[PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults

[Mike Rapoport]

When get_user_pages*() is called with pages = NULL, the processing of
VM_FAULT_RETRY terminates early without actually retrying to fault-in all
the pages.

If the pages in the requested range belong to a VMA that has userfaultfd
registered, handle_userfault() returns VM_FAULT_RETRY *after* user space
has populated the page, but for the gup pre-fault case there's no actual
retry and the caller will get no pages although they are present.

This issue was uncovered when running post-copy memory restore in CRIU
after commit d9c9ce34ed5c ("x86/fpu: Fault-in user stack if
copy_fpstate_to_sigframe() fails").

After this change, the copying of FPU state to the sigframe switched from
copy_to_user() variants which caused a real page fault to get_user_pages()
with pages parameter set to NULL.

In post-copy mode of CRIU, the destination memory is managed with
userfaultfd and lack of the retry for pre-fault case in get_user_pages()
causes a crash of the restored process.

Making the pre-fault behavior of get_user_pages() the same as the "normal"
one fixes the issue.

Fixes: d9c9ce34ed5c ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")
Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
---
 mm/gup.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 91819b8..c32ae5a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -936,10 +936,6 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 			BUG_ON(ret >= nr_pages);
 		}
 
-		if (!pages)
-			/* If it's a prefault don't insist harder */
-			return ret;
-
 		if (ret > 0) {
 			nr_pages -= ret;
 			pages_done += ret;
@@ -955,8 +951,12 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 				pages_done = ret;
 			break;
 		}
-		/* VM_FAULT_RETRY triggered, so seek to the faulting offset */
-		pages += ret;
+		/*
+		 * VM_FAULT_RETRY triggered, so seek to the faulting offset.
+		 * For the prefault case (!pages) we only update counts.
+		 */
+		if (likely(pages))
+			pages += ret;
 		start += ret << PAGE_SHIFT;
 
 		/*
@@ -979,7 +979,8 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 		pages_done++;
 		if (!nr_pages)
 			break;
-		pages++;
+		if (likely(pages))
+			pages++;
 		start += PAGE_SIZE;
 	}
 	if (lock_dropped && *locked) {
-- 
2.7.4