Commit df17277b authored by Mike Rapoport's avatar Mike Rapoport Committed by Linus Torvalds
Browse files

mm/gup: continue VM_FAULT_RETRY processing even for pre-faults

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 d9c9ce34 ("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: d9c9ce34

 ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")
Signed-off-by: default avatarMike Rapoport <>
Tested-by: Andrei Vagin <> [

Tested-by: default avatarHugh Dickins <>
Cc: Andrea Arcangeli <>
Cc: Sebastian Andrzej Siewior <>
Cc: Borislav Petkov <>
Cc: Pavel Machek <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent b9fba67b
......@@ -1042,10 +1042,6 @@ 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;
......@@ -1061,8 +1057,12 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
pages_done = ret;
/* 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;
......@@ -1085,7 +1085,8 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
if (!nr_pages)
if (likely(pages))
start += PAGE_SIZE;
if (lock_dropped && *locked) {
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