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 > > -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html