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

[Lars Persson]

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).

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
>