[PATCH 1/1] MM: swapfile, fix crash on double swapon

[PATCH 1/1] MM: swapfile, fix crash on double swapon

[Jiri Slaby]

Double swapon on a device causes a crash:
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
PGD 1dc0b067 PUD 1dc09067 PMD 0
Oops: 0000 [#1] SMP
last sysfs file:
CPU 1
Modules linked in:
Pid: 562, comm: swapon Tainted: G        W  2.6.32-rc5-mm1_64 #867
RIP: 0010:[<ffffffff810af160>]  [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
...

It is due to swap_info_struct->first_swap_extent.list not being
initialized. ->next is NULL in such a situation and
destroy_swap_extents fails to iterate over the list with the BUG
above.

Introduced by swap_info-include-first_swap_extent.patch. Revert the
INIT_LIST_HEAD move.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
---
 mm/swapfile.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 93e71cf..26ef6a2 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1313,7 +1313,6 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
 	if (start_page == 0) {
 		se = &sis->first_swap_extent;
 		sis->curr_swap_extent = se;
-		INIT_LIST_HEAD(&se->list);
 		se->start_page = 0;
 		se->nr_pages = nr_pages;
 		se->start_block = start_block;
@@ -1769,6 +1768,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		kfree(p);
 		goto out;
 	}
+	INIT_LIST_HEAD(&p->first_swap_extent.list);
 	if (type >= nr_swapfiles) {
 		p->type = type;
 		swap_info[type] = p;
-- 
1.6.4.2

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Re: [PATCH 1/1] MM: swapfile, fix crash on double swapon

[Hugh Dickins]

On Mon, 2 Nov 2009, Jiri Slaby wrote:

> Double swapon on a device causes a crash:
> BUG: unable to handle kernel NULL pointer dereference at (null)

Thanks a lot for finding that: it doesn't just happen with a double
swapon of the same device, it happens with most kinds of error in
the swapon sequence.  I thought I was being nice and tidy moving
that initialization, but actually I was just being careless.

> IP: [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
> PGD 1dc0b067 PUD 1dc09067 PMD 0
> Oops: 0000 [#1] SMP
> last sysfs file:
> CPU 1
> Modules linked in:
> Pid: 562, comm: swapon Tainted: G        W  2.6.32-rc5-mm1_64 #867
> RIP: 0010:[<ffffffff810af160>]  [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
> ...
> 
> It is due to swap_info_struct->first_swap_extent.list not being
> initialized. ->next is NULL in such a situation and
> destroy_swap_extents fails to iterate over the list with the BUG
> above.
> 
> Introduced by swap_info-include-first_swap_extent.patch. Revert the
> INIT_LIST_HEAD move.
> 
> Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Rik van Riel <riel@redhat.com>

Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

> ---
>  mm/swapfile.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 93e71cf..26ef6a2 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1313,7 +1313,6 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
>  	if (start_page == 0) {
>  		se = &sis->first_swap_extent;
>  		sis->curr_swap_extent = se;
> -		INIT_LIST_HEAD(&se->list);
>  		se->start_page = 0;
>  		se->nr_pages = nr_pages;
>  		se->start_block = start_block;
> @@ -1769,6 +1768,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  		kfree(p);
>  		goto out;
>  	}
> +	INIT_LIST_HEAD(&p->first_swap_extent.list);
>  	if (type >= nr_swapfiles) {
>  		p->type = type;
>  		swap_info[type] = p;
> -- 
> 1.6.4.2

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[PATCH mmotm] fix to swap_info-include-first_swap_extent-fix.patch

[Hugh Dickins]

Sorry, just noticed what the diff contexts don't show: Jiri's patch
is initializing p->first_swap_extent.list at a point before p has
been decided - we may kfree that newly allocated p and go on to
reuse an existing free entry for p.

Now, the patch is not actually wrong: an existing free entry will have
a good empty first_swap_extent.list; but it looks suspicious, it seems
strange to initialize a field in something we're about to kfree, and
I'd rather we put that initialization back to where it was in 2.6.32.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
---

 mm/swapfile.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- mmotm/mm/swapfile.c	2009-11-03 18:05:36.000000000 +0000
+++ linux/mm/swapfile.c	2009-11-03 18:08:26.000000000 +0000
@@ -1768,7 +1768,6 @@ SYSCALL_DEFINE2(swapon, const char __use
 		kfree(p);
 		goto out;
 	}
-	INIT_LIST_HEAD(&p->first_swap_extent.list);
 	if (type >= nr_swapfiles) {
 		p->type = type;
 		swap_info[type] = p;
@@ -1787,6 +1786,7 @@ SYSCALL_DEFINE2(swapon, const char __use
 		 * would be relying on p->type to remain valid.
 		 */
 	}
+	INIT_LIST_HEAD(&p->first_swap_extent.list);
 	p->flags = SWP_USED;
 	p->next = -1;
 	spin_unlock(&swap_lock);

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