Patch "mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-huge_memory.c-respect-foll_force-foll_cow-for-thp.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Keno Fischer <[email protected]>
Date: Tue, 24 Jan 2017 15:17:48 -0800
Subject: mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp

From: Keno Fischer <[email protected]>

commit 8310d48b125d19fcd9521d83b8293e63eb1646aa upstream.

In commit 19be0eaffa3a ("mm: remove gup_flags FOLL_WRITE games from __get_user_pages()"), the mm code was changed from unsetting FOLL_WRITE after a COW was resolved to setting the (newly introduced) FOLL_COW instead. Simultaneously, the check in gup.c was updated to still allow writes with FOLL_FORCE set if FOLL_COW had also been set.

However, a similar check in huge_memory.c was forgotten. As a result, remote memory writes to ro regions of memory backed by transparent huge pages cause an infinite loop in the kernel (handle_mm_fault sets FOLL_COW and returns 0 causing a retry, but follow_trans_huge_pmd bails out immidiately because (flags & FOLL_WRITE) && !pmd_write(*pmd) is true.

While in this state the process is stil SIGKILLable, but little else works (e.g. no ptrace attach, no other signals). This is easily reproduced with the following code (assuming thp are set to always):

    #include <assert.h>
    #include <fcntl.h>
    #include <stdint.h>
    #include <stdio.h>
    #include <string.h>
    #include <sys/mman.h>
    #include <sys/stat.h>
    #include <sys/types.h>
    #include <sys/wait.h>
    #include <unistd.h>

    #define TEST_SIZE 5 * 1024 * 1024

    int main(void) {
      int status;
      pid_t child;
      int fd = open("/proc/self/mem", O_RDWR);
      void *addr = mmap(NULL, TEST_SIZE, PROT_READ,
                        MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
      assert(addr != MAP_FAILED);
      pid_t parent_pid = getpid();
      if ((child = fork()) == 0) {
        void *addr2 = mmap(NULL, TEST_SIZE, PROT_READ | PROT_WRITE,
                           MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
        assert(addr2 != MAP_FAILED);
        memset(addr2, 'a', TEST_SIZE);
        pwrite(fd, addr2, TEST_SIZE, (uintptr_t)addr);
        return 0;
      }
      assert(child == waitpid(child, &status, 0));
      assert(WIFEXITED(status) && WEXITSTATUS(status) == 0);
      return 0;
    }

Fix this by updating follow_trans_huge_pmd in huge_memory.c analogously to the update in gup.c in the original commit. The same pattern exists in follow_devmap_pmd. However, we should not be able to reach that check with FOLL_COW set, so add WARN_ONCE to make sure we notice if we ever do.

---
 mm/huge_memory.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

@@ -772,6 +772,12 @@ struct page *follow_devmap_pmd(struct vm
 
 	assert_spin_locked(pmd_lockptr(mm, pmd));
 
+	/*
+	 * When we COW a devmap PMD entry, we split it into PTEs, so we should
+	 * not be in this function with `flags & FOLL_COW` set.
+	 */
+	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
+
 	if (flags & FOLL_WRITE && !pmd_write(*pmd))
 		return NULL;
 

@@ -1118,6 +1124,16 @@ out_unlock:
 	return ret;
 }
 
+/*
+ * FOLL_FORCE can write to even unwritable pmd's, but only
+ * after we've gone through a COW cycle and they are dirty.
+ */
+static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
+{
+	return pmd_write(pmd) ||
+	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+}
+
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 				   unsigned long addr,
 				   pmd_t *pmd,

@@ -1128,7 +1144,7 @@ struct page *follow_trans_huge_pmd(struc
 
 	assert_spin_locked(pmd_lockptr(mm, pmd));
 
-	if (flags & FOLL_WRITE && !pmd_write(*pmd))
+	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
 		goto out;
 
 	/* Avoid dumping huge zero page */

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mm-huge_memory.c-respect-foll_force-foll_cow-for-thp.patch