Re: fs/exec.c: fix minor memory leak

Re: fs/exec.c: fix minor memory leak

[Oleg Nesterov]

Andrew, Vlastimil,

I found this patch by accident when I was looking at http://marc.info/?l=linux-mm
and I can't resist ;)

> On 04/21/2016 11:15 PM, Andrew Morton wrote:
> >
> > Could someone please double-check this?
>
> Looks OK to me.
>
> > From: Andrew Morton <akpm@linux-foundation.org>
> > Subject: fs/exec.c: fix minor memory leak
> >
> > When the to-be-removed argument's trailing '\0' is the final byte in the
> > page, remove_arg_zero()'s logic will avoid freeing the page, will break
> > from the loop and will then advance bprm->p to point at the first byte in
> > the next page.  Net result: the final page for the zeroeth argument is
> > unfreed.
> >
> > It isn't a very important leak - that page will be freed later by the
> > bprm-wide sweep in free_arg_pages().

And so I think we should just remove this free_arg_page(), it (and the patch)
only adds the unnecessary confusion.

Note that today free_arg_page() is nop if CONFIG_MMU. At the same time, the
only reason for this free_arg_page() was that (until the commit b6a2fea39)
CONFIG_MMU did install_arg_page() for every page != NULL in bprm->page[].

So we simply do not need it today. And note that the caller is going to do
copy_strings_kernel(), so if we do free_arg_page() with CONFIG_MMU=n we will
likely have to re-allocate this page right after free.

And note that this code is actually wrong! remove_arg_zero() assumes that
argv[0] is null-terminated but this is not necessarily true. copy_strings()
does:

	len = strnlen_user(...);
	...
	copy_from_user(..., len);

another thread or debugger can change the memory in between. Fortunately
nothing really bad can happen (afaics) even if CONFIG_MMU=n, bprm->filename
must be always zero-terminated and it was copied by the 1st copy_strings_kernel().
Still perhaps it makes sense to check "bprm->p < bprm->exec" in the main loop.

Oleg.

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Re: fs/exec.c: fix minor memory leak

[Andrew Morton]

On Mon, 16 May 2016 22:43:39 +0200 Oleg Nesterov <oleg@redhat.com> wrote:

> Andrew, Vlastimil,
> 
> I found this patch by accident when I was looking at http://marc.info/?l=linux-mm
> and I can't resist ;)
> 
> > On 04/21/2016 11:15 PM, Andrew Morton wrote:
> > >
> > > Could someone please double-check this?
> >
> > Looks OK to me.
> >
> > > From: Andrew Morton <akpm@linux-foundation.org>
> > > Subject: fs/exec.c: fix minor memory leak
> > >
> > > When the to-be-removed argument's trailing '\0' is the final byte in the
> > > page, remove_arg_zero()'s logic will avoid freeing the page, will break
> > > from the loop and will then advance bprm->p to point at the first byte in
> > > the next page.  Net result: the final page for the zeroeth argument is
> > > unfreed.
> > >
> > > It isn't a very important leak - that page will be freed later by the
> > > bprm-wide sweep in free_arg_pages().
> 
> And so I think we should just remove this free_arg_page(), it (and the patch)
> only adds the unnecessary confusion.
> 

Send patch :)

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[PATCH] exec: remove the no longer needed remove_arg_zero()->free_arg_page()

[Oleg Nesterov]

remove_arg_zero() does free_arg_page() for no reason. This was needed
before and only if CONFIG_MMU=y: see the commit 4fc75ff4 ("exec: fix
remove_arg_zero"), install_arg_page() was called for every page != NULL
in bprm->page[] array. Today install_arg_page() has already gone and
free_arg_page() is nop after another commit b6a2fea39 ("mm: variable
length argument support").

CONFIG_MMU=n does free_arg_pages() in free_bprm() and thus it doesn't
need remove_arg_zero()->free_arg_page() too; apart from get_arg_page()
it never checks if the page in bprm->page[] was allocated or not, so
the "extra" non-freed page is fine. OTOH, this free_arg_page() can add
the minor pessimization, the caller is going to do copy_strings_kernel()
right after remove_arg_zero() which will likely need to re-allocate the
same page again.

And as Hujunjie pointed out, the "offset == PAGE_SIZE" check is wrong
because we are going to increment bprm->p once again before return, so
CONFIG_MMU=n "leaks" the page anyway if '\0' is the final byte in this
page.

NOTE: remove_arg_zero() assumes that argv[0] is null-terminated but this
is not necessarily true. copy_strings() does "len = strnlen_user(...)",
then copy_from_user(len) but another thread or debuger can overwrite the
trailing '\0' in between. Afaics nothing really bad can happen because
we must always have the null-terminated bprm->filename copied by the 1st
copy_strings_kernel(), but perhaps we should change this code to check
"bprm->p < bprm->exec" anyway, and/or change copy_strings() to ensure
that the last byte in string is always zero.

Reported by: hujunjie <jj.net@163.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/exec.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index c4010b8..9b85c4d 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -243,10 +243,6 @@ static void put_arg_page(struct page *page)
 	put_page(page);
 }
 
-static void free_arg_page(struct linux_binprm *bprm, int i)
-{
-}
-
 static void free_arg_pages(struct linux_binprm *bprm)
 {
 }
@@ -1481,9 +1477,6 @@ int remove_arg_zero(struct linux_binprm *bprm)
 
 		kunmap_atomic(kaddr);
 		put_arg_page(page);
-
-		if (offset == PAGE_SIZE)
-			free_arg_page(bprm, (bprm->p >> PAGE_SHIFT) - 1);
 	} while (offset == PAGE_SIZE);
 
 	bprm->p++;
-- 
2.5.0


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