path: root/fs/sysfs
diff options
authorNeilBrown <>2017-04-03 11:30:34 +1000
committerGreg Kroah-Hartman <>2017-04-08 17:33:32 +0200
commitc8a139d001a1aab1ea8734db14b22dac9dd143b6 (patch)
treec036998a6ca7946d3bac2485838245edac07ee8d /fs/sysfs
parentcf903e9d3a97f89b224d2d07be37c0f160db8192 (diff)
sysfs: be careful of error returns from ops->show()
ops->show() can return a negative error code. Commit 65da3484d9be ("sysfs: correctly handle short reads on PREALLOC attrs.") (in v4.4) caused this to be stored in an unsigned 'size_t' variable, so errors would look like large numbers. As a result, if an error is returned, sysfs_kf_read() will return the value of 'count', typically 4096. Commit 17d0774f8068 ("sysfs: correctly handle read offset on PREALLOC attrs") (in v4.8) extended this error to use the unsigned large 'len' as a size for memmove(). Consequently, if ->show returns an error, then the first read() on the sysfs file will return 4096 and could return uninitialized memory to user-space. If the application performs a subsequent read, this will trigger a memmove() with extremely large count, and is likely to crash the machine is bizarre ways. This bug can currently only be triggered by reading from an md sysfs attribute declared with __ATTR_PREALLOC() during the brief period between when mddev_put() deletes an mddev from the ->all_mddevs list, and when mddev_delayed_delete() - which is scheduled on a workqueue - completes. Before this, an error won't be returned by the ->show() After this, the ->show() won't be called. I can reproduce it reliably only by putting delay like usleep_range(500000,700000); early in mddev_delayed_delete(). Then after creating an md device md0 run echo clear > /sys/block/md0/md/array_state; cat /sys/block/md0/md/array_state The bug can be triggered without the usleep. Fixes: 65da3484d9be ("sysfs