From: Minchan Kim <minchan@xxxxxxxxxx> Subject: mm: fix KSM data corruption Nadav reported KSM can corrupt the user data by the TLB batching race[1]. That means data user written can be lost. Quote from Nadav Amit " For this race we need 4 CPUs: CPU0: Caches a writable and dirty PTE entry, and uses the stale value for write later. CPU1: Runs madvise_free on the range that includes the PTE. It would clear the dirty-bit. It batches TLB flushes. CPU2: Writes 4 to /proc/PID/clear_refs , clearing the PTEs soft-dirty. We care about the fact that it clears the PTE write-bit, and of course, batches TLB flushes. CPU3: Runs KSM. Our purpose is to pass the following test in write_protect_page(): if (pte_write(*pvmw.pte) || pte_dirty(*pvmw.pte) || (pte_protnone(*pvmw.pte) && pte_savedwrite(*pvmw.pte))) Since it will avoid TLB flush. And we want to do it while the PTE is stale. Later, and before replacing the page, we would be able to change the page. Note that all the operations the CPU1-3 perform canhappen in parallel since they only acquire mmap_sem for read. We start with two identical pages. Everything below regards the same page/PTE. CPU0 CPU1 CPU2 CPU3 ---- ---- ---- ---- Write the same value on page [cache PTE as dirty in TLB] MADV_FREE pte_mkclean() 4 > clear_refs pte_wrprotect() write_protect_page() [ success, no flush ] pages_indentical() [ ok ] Write to page different value [Ok, using stale PTE] replace_page() Later, CPU1, CPU2 and CPU3 would flush the TLB, but that is too late. CPU0 already wrote on the page, but KSM ignored this write, and it got lost. " In above scenario, MADV_FREE is fixed by changing TLB batching API including [set|clear]_tlb_flush_pending. Remained thing is soft-dirty part. This patch changes soft-dirty uses TLB batching API instead of flush_tlb_mm and KSM checks pending TLB flush by using mm_tlb_flush_pending so that it will flush TLB to avoid data lost if there are other parallel threads pending TLB flush. [1] http://lkml.kernel.org/r/BD3A0EBE-ECF4-41D4-87FA-C755EA9AB6BD@xxxxxxxxx Link: http://lkml.kernel.org/r/20170802000818.4760-8-namit@xxxxxxxxxx Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx> Reported-by: Nadav Amit <namit@xxxxxxxxxx> Tested-by: Nadav Amit <namit@xxxxxxxxxx> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Jeff Dike <jdike@xxxxxxxxxxx> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Cc: Nadav Amit <nadav.amit@xxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Russell King <linux@xxxxxxxxxxxxxxx> Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> Cc: Tony Luck <tony.luck@xxxxxxxxx> Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/proc/task_mmu.c | 7 +++++-- mm/ksm.c | 3 ++- 2 files changed, 7 insertions(+), 3 deletions(-) diff -puN fs/proc/task_mmu.c~mm-fix-ksm-data-corruption fs/proc/task_mmu.c --- a/fs/proc/task_mmu.c~mm-fix-ksm-data-corruption +++ a/fs/proc/task_mmu.c @@ -16,9 +16,10 @@ #include <linux/mmu_notifier.h> #include <linux/page_idle.h> #include <linux/shmem_fs.h> +#include <linux/uaccess.h> #include <asm/elf.h> -#include <linux/uaccess.h> +#include <asm/tlb.h> #include <asm/tlbflush.h> #include "internal.h" @@ -1008,6 +1009,7 @@ static ssize_t clear_refs_write(struct f struct mm_struct *mm; struct vm_area_struct *vma; enum clear_refs_types type; + struct mmu_gather tlb; int itype; int rv; @@ -1054,6 +1056,7 @@ static ssize_t clear_refs_write(struct f } down_read(&mm->mmap_sem); + tlb_gather_mmu(&tlb, mm, 0, -1); if (type == CLEAR_REFS_SOFT_DIRTY) { for (vma = mm->mmap; vma; vma = vma->vm_next) { if (!(vma->vm_flags & VM_SOFTDIRTY)) @@ -1075,7 +1078,7 @@ static ssize_t clear_refs_write(struct f walk_page_range(0, mm->highest_vm_end, &clear_refs_walk); if (type == CLEAR_REFS_SOFT_DIRTY) mmu_notifier_invalidate_range_end(mm, 0, -1); - flush_tlb_mm(mm); + tlb_finish_mmu(&tlb, 0, -1); up_read(&mm->mmap_sem); out_mm: mmput(mm); diff -puN mm/ksm.c~mm-fix-ksm-data-corruption mm/ksm.c --- a/mm/ksm.c~mm-fix-ksm-data-corruption +++ a/mm/ksm.c @@ -1038,7 +1038,8 @@ static int write_protect_page(struct vm_ goto out_unlock; if (pte_write(*pvmw.pte) || pte_dirty(*pvmw.pte) || - (pte_protnone(*pvmw.pte) && pte_savedwrite(*pvmw.pte))) { + (pte_protnone(*pvmw.pte) && pte_savedwrite(*pvmw.pte)) || + mm_tlb_flush_pending(mm)) { pte_t entry; swapped = PageSwapCache(page); _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
