+ fork-lock-vmas-of-the-parent-process-when-forking-v3.patch added to mm-hotfixes-unstable branch

+ fork-lock-vmas-of-the-parent-process-when-forking-v3.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

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The patch titled
     Subject: fork: lock VMAs of the parent process when forking
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     fork-lock-vmas-of-the-parent-process-when-forking-v3.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fork-lock-vmas-of-the-parent-process-when-forking-v3.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Suren Baghdasaryan <surenb@google.com>
Subject: fork: lock VMAs of the parent process when forking
Date: Wed, 5 Jul 2023 10:12:11 -0700

Patch series "Avoid memory corruption caused by per-VMA locks", v3.

A memory corruption was reported in [1] with bisection pointing to the
patch [2] enabling per-VMA locks for x86.  Based on the reproducer
provided in [1] we suspect this is caused by the lack of VMA locking while
forking a child process.

Patch 1/2 in the series implements proper VMA locking during fork.  I
tested the fix locally using the reproducer and was unable to reproduce
the memory corruption problem.  This fix can potentially regress some
fork-heavy workloads.  Kernel build time did not show noticeable
regression on a 56-core machine while a stress test mapping 10000 VMAs and
forking 5000 times in a tight loop shows ~5% regression.  If such fork
time regression is unacceptable, disabling CONFIG_PER_VMA_LOCK should
restore its performance.  Further optimizations are possible if this
regression proves to be problematic.


This patch (of 2):

When forking a child process, parent write-protects an anonymous page and
COW-shares it with the child being forked using copy_present_pte(). 
Parent's TLB is flushed right before we drop the parent's mmap_lock in
dup_mmap().  If we get a write-fault before that TLB flush in the parent,
and we end up replacing that anonymous page in the parent process in
do_wp_page() (because, COW-shared with the child), this might lead to some
stale writable TLB entries targeting the wrong (old) page.  Similar issue
happened in the past with userfaultfd (see flush_tlb_page() call inside
do_wp_page()).

Lock VMAs of the parent process when forking a child, which prevents
concurrent page faults during fork operation and avoids this issue.  This
fix can potentially regress some fork-heavy workloads.  Kernel build time
did not show noticeable regression on a 56-core machine while a stress
test mapping 10000 VMAs and forking 5000 times in a tight loop shows ~5%
regression.  If such fork time regression is unacceptable, disabling
CONFIG_PER_VMA_LOCK should restore its performance.  Further optimizations
are possible if this regression proves to be problematic.

Link: https://lkml.kernel.org/r/20230705171213.2843068-2-surenb@google.com
Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first"=
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@ke=
rnel.org/
Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@ap=
plied-asynchrony.com/
Reported-by: Jacob Young <jacobly.alt@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=3D217624
)
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Li <chriscli@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: David Rientjes <rientjes@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Laurent Dufour <ldufour@linux.ibm.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michel Lespinasse <michel@lespinasse.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@google.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: <peterz@infradead.org>
Cc: Punit Agrawal <punit.agrawal@bytedance.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/fork.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/kernel/fork.c~fork-lock-vmas-of-the-parent-process-when-forking-v3
+++ a/kernel/fork.c
@@ -658,6 +658,12 @@ static __latent_entropy int dup_mmap(str
 		retval = -EINTR;
 		goto fail_uprobe_end;
 	}
+#ifdef CONFIG_PER_VMA_LOCK
+	/* Disallow any page faults before calling flush_cache_dup_mm */
+	for_each_vma(old_vmi, mpnt)
+		vma_start_write(mpnt);
+	vma_iter_init(&old_vmi, oldmm, 0);
+#endif
 	flush_cache_dup_mm(oldmm);
 	uprobe_dup_mmap(oldmm, mm);
 	/*
_

Patches currently in -mm which might be from surenb@google.com are

fork-lock-vmas-of-the-parent-process-when-forking-v3.patch
mm-disable-config_per_vma_lock-until-its-fixed.patch
swap-remove-remnants-of-polling-from-read_swap_cache_async.patch
mm-add-missing-vm_fault_result_trace-name-for-vm_fault_completed.patch
mm-drop-per-vma-lock-when-returning-vm_fault_retry-or-vm_fault_completed.patch
mm-change-folio_lock_or_retry-to-use-vm_fault-directly.patch
mm-handle-swap-page-faults-under-per-vma-lock.patch
mm-handle-userfaults-under-vma-lock.patch