mm: always flush VMA ranges affected by zap_page_range
Nadav Amit report zap_page_range only specifies that the caller protect the VMA list but does not specify whether it is held for read or write with callers using either. madvise holds mmap_sem for read meaning that a parallel zap operation can unmap PTEs which are then potentially skipped by madvise which potentially returns with stale TLB entries present. While the API could be extended, it would be a difficult API to use. This patch causes zap_page_range() to always consider flushing the full affected range. For small ranges or sparsely populated mappings, this may result in one additional spurious TLB flush. For larger ranges, it is possible that the TLB has already been flushed and the overhead is negligible. Either way, this approach is safer overall and avoids stale entries being present when madvise returns. This can be illustrated with the following program provided by Nadav Amit and slightly modified. With the patch applied, it has an exit code of 0 indicating a stale TLB entry did not leak to userspace. ---8<--- volatile int sync_step = 0; volatile char *p; static inline unsigned long rdtsc() { unsigned long hi, lo; __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi)); return lo | (hi << 32); } static inline void wait_rdtsc(unsigned long cycles) { unsigned long tsc = rdtsc(); while (rdtsc() - tsc < cycles); } void *big_madvise_thread(void *ign) { sync_step = 1; while (sync_step != 2); madvise((void*)p, PAGE_SIZE * N_PAGES, MADV_DONTNEED); } int main(void) { pthread_t aux_thread; p = mmap(0, PAGE_SIZE * N_PAGES, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); memset((void*)p, 8, PAGE_SIZE * N_PAGES); pthread_create(&aux_thread, NULL, big_madvise_thread, NULL); while (sync_step != 1); *p = 8; // Cache in TLB sync_step = 2; wait_rdtsc(100000); madvise((void*)p, PAGE_SIZE, MADV_DONTNEED); printf("data: %d (%s)\n", *p, (*p == 8 ? "stale, broken" : "cleared, fine")); return *p == 8 ? -1 : 0; } ---8<--- Link: http://lkml.kernel.org/r/20170725101230.5v7gvnjmcnkzzql3@techsingularity.netSigned-off-by: Mel Gorman <mgorman@suse.de> Reported-by: Nadav Amit <nadav.amit@gmail.com> Cc: Andy Lutomirski <luto@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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