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Subject[RFC PATCH] mm: fork: Prevent a NULL deref by getting mm only if the refcount isn't 0
Date
We've seen a number of crashes with the following signature:

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
...
Oops: 0000 [#1] SMP PTI
...
RIP: 0010:__rb_erase_color+0xc2/0x260
...
Call Trace:
unlink_file_vma+0x36/0x50
free_pgtables+0x62/0x110
exit_mmap+0xd5/0x160
? put_dec+0x3a/0x90
? num_to_str+0xa8/0xc0
mmput+0x11/0xb0
do_task_stat+0x940/0xc80
proc_single_show+0x49/0x80
? __check_object_size+0xcc/0x1a0
seq_read+0xd3/0x400
vfs_read+0x72/0xb0
ksys_read+0x9c/0xd0
do_syscall_64+0x69/0x400
? schedule+0x2a/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xa9
...

This happens when a process goes through the tasks stats in procfs while
another is exiting. This looks like a race where the process that's
exiting drops the last reference on the mm (with mmput) while the other
increases it (with mmget). By only increasing when the reference isn't
0 to begin with, we prevent this from happening.

Signed-off-by: Filippo Sironi <sironi@amazon.de>
---
kernel/fork.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index d3171e8e88e5..a7541a85e5a9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1209,10 +1209,8 @@ struct mm_struct *get_task_mm(struct task_struct *task)
task_lock(task);
mm = task->mm;
if (mm) {
- if (task->flags & PF_KTHREAD)
+ if (task->flags & PF_KTHREAD || !mmget_not_zero(mm))
mm = NULL;
- else
- mmget(mm);
}
task_unlock(task);
return mm;
--
2.17.1



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