mm: fix potential data race in SyS_swapon

While running KernelThreadSanitizer (ktsan) on upstream kernel with
trinity, we got a few reports from SyS_swapon, here is one of them:

Read of size 8 by thread T307 (K7621):
 [<     inlined    >] SyS_swapon+0x3c0/0x1850 SYSC_swapon mm/swapfile.c:2395
 [<ffffffff812242c0>] SyS_swapon+0x3c0/0x1850 mm/swapfile.c:2345
 [<ffffffff81e97c8a>] ia32_do_call+0x1b/0x25

Looks like the swap_lock should be taken when iterating through the
swap_info array on lines 2392 - 2401: q->swap_file may be reset to
NULL by another thread before it is dereferenced for f_mapping.

But why is that iteration needed at all?  Doesn't the claim_swapfile()
which follows do all that is needed to check for a duplicate entry -
FMODE_EXCL on a bdev, testing IS_SWAPFILE under i_mutex on a regfile?

Well, not quite: bd_may_claim() allows the same "holder" to claim the
bdev again, so we do need to use a different holder than "sys_swapon";
and we should not replace appropriate -EBUSY by inappropriate -EINVAL.

Index i was reused in a cpu loop further down: renamed cpu there.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 7 insertions, 18 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 41e4581af7c5..aebc2dd6e649 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2143,11 +2143,10 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
         if (S_ISBLK(inode->i_mode)) {
                 p->bdev = bdgrab(I_BDEV(inode));
                 error = blkdev_get(p->bdev,
-                                   FMODE_READ | FMODE_WRITE | FMODE_EXCL,
-                                   sys_swapon);
+                                   FMODE_READ | FMODE_WRITE | FMODE_EXCL, p);
                 if (error < 0) {
                         p->bdev = NULL;
-                        return -EINVAL;
+                        return error;
                 }
                 p->old_block_size = block_size(p->bdev);
                 error = set_blocksize(p->bdev, PAGE_SIZE);
@@ -2348,7 +2347,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
         struct filename *name;
         struct file *swap_file = NULL;
         struct address_space *mapping;
-        int i;
         int prio;
         int error;
         union swap_header *swap_header;
@@ -2388,19 +2386,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
         p->swap_file = swap_file;
         mapping = swap_file->f_mapping;
-
-        for (i = 0; i < nr_swapfiles; i++) {
-                struct swap_info_struct *q = swap_info[i];
-
-                if (q == p || !q->swap_file)
-                        continue;
-                if (mapping == q->swap_file->f_mapping) {
-                        error = -EBUSY;
-                        goto bad_swap;
-                }
-        }
-
         inode = mapping->host;
+
         /* If S_ISREG(inode->i_mode) will do mutex_lock(&inode->i_mutex); */
         error = claim_swapfile(p, inode);
         if (unlikely(error))
@@ -2433,6 +2420,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
                 goto bad_swap;
         }
         if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
+                int cpu;
+
                 p->flags |= SWP_SOLIDSTATE;
                 /*
                  * select a random position to start with to help wear leveling
@@ -2451,9 +2440,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
                         error = -ENOMEM;
                         goto bad_swap;
                 }
-                for_each_possible_cpu(i) {
+                for_each_possible_cpu(cpu) {
                         struct percpu_cluster *cluster;
-                        cluster = per_cpu_ptr(p->percpu_cluster, i);
+                        cluster = per_cpu_ptr(p->percpu_cluster, cpu);
                         cluster_set_null(&cluster->index);
                 }
         }