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BUG: KCSAN: data-race in __anon_vma_prepare / expand_downwards

read to 0xffff88809ebcb9e8 of 8 bytes by task 12018 on cpu 1:
 anon_vma_prepare include/linux/rmap.h:150 [inline]
 expand_downwards+0xd9/0x9b0 mm/mmap.c:2461
 expand_stack mm/mmap.c:2555 [inline]
 find_extend_vma+0xc4/0x140 mm/mmap.c:2576
 __get_user_pages+0x150/0x1160 mm/gup.c:810
 __get_user_pages_locked mm/gup.c:1023 [inline]
 get_user_pages_unlocked+0x221/0x3f0 mm/gup.c:1688
 __gup_longterm_unlocked mm/gup.c:2375 [inline]
 get_user_pages_fast+0x307/0x340 mm/gup.c:2430
 iov_iter_get_pages+0x1d3/0x7c0 lib/iov_iter.c:1287
 af_alg_make_sg+0x58/0x2a0 crypto/af_alg.c:397
 af_alg_get_rsgl+0x1a2/0x470 crypto/af_alg.c:1144
 _aead_recvmsg crypto/algif_aead.c:159 [inline]
 aead_recvmsg+0x2b7/0xca0 crypto/algif_aead.c:330
 sock_recvmsg_nosec net/socket.c:871 [inline]
 sock_recvmsg net/socket.c:889 [inline]
 sock_recvmsg+0x92/0xb0 net/socket.c:885
 ___sys_recvmsg+0x1a0/0x3e0 net/socket.c:2480
 __sys_recvmsg+0x9d/0x160 net/socket.c:2537
 __do_sys_recvmsg net/socket.c:2547 [inline]
 __se_sys_recvmsg net/socket.c:2544 [inline]
 __x64_sys_recvmsg+0x51/0x70 net/socket.c:2544
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff88809ebcb9e8 of 8 bytes by task 12019 on cpu 0:
 __anon_vma_prepare+0xe0/0x2b0 mm/rmap.c:201
 anon_vma_prepare include/linux/rmap.h:153 [inline]
 do_anonymous_page mm/memory.c:3000 [inline]
 handle_pte_fault mm/memory.c:3847 [inline]
 __handle_mm_fault+0x2af3/0x2c70 mm/memory.c:3973
 handle_mm_fault+0x21b/0x530 mm/memory.c:4010
 faultin_page mm/gup.c:640 [inline]
 __get_user_pages+0x485/0x1160 mm/gup.c:845
 populate_vma_page_range+0xe6/0x100 mm/gup.c:1223
 __mm_populate+0x168/0x2a0 mm/gup.c:1271
 mm_populate include/linux/mm.h:2357 [inline]
 vm_mmap_pgoff+0x181/0x190 mm/util.c:501
 ksys_mmap_pgoff+0x99/0x420 mm/mmap.c:1629
 __do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
 __se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
 __x64_sys_mmap+0x2e/0x40 arch/x86/kernel/sys_x86_64.c:91
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 12019 Comm: syz-executor.2 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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