mm: /proc/pid/smaps_rollup: fix no vma's null-deref

stable inclusion
from stable-v4.19.264
commit dbe863bce7679c7f5ec0e993d834fe16c5e687b5
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I63UEU
CVE: NA

--------------------------------

Commit 258f669e ("mm: /proc/pid/smaps_rollup: convert to single value
seq_file") introduced a null-deref if there are no vma's in the task in
show_smaps_rollup.

Fixes: 258f669e ("mm: /proc/pid/smaps_rollup: convert to single value seq_file")
Signed-off-by: NSeth Jenkins <sethjenkins@google.com>
Reviewed-by: NAlexey Dobriyan <adobriyan@gmail.com>
Tested-by: NAlexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

fs/proc/task_mmu.c

@@ -851,7 +851,7 @@ static int show_smaps_rollup(struct seq_file *m, void *v)
 		last_vma_end = vma->vm_end;
 	}
 
-	show_vma_header_prefix(m, priv->mm->mmap->vm_start,
+	show_vma_header_prefix(m, priv->mm->mmap ? priv->mm->mmap->vm_start : 0,
 			       last_vma_end, 0, 0, 0, 0);
 	seq_pad(m, ' ');
 	seq_puts(m, "[rollup]\n");