fs/exec.c: fix minor memory leak

fs/exec.c: fix minor memory leak

[Andrew Morton]

Could someone please double-check this?


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().

Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=116841
Reported by: hujunjie <jj.net@163.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/exec.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff -puN fs/exec.c~fs-execc-fix-minor-memory-leak fs/exec.c
--- a/fs/exec.c~fs-execc-fix-minor-memory-leak
+++ a/fs/exec.c
@@ -1482,8 +1482,15 @@ int remove_arg_zero(struct linux_binprm
 		kunmap_atomic(kaddr);
 		put_arg_page(page);
 
-		if (offset == PAGE_SIZE)
+		if (offset == PAGE_SIZE) {
 			free_arg_page(bprm, (bprm->p >> PAGE_SHIFT) - 1);
+		} else if (offset == PAGE_SIZE - 1) {
+			/*
+			 * The trailing '\0' is the last byte in a page - we're
+			 * about to advance past that byte so free its page now
+			 */
+			free_arg_page(bprm, (bprm->p >> PAGE_SHIFT));
+		}
 	} while (offset == PAGE_SIZE);
 
 	bprm->p++;
_

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

[Vlastimil Babka]

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().
>
> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=116841
> Reported by: hujunjie <jj.net@163.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>
>   fs/exec.c |    9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff -puN fs/exec.c~fs-execc-fix-minor-memory-leak fs/exec.c
> --- a/fs/exec.c~fs-execc-fix-minor-memory-leak
> +++ a/fs/exec.c
> @@ -1482,8 +1482,15 @@ int remove_arg_zero(struct linux_binprm
>   		kunmap_atomic(kaddr);
>   		put_arg_page(page);
>
> -		if (offset == PAGE_SIZE)
> +		if (offset == PAGE_SIZE) {
>   			free_arg_page(bprm, (bprm->p >> PAGE_SHIFT) - 1);
> +		} else if (offset == PAGE_SIZE - 1) {
> +			/*
> +			 * The trailing '\0' is the last byte in a page - we're
> +			 * about to advance past that byte so free its page now
> +			 */
> +			free_arg_page(bprm, (bprm->p >> PAGE_SHIFT));
> +		}
>   	} while (offset == PAGE_SIZE);
>
>   	bprm->p++;
> _
>
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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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