> > 
> >    CPU0                            CPU1
> > 
> >    write to page
> >    flush_dcache_page
> >    flush_icache_page
> >    set_pte_at
> >                                    map page
> >    update_mmu_cache
> > 
> > If CPU1 maps the page in between CPU0's set_pte_at, which marks it valid
> > & visible, and update_mmu_cache where the dcache flush occurs then CPU1s
> > icache will fill from stale data (unless it fills from the dcache, in
> > which case all is good, but most MIPS CPUs don't have this property).
> > Commit 4d46a67a3eb8 ("MIPS: Fix race condition in lazy cache flushing.")
> > attempted to fix that by performing the dcache flush in
> > flush_icache_page such that it occurs before the set_pte_at call makes
> > the page visible. However it has the problem that not all code that
> > writes to pages exposed to userland call flush_icache_page. There are
> > many callers of set_pte_at under mm/ and only 2 of them do call
> > flush_icache_page. Thus the race window between a page becoming visible
> > & being coherent between the icache & dcache remains open in some cases.
> > 
> > To illustrate some of the cases, a WARN was added to __update_cache with
> > this patch applied that triggered in cases where a page about to be
> > flushed from the dcache was not the last page provided to
> > flush_icache_page. That is, backtraces were obtained for cases in which
> > the race window is left open without this patch. The 2 standout examples
> > follow.
> > 
> > When forking a process:
> > 
> > [   15.271842] [<80417630>] __update_cache+0xcc/0x188
> > [   15.277274] [<80530394>] copy_page_range+0x56c/0x6ac
> > [   15.282861] [<8042936c>] copy_process.part.54+0xd40/0x17ac
> > [   15.289028] [<80429f80>] do_fork+0xe4/0x420
> > [   15.293747] [<80413808>] handle_sys+0x128/0x14c
> > 
> > When exec'ing an ELF binary:
> > 
> > [   14.445964] [<80417630>] __update_cache+0xcc/0x188
> > [   14.451369] [<80538d88>] move_page_tables+0x414/0x498
> > [   14.457075] [<8055d848>] setup_arg_pages+0x220/0x318
> > [   14.462685] [<805b0f38>] load_elf_binary+0x530/0x12a0
> > [   14.468374] [<8055ec3c>] search_binary_handler+0xbc/0x214
> > [   14.474444] [<8055f6c0>] do_execveat_common+0x43c/0x67c
> > [   14.480324] [<8055f938>] do_execve+0x38/0x44
> > [   14.485137] [<80413808>] handle_sys+0x128/0x14c
> > 
> > These code paths write into a page, call flush_dcache_page then call
> > set_pte_at without flush_icache_page inbetween. The end result is that
> > the icache can become corrupted & userland processes may execute
> > unexpected or invalid code, typically resulting in a reserved
> > instruction exception, a trap or a segfault.
> > 
> > Fix this race condition fully by performing any cache maintenance
> > required to keep the icache & dcache in sync in set_pte_at, before the
> > page is made valid. This has the added bonus of ensuring the cache
> > maintenance always happens in one location, rather than being duplicated
> > in flush_icache_page & update_mmu_cache. It also matches the way other
> > architectures solve the same problem (see arm, ia64 & powerpc).
> > 
> > Signed-off-by: Paul Burton <paul.burton@xxxxxxxxxx>
> > Reported-by: Ionela Voinescu <ionela.voinescu@xxxxxxxxxx>
> > Cc: Lars Persson <lars.persson@xxxxxxxx>
> > Cc: stable <stable@xxxxxxxxxxxxxxx> # v4.1+
> > Fixes: 4d46a67a3eb8 ("MIPS: Fix race condition in lazy cache flushing.")
> > 
> > ---
> > 
> > arch/mips/include/asm/cacheflush.h |  6 ------
> > arch/mips/include/asm/pgtable.h    | 26 +++++++++++++++++++++-----
> > arch/mips/mm/cache.c               | 19 +++----------------
> > 3 files changed, 24 insertions(+), 27 deletions(-)
> > 
> > diff --git a/arch/mips/include/asm/cacheflush.h b/arch/mips/include/asm/cacheflush.h
> > index 7e9f468..34ed22e 100644
> > --- a/arch/mips/include/asm/cacheflush.h
> > +++ b/arch/mips/include/asm/cacheflush.h
> > @@ -51,7 +51,6 @@ extern void (*flush_cache_range)(struct vm_area_struct *vma,
> >    unsigned long start, unsigned long end);
> > extern void (*flush_cache_page)(struct vm_area_struct *vma, unsigned long page, unsigned long pfn);
> > extern void __flush_dcache_page(struct page *page);
> > -extern void __flush_icache_page(struct vm_area_struct *vma, struct page *page);
> > 
> > #define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 1
> > static inline void flush_dcache_page(struct page *page)
> > @@ -77,11 +76,6 @@ static inline void flush_anon_page(struct vm_area_struct *vma,
> > static inline void flush_icache_page(struct vm_area_struct *vma,
> >    struct page *page)
> > {
> > -    if (!cpu_has_ic_fills_f_dc && (vma->vm_flags & VM_EXEC) &&
