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mm, swap: bounds check swap_info array accesses to avoid NULL derefs

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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 other places in swapfile.c 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.

Link: http://lkml.kernel.org/r/20190131024410.29859-1-daniel.m.jordan@oracle.com
Fixes: ec8acf20af ("swap: add per-partition lock for swapfile")
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Daniel Jordan 2019-03-05 15:48:19 -08:00 committed by Linus Torvalds
parent 060f005f07
commit c10d38cc8d
1 changed files with 29 additions and 22 deletions
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51
mm/swapfile.c
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@ -98,6 +98,15 @@ 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 */
@ -1044,12 +1053,14 @@ 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);
@ -1060,6 +1071,7 @@ 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};
}
@ -1071,9 +1083,9 @@ 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);
@ -1697,10 +1709,9 @@ 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);
}
@ -2151,7 +2162,7 @@ static sector_t map_swap_entry(swp_entry_t entry, struct block_device **bdev)
struct swap_extent *se;
pgoff_t offset;
sis = swap_info[swp_type(entry)];
sis = swp_swap_info(entry);
*bdev = sis->bdev;
offset = swp_offset(entry);
@ -2593,9 +2604,7 @@ static void *swap_start(struct seq_file *swap, loff_t *pos)
if (!l)
return SEQ_START_TOKEN;
for (type = 0; type < nr_swapfiles; type++) {
smp_rmb(); /* read nr_swapfiles before swap_info[type] */
si = swap_info[type];
for (type = 0; (si = swap_type_to_swap_info(type)); type++) {
if (!(si->flags & SWP_USED) || !si->swap_map)
continue;
if (!--l)
@ -2615,9 +2624,7 @@ static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
else
type = si->type + 1;
for (; type < nr_swapfiles; type++) {
smp_rmb(); /* read nr_swapfiles before swap_info[type] */
si = swap_info[type];
for (; (si = swap_type_to_swap_info(type)); type++) {
if (!(si->flags & SWP_USED) || !si->swap_map)
continue;
++*pos;
@ -2724,14 +2731,14 @@ 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];
@ -3251,7 +3258,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
{
struct swap_info_struct *p;
struct swap_cluster_info *ci;
unsigned long offset, type;
unsigned long offset;
unsigned char count;
unsigned char has_cache;
int err = -EINVAL;
@ -3259,10 +3266,10 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
if (non_swap_entry(entry))
goto out;
type = swp_type(entry);
if (type >= nr_swapfiles)
p = swp_swap_info(entry);
if (!p)
goto bad_file;
p = swap_info[type];
offset = swp_offset(entry);
if (unlikely(offset >= p->max))
goto out;
@ -3359,7 +3366,7 @@ int swapcache_prepare(swp_entry_t entry)
struct swap_info_struct *swp_swap_info(swp_entry_t entry)
{
return swap_info[swp_type(entry)];
return swap_type_to_swap_info(swp_type(entry));
}
struct swap_info_struct *page_swap_info(struct page *page)