[Devel] [PATCH RHEL7 COMMIT] ms/mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-514.26.1.vz7.35.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-514.26.1.vz7.35.5
------>
commit f5b413ea4e53d819c8b4e4a4927fb563bd3ec24f
Author: Keno Fischer <keno at juliacomputing.com>
Date:   Thu Aug 31 17:51:25 2017 +0300

    ms/mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp
    
    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.
    
    [akpm at linux-foundation.org: coding-style fixes]
    Link: http://lkml.kernel.org/r/20170106015025.GA38411@juliacomputing.com
    Signed-off-by: Keno Fischer <keno at juliacomputing.com>
    Acked-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
    Cc: Greg Thelen <gthelen at google.com>
    Cc: Nicholas Piggin <npiggin at gmail.com>
    Cc: Willy Tarreau <w at 1wt.eu>
    Cc: Oleg Nesterov <oleg at redhat.com>
    Cc: Kees Cook <keescook at chromium.org>
    Cc: Andy Lutomirski <luto at kernel.org>
    Cc: Michal Hocko <mhocko at suse.com>
    Cc: Hugh Dickins <hughd at google.com>
    Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    [bwh: Backported to 3.2:
     - Drop change to follow_devmap_pmd()
     - pmd_dirty() is not available; check the page flags as in
       can_follow_write_pte()
     - Adjust context]
    Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
    [mhocko:
      This has been forward ported from the 3.2 stable tree.
      And fixed to return NULL.]
    Reviewed-by: Michal Hocko <mhocko at suse.cz>
    Signed-off-by: Jiri Slaby <jslaby at suse.cz>
    Signed-off-by: Willy Tarreau <w at 1wt.eu>
    
    https://jira.sw.ru/browse/PSBM-70151
    
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/huge_memory.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 477610d..5a07e76 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1317,6 +1317,18 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	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, struct page *page,
+					unsigned int flags)
+{
+	return pmd_write(pmd) ||
+		((flags & FOLL_FORCE) && (flags & FOLL_COW) &&
+		 page && PageAnon(page));
+}
+
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 				   unsigned long addr,
 				   pmd_t *pmd,
@@ -1327,9 +1339,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
 	assert_spin_locked(pmd_lockptr(mm, pmd));
 
-	if (flags & FOLL_WRITE && !pmd_write(*pmd))
-		goto out;
-
 	/* Avoid dumping huge zero page */
 	if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
 		return ERR_PTR(-EFAULT);
@@ -1340,6 +1349,10 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
 	page = pmd_page(*pmd);
 	VM_BUG_ON_PAGE(!PageHead(page), page);
+
+	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, page, flags))
+		return NULL;
+
 	if (flags & FOLL_TOUCH) {
 		pmd_t _pmd;
 		/*