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.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.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@xxxxxxxxxx>
Subject: fork: lock VMAs of the parent process when forking
Date: Wed, 5 Jul 2023 18:13:59 -0700
Patch series "Avoid memory corruption caused by per-VMA locks", v4.
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 ~7% 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.
Patch 2/2 disables per-VMA locks until the fix is tested and verified.
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 ~7%
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/20230706011400.2949242-1-surenb@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20230706011400.2949242-2-surenb@xxxxxxxxxx
Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first")
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Reported-by: Jiri Slaby <jirislaby@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@xxxxxxxxxx/
Reported-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@xxxxxxxxxxxxxxxxxxxxxx/
Reported-by: Jacob Young <jacobly.alt@xxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=3D217624
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/fork.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/kernel/fork.c~fork-lock-vmas-of-the-parent-process-when-forking
+++ 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_set(&old_vmi, 0);
+#endif
flush_cache_dup_mm(oldmm);
uprobe_dup_mmap(oldmm, mm);
/*
_
Patches currently in -mm which might be from surenb@xxxxxxxxxx are
fork-lock-vmas-of-the-parent-process-when-forking.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
