Commit df17277b authored May 31, 2019 by Mike Rapoport Committed by Linus Torvalds Jun 01, 2019

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.

Link: http://lkml.kernel.org/r/1557844195-18882-1-git-send-email-rppt@linux.ibm.com


Fixes: d9c9ce34 ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Tested-by: Andrei Vagin <avagin@gmail.com> [https://travis-ci.org/avagin/linux/builds/533184940

]
Tested-by: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent b9fba67b

mm/gup.c

+8 −7

Original line number	Diff line number	Diff line
		@@ -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;
		@@ -1061,7 +1057,11 @@ 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 */
		/*
		* 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;

		@@ -1085,6 +1085,7 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
		pages_done++;
		if (!nr_pages)
		break;
		if (likely(pages))
		pages++;
		start += PAGE_SIZE;
		}

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