2b4f3b4987
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@google.com Link: https://lkml.kernel.org/r/20230706011400.2949242-2-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@kernel.org/ Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com> Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/ Reported-by: Jacob Young <jacobly.alt@gmail.com> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=3D217624 Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com> Acked-by: David Hildenbrand <david@redhat.com> Tested-by: Holger Hoffsttte <holger@applied-asynchrony.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.