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Subject[PATCH -next] fork: annotate a data race in vm_area_dup()
Date
struct vm_area_struct could be accessed concurrently as noticed by
KCSAN,

write to 0xffff9cf8bba08ad8 of 8 bytes by task 14263 on cpu 35:
vma_interval_tree_insert+0x101/0x150:
rb_insert_augmented_cached at include/linux/rbtree_augmented.h:58
(inlined by) vma_interval_tree_insert at mm/interval_tree.c:23
__vma_link_file+0x6e/0xe0
__vma_link_file at mm/mmap.c:629
vma_link+0xa2/0x120
mmap_region+0x753/0xb90
do_mmap+0x45c/0x710
vm_mmap_pgoff+0xc0/0x130
ksys_mmap_pgoff+0x1d1/0x300
__x64_sys_mmap+0x33/0x40
do_syscall_64+0x91/0xc44
entry_SYSCALL_64_after_hwframe+0x49/0xbe

read to 0xffff9cf8bba08a80 of 200 bytes by task 14262 on cpu 122:
vm_area_dup+0x6a/0xe0
vm_area_dup at kernel/fork.c:362
__split_vma+0x72/0x2a0
__split_vma at mm/mmap.c:2661
split_vma+0x5a/0x80
mprotect_fixup+0x368/0x3f0
do_mprotect_pkey+0x263/0x420
__x64_sys_mprotect+0x51/0x70
do_syscall_64+0x91/0xc44
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The write is holding mmap_sem while changing vm_area_struct.shared.rb.
Even though the read is lockless while making a copy, the clone will
have its own shared.rb reinitialized. Thus, mark it as an intentional
data race using the data_race() macro.

Signed-off-by: Qian Cai <cai@lca.pw>
---
kernel/fork.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 41f784b6203a..81bdc6e8a6cf 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -359,7 +359,11 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
struct vm_area_struct *new = kmem_cache_alloc(vm_area_cachep, GFP_KERNEL);

if (new) {
- *new = *orig;
+ /*
+ * @orig may be modified concurrently, but the clone will be
+ * reinitialized.
+ */
+ *new = data_race(*orig);
INIT_LIST_HEAD(&new->anon_vma_chain);
}
return new;
--
1.8.3.1
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