[RFC PATCH] mm: fork: Prevent a NULL deref by getting mm only if the refcount isn't 0

[Filippo Sironi <sironi@amazon.de>]

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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