Re: [PATCH] mm, swap: bounds check swap_info accesses to avoid NULL derefs

Andrea Parri
2019-01-15T01:17:12Z

On Mon, Jan 14, 2019 at 07:23:05PM -0500, Daniel Jordan wrote:
> Dan Carpenter reports a potential NULL dereference in
> get_swap_page_of_type:
>
> Smatch complains that the NULL checks on "si" aren't consistent. This
> seems like a real bug because we have not ensured that the type is
> valid and so "si" can be NULL.
>
> Add the missing check for NULL, taking care to use a read barrier to
> ensure CPU1 observes CPU0's updates in the correct order:
>
> CPU0 CPU1
> alloc_swap_info() if (type >= nr_swapfiles)
> swap_info[type] = p /* handle invalid entry */
> smp_wmb() smp_rmb()
> ++nr_swapfiles p = swap_info[type]
>
> Without smp_rmb, CPU1 might observe CPU0's write to nr_swapfiles before
> CPU0's write to swap_info[type] and read NULL from swap_info[type].
>
> Ying Huang noticed that other places don't order these reads properly.
> Introduce swap_type_to_swap_info to encourage correct usage.
>
> Use READ_ONCE and WRITE_ONCE to follow the Linux Kernel Memory Model
> (see tools/memory-model/Documentation/explanation.txt).
>
> This ordering need not be enforced in places where swap_lock is held
> (e.g. si_swapinfo) because swap_lock serializes updates to nr_swapfiles
> and the swap_info array.
>
> This is a theoretical problem, no actual reports of it exist.
>
> Fixes: ec8acf20afb8 ("swap: add per-partition lock for swapfile")
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Omar Sandoval <osandov@fb.com>
> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Shaohua Li <shli@kernel.org>
> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Will Deacon <will.deacon@arm.com>

Please feel free to add:

Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>

Andrea

>
> ---
>
> I'd appreciate it if someone more familiar with memory barriers could
> check this over. Thanks.
>
> Probably no need for stable, this is all theoretical.
>
> Against linux-mmotm tag v5.0-rc1-mmotm-2019-01-09-13-40
>
> mm/swapfile.c | 43 +++++++++++++++++++++++++++----------------
> 1 file changed, 27 insertions(+), 16 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index f0edf7244256..dad52fc67045 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -99,6 +99,15 @@ static atomic_t proc_poll_event = ATOMIC_INIT(0);
>
> atomic_t nr_rotate_swap = ATOMIC_INIT(0);
>
> +static struct swap_info_struct *swap_type_to_swap_info(int type)
> +{
> + if (type >= READ_ONCE(nr_swapfiles))
> + return NULL;
> +
> + smp_rmb(); /* Pairs with smp_wmb in alloc_swap_info. */
> + return READ_ONCE(swap_info[type]);
> +}
> +
> static inline unsigned char swap_count(unsigned char ent)
> {
> return ent & ~SWAP_HAS_CACHE; /* may include COUNT_CONTINUED flag */
> @@ -1045,12 +1054,14 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> /* The only caller of this function is now suspend routine */
> swp_entry_t get_swap_page_of_type(int type)
> {
> - struct swap_info_struct *si;
> + struct swap_info_struct *si = swap_type_to_swap_info(type);
> pgoff_t offset;
>
> - si = swap_info[type];
> + if (!si)
> + goto fail;
> +
> spin_lock(&si->lock);
> - if (si && (si->flags & SWP_WRITEOK)) {
> + if (si->flags & SWP_WRITEOK) {
> atomic_long_dec(&nr_swap_pages);
> /* This is called for allocating swap entry, not cache */
> offset = scan_swap_map(si, 1);
> @@ -1061,6 +1072,7 @@ swp_entry_t get_swap_page_of_type(int type)
> atomic_long_inc(&nr_swap_pages);
> }
> spin_unlock(&si->lock);
> +fail:
> return (swp_entry_t) {0};
> }
>
> @@ -1072,9 +1084,9 @@ static struct swap_info_struct *__swap_info_get(swp_entry_t entry)
> if (!entry.val)
> goto out;
> type = swp_type(entry);
> - if (type >= nr_swapfiles)
> + p = swap_type_to_swap_info(type);
> + if (!p)
> goto bad_nofile;
> - p = swap_info[type];
> if (!(p->flags & SWP_USED))
> goto bad_device;
> offset = swp_offset(entry);
> @@ -1212,9 +1224,9 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
> if (!entry.val)
> goto out;
> type = swp_type(entry);
> - if (type >= nr_swapfiles)
> + si = swap_type_to_swap_info(type);
> + if (!si)
> goto bad_nofile;
> - si = swap_info[type];
>
> preempt_disable();
> if (!(si->flags & SWP_VALID))
> @@ -1765,10 +1777,9 @@ int swap_type_of(dev_t device, sector_t offset, struct block_device **bdev_p)
> sector_t swapdev_block(int type, pgoff_t offset)
> {
> struct block_device *bdev;
> + struct swap_info_struct *si = swap_type_to_swap_info(type);
>
> - if ((unsigned int)type >= nr_swapfiles)
> - return 0;
> - if (!(swap_info[type]->flags & SWP_WRITEOK))
> + if (!si || !(si->flags & SWP_WRITEOK))
> return 0;
> return map_swap_entry(swp_entry(type, offset), &bdev);
> }
> @@ -2799,9 +2810,9 @@ static void *swap_start(struct seq_file *swap, loff_t *pos)
> if (!l)
> return SEQ_START_TOKEN;
>
> - for (type = 0; type < nr_swapfiles; type++) {
> + for (type = 0; type < READ_ONCE(nr_swapfiles); type++) {
> smp_rmb(); /* read nr_swapfiles before swap_info[type] */
> - si = swap_info[type];
> + si = READ_ONCE(swap_info[type]);
> if (!(si->flags & SWP_USED) || !si->swap_map)
> continue;
> if (!--l)
> @@ -2821,9 +2832,9 @@ static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
> else
> type = si->type + 1;
>
> - for (; type < nr_swapfiles; type++) {
> + for (; type < READ_ONCE(nr_swapfiles); type++) {
> smp_rmb(); /* read nr_swapfiles before swap_info[type] */
> - si = swap_info[type];
> + si = READ_ONCE(swap_info[type]);
> if (!(si->flags & SWP_USED) || !si->swap_map)
> continue;
> ++*pos;
> @@ -2930,14 +2941,14 @@ static struct swap_info_struct *alloc_swap_info(void)
> }
> if (type >= nr_swapfiles) {
> p->type = type;
> - swap_info[type] = p;
> + WRITE_ONCE(swap_info[type], p);
> /*
> * Write swap_info[type] before nr_swapfiles, in case a
> * racing procfs swap_start() or swap_next() is reading them.
> * (We never shrink nr_swapfiles, we never free this entry.)
> */
> smp_wmb();
> - nr_swapfiles++;
> + WRITE_ONCE(nr_swapfiles, nr_swapfiles + 1);
> } else {
> kvfree(p);
> p = swap_info[type];
> --
> 2.20.0
>