From 43974893607c2aea6819b79b51350e2e5402cbc2 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Mon, 25 Jul 2022 10:20:46 -0400 Subject: [PATCH] mm/mprotect: fix soft-dirty check in can_change_pte_writable() Patch series "mm/mprotect: Fix soft-dirty checks", v4. This patch (of 3): The check wanted to make sure when soft-dirty tracking is enabled we won't grant write bit by accident, as a page fault is needed for dirty tracking. The intention is correct but we didn't check it right because VM_SOFTDIRTY set actually means soft-dirty tracking disabled. Fix it. There's another thing tricky about soft-dirty is that, we can't check the vma flag !(vma_flags & VM_SOFTDIRTY) directly but only check it after we checked CONFIG_MEM_SOFT_DIRTY because otherwise VM_SOFTDIRTY will be defined as zero, and !(vma_flags & VM_SOFTDIRTY) will constantly return true. To avoid misuse, introduce a helper for checking whether vma has soft-dirty tracking enabled. We can easily verify this with any exclusive anonymous page, like program below: =======8<====== #include #include #include #include #include #include #include #include #include #include #include #include #include #define BIT_ULL(nr) (1ULL << (nr)) #define PM_SOFT_DIRTY BIT_ULL(55) unsigned int psize; char *page; uint64_t pagemap_read_vaddr(int fd, void *vaddr) { uint64_t value; int ret; ret = pread(fd, &value, sizeof(uint64_t), ((uint64_t)vaddr >> 12) * sizeof(uint64_t)); assert(ret == sizeof(uint64_t)); return value; } void clear_refs_write(void) { int fd = open("/proc/self/clear_refs", O_RDWR); assert(fd >= 0); write(fd, "4", 2); close(fd); } #define check_soft_dirty(str, expect) do { \ bool dirty = pagemap_read_vaddr(fd, page) & PM_SOFT_DIRTY; \ if (dirty != expect) { \ printf("ERROR: %s, soft-dirty=%d (expect: %d) ", str, dirty, expect); \ exit(-1); \ } \ } while (0) int main(void) { int fd = open("/proc/self/pagemap", O_RDONLY); assert(fd >= 0); psize = getpagesize(); page = mmap(NULL, psize, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0); assert(page != MAP_FAILED); *page = 1; check_soft_dirty("Just faulted in page", 1); clear_refs_write(); check_soft_dirty("Clear_refs written", 0); mprotect(page, psize, PROT_READ); check_soft_dirty("Marked RO", 0); mprotect(page, psize, PROT_READ|PROT_WRITE); check_soft_dirty("Marked RW", 0); *page = 2; check_soft_dirty("Wrote page again", 1); munmap(page, psize); close(fd); printf("Test passed. "); return 0; } =======8<====== Here we attach a Fixes to commit 14eb6ac2c0a5 only for easy tracking, as this patch won't apply to a tree before that point. However the commit wasn't the source of problem, but instead ed2b8b4c23df. It's just that after 14eb6ac2c0a5 anonymous memory will also suffer from this problem with mprotect(). Link: https://lkml.kernel.org/r/20220725142048.30450-1-peterx@redhat.com Link: https://lkml.kernel.org/r/20220725142048.30450-2-peterx@redhat.com Fixes: ed2b8b4c23df ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared") Fixes: 14eb6ac2c0a5 ("mm/mprotect: try avoiding write faults for exclusive anonymous pages when changing protection") Signed-off-by: Peter Xu Reviewed-by: David Hildenbrand Cc: Nadav Amit Cc: Andrea Arcangeli Signed-off-by: Andrew Morton --- mm/internal.h | 18 ++++++++++++++++++ mm/mmap.c | 2 +- mm/mprotect.c | 2 +- 3 files changed, 20 insertions(+), 2 deletions(-) diff --git a/mm/internal.h b/mm/internal.h index 899dab512c5a7..caebaeb2e5c99 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -862,4 +862,22 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags); DECLARE_PER_CPU(struct per_cpu_nodestat, boot_nodestats); +static inline bool vma_soft_dirty_enabled(struct vm_area_struct *vma) +{ + /* + * NOTE: we must check this before VM_SOFTDIRTY on soft-dirty + * enablements, because when without soft-dirty being compiled in, + * VM_SOFTDIRTY is defined as 0x0, then !(vm_flags & VM_SOFTDIRTY) + * will be constantly true. + */ + if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY)) + return false; + + /* + * Soft-dirty is kind of special: its tracking is enabled when the + * vma flags not set. + */ + return !(vma->vm_flags & VM_SOFTDIRTY); +} + #endif /* __MM_INTERNAL_H */ diff --git a/mm/mmap.c b/mm/mmap.c index ec4e0d53a388a..c035020d0c896 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1647,7 +1647,7 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot) return 0; /* Do we need to track softdirty? */ - if (IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) && !(vm_flags & VM_SOFTDIRTY)) + if (vma_soft_dirty_enabled(vma)) return 1; /* Specialty mapping? */ diff --git a/mm/mprotect.c b/mm/mprotect.c index 8250c1315d9c7..3a23dde73723b 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -49,7 +49,7 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma, return false; /* Do we need write faults for softdirty tracking? */ - if ((vma->vm_flags & VM_SOFTDIRTY) && !pte_soft_dirty(pte)) + if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte)) return false; /* Do we need write faults for uffd-wp tracking? */ -- 2.39.5