On 08/24, Eric Biggers wrote:

On Thu, Aug 24, 2017 at 03:20:41PM +0200, Oleg Nesterov wrote:

quoted

On 08/23, Eric Biggers wrote:

quoted

From: Eric Biggers <redacted>

Commit 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for
write killable") made it possible to kill a forking task while it is
waiting to acquire its ->mmap_sem for write, in dup_mmap().  However, it
was overlooked that this introduced an new error path before a reference
is taken on the mm_struct's ->exe_file.

Hmm. Unless I am totally confused, the same problem with mm->exol_area?
I'll recheck....

I'm not sure what you mean by ->exol_area.

I meant mm->uprobes_state.xol_area, sorry

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--- a/kernel/fork.c
+++ b/kernel/fork.c

@@ -806,6 +806,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	mm_init_cpumask(mm);
 	mm_init_aio(mm);
 	mm_init_owner(mm, p);
+	RCU_INIT_POINTER(mm->exe_file, NULL);

Can't we simply move

	RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));

from dup_mmap() here? Afaics this doesn't need mmap_sem.

Two problems, even assuming that get_mm_exe_file() doesn't require mmap_sem:

- If mm_alloc_pgd() or init_new_context() in mm_init() fails, mm_init() doesn't
  do the full mmput(), so the file reference would not be dropped.  So it would
  need to be changed to drop the file reference too.

Ah yes, I forgot that mm_init() can fail after that, thanks for correcting me.

Oleg.