From: Vlastimil Babka vbabka@suse.cz

The x86 version of get_user_pages_fast() relies on disabled interrupts to synchronize gup_pte_range() between gup_get_pte(ptep); and get_page() against a parallel munmap. The munmap side nulls the pte, then flushes TLBs, then releases the page. As TLB flush is done synchronously via IPI disabling interrupts blocks the page release, and get_page(), which assumes existing reference on page, is thus safe. However when TLB flush is done by a hypercall, e.g. in a Xen PV guest, there is no blocking thanks to disabled interrupts, and get_page() can succeed on a page that was already freed or even reused.

We have recently seen this happen with our 4.4 and 4.12 based kernels, with userspace (java) that exits a thread, where mm_release() performs a futex_wake() on tsk->clear_child_tid, and another thread in parallel unmaps the page where tsk->clear_child_tid points to. The spurious get_page() succeeds, but futex code immediately releases the page again, while it's already on a freelist. Symptoms include a bad page state warning, general protection faults acessing a poisoned list prev/next pointer in the freelist, or free page pcplists of two cpus joined together in a single list. Oscar has also reproduced this scenario, with a patch inserting delays before the get_page() to make the race window larger.

Fix this by removing the dependency on TLB flush interrupts the same way as the generic get_user_pages_fast() code by using page_cache_add_speculative() and revalidating the PTE contents after pinning the page. Mainline is safe since 4.13 where the x86 gup code was removed in favor of the common code. Accessing the page table itself safely also relies on disabled interrupts and TLB flush IPIs that don't happen with hypercalls, which was acknowledged in commit 9e52fc2b50de ("x86/mm: Enable RCU based page table freeing (CONFIG_HAVE_RCU_TABLE_FREE=y)"). That commit with follups should also be backported for full safety, although our reproducer didn't hit a problem without that backport.

Reproduced-by: Oscar Salvador osalvador@suse.de Signed-off-by: Vlastimil Babka vbabka@suse.cz Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@redhat.com Cc: Peter Zijlstra peterz@infradead.org Cc: Juergen Gross jgross@suse.com Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Vitaly Kuznetsov vkuznets@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Borislav Petkov bp@alien8.de Cc: Dave Hansen dave.hansen@linux.intel.com Cc: Andy Lutomirski luto@kernel.org

Signed-off-by: Vlastimil Babka vbabka@suse.cz --- arch/x86/mm/gup.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c index 6612d532e42e..6379a4883c0a 100644 --- a/arch/x86/mm/gup.c +++ b/arch/x86/mm/gup.c @@ -9,6 +9,7 @@ #include <linux/vmstat.h> #include <linux/highmem.h> #include <linux/swap.h> +#include <linux/pagemap.h>

#include <asm/pgtable.h>

@@ -95,10 +96,23 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr, } VM_BUG_ON(!pfn_valid(pte_pfn(pte))); page = pte_page(pte); - if (unlikely(!try_get_page(page))) { + + if (WARN_ON_ONCE(page_ref_count(page) < 0)) { + pte_unmap(ptep); + return 0; + } + + if (!page_cache_get_speculative(page)) { pte_unmap(ptep); return 0; } + + if (unlikely(pte_val(pte) != pte_val(*ptep))) { + put_page(page); + pte_unmap(ptep); + return 0; + } + SetPageReferenced(page); pages[*nr] = page; (*nr)++;