> > -        Page_dcache_dirty(page)) {
> > -        __flush_icache_page(vma, page);
> > -        ClearPageDcacheDirty(page);
> > -    }
> > }
> > 
> > extern void (*flush_icache_range)(unsigned long start, unsigned long end);
> > diff --git a/arch/mips/include/asm/pgtable.h b/arch/mips/include/asm/pgtable.h
> > index 9a4fe01..65bf2c0 100644
> > --- a/arch/mips/include/asm/pgtable.h
> > +++ b/arch/mips/include/asm/pgtable.h
> > @@ -127,10 +127,14 @@ do {                                    \
> >    }                                \
> > } while(0)
> > 
> > +static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> > +                  pte_t *ptep, pte_t pteval);
> > +
> > #if defined(CONFIG_PHYS_ADDR_T_64BIT) && defined(CONFIG_CPU_MIPS32)
> > 
> > #define pte_none(pte)        (!(((pte).pte_high) & ~_PAGE_GLOBAL))
> > #define pte_present(pte)    ((pte).pte_low & _PAGE_PRESENT)
> > +#define pte_no_exec(pte)    ((pte).pte_low & _PAGE_NO_EXEC)
> > 
> > static inline void set_pte(pte_t *ptep, pte_t pte)
> > {
> > @@ -148,7 +152,6 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
> >            buddy->pte_high |= _PAGE_GLOBAL;
> >    }
> > }
> > -#define set_pte_at(mm, addr, ptep, pteval) set_pte(ptep, pteval)
> > 
> > static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
> > {
> > @@ -166,6 +169,7 @@ static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *pt
> > 
> > #define pte_none(pte)        (!(pte_val(pte) & ~_PAGE_GLOBAL))
> > #define pte_present(pte)    (pte_val(pte) & _PAGE_PRESENT)
> > +#define pte_no_exec(pte)    (pte_val(pte) & _PAGE_NO_EXEC)
> > 
> > /*
> >  * Certain architectures need to do special things when pte's
> > @@ -218,7 +222,6 @@ static inline void set_pte(pte_t *ptep, pte_t pteval)
> >    }
> > #endif
> > }
> > -#define set_pte_at(mm, addr, ptep, pteval) set_pte(ptep, pteval)
> > 
> > static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
> > {
> > @@ -234,6 +237,22 @@ static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *pt
> > }
> > #endif
> > 
> > +static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> > +                  pte_t *ptep, pte_t pteval)
> > +{
> > +    extern void __update_cache(unsigned long address, pte_t pte);
> > +
> > +    if (!pte_present(pteval))
> > +        goto cache_sync_done;
> > +
> > +    if (pte_present(*ptep) && (pte_pfn(*ptep) == pte_pfn(pteval)))
> > +        goto cache_sync_done;
> > +
> > +    __update_cache(addr, pteval);
> > +cache_sync_done:
> > +    set_pte(ptep, pteval);
> > +}
> > +
> > /*
> >  * (pmds are folded into puds so this doesn't get actually called,
> >  * but the define is needed for a generic inline function.)
> > @@ -430,15 +449,12 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
> > 
> > extern void __update_tlb(struct vm_area_struct *vma, unsigned long address,
> >    pte_t pte);
> > -extern void __update_cache(struct vm_area_struct *vma, unsigned long address,
> > -    pte_t pte);
> > 
> > static inline void update_mmu_cache(struct vm_area_struct *vma,
> >    unsigned long address, pte_t *ptep)
> > {
> >    pte_t pte = *ptep;
> >    __update_tlb(vma, address, pte);
> > -    __update_cache(vma, address, pte);
> > }
> > 
> > static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
> > diff --git a/arch/mips/mm/cache.c b/arch/mips/mm/cache.c
> > index 8befa55..bf04c6c 100644
> > --- a/arch/mips/mm/cache.c
> > +++ b/arch/mips/mm/cache.c
> > @@ -125,30 +125,17 @@ void __flush_anon_page(struct page *page, unsigned long vmaddr)
> > 
> > EXPORT_SYMBOL(__flush_anon_page);
> > 
> > -void __flush_icache_page(struct vm_area_struct *vma, struct page *page)
> > -{
> > -    unsigned long addr;
> > -
> > -    if (PageHighMem(page))
> > -        return;
> > -
> > -    addr = (unsigned long) page_address(page);
> > -    flush_data_cache_page(addr);
> > -}
> > -EXPORT_SYMBOL_GPL(__flush_icache_page);
> > -
> > -void __update_cache(struct vm_area_struct *vma, unsigned long address,
> > -    pte_t pte)
> > +void __update_cache(unsigned long address, pte_t pte)
> > {
> >    struct page *page;
> >    unsigned long pfn, addr;
> > -    int exec = (vma->vm_flags & VM_EXEC) && !cpu_has_ic_fills_f_dc;
> > +    int exec = !pte_no_exec(pte) && !cpu_has_ic_fills_f_dc;
> > 
> >    pfn = pte_pfn(pte);
> >    if (unlikely(!pfn_valid(pfn)))
> >        return;
> >    page = pfn_to_page(pfn);
> > -    if (page_mapping(page) && Page_dcache_dirty(page)) {
> > +    if (Page_dcache_dirty(page)) {
> >        if (PageHighMem(page))
> >            addr = (unsigned long)kmap_atomic(page);
> >        else
> > -- 
> > 2.7.1
> >