From: Daniel Jordan daniel.m.jordan@oracle.com
[ Upstream commit c10d38cc8d3e43f946b6c2bf4602c86791587f30 ]
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: ec8acf20afb8 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- mm/swapfile.c | 51 +++++++++++++++++++++++++++++---------------------- 1 file changed, 29 insertions(+), 22 deletions(-)
diff --git a/mm/swapfile.c b/mm/swapfile.c index 340ef3177686..0047dcaf9369 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -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 */ @@ -1030,12 +1039,14 @@ 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); @@ -1046,6 +1057,7 @@ 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}; }
@@ -1057,9 +1069,9 @@ 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); @@ -1708,10 +1720,9 @@ 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); } @@ -2269,7 +2280,7 @@ 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); @@ -2707,9 +2718,7 @@ 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) @@ -2729,9 +2738,7 @@ 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; @@ -2838,14 +2845,14 @@ 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]; @@ -3365,7 +3372,7 @@ 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; @@ -3373,10 +3380,10 @@ 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; @@ -3473,7 +3480,7 @@ 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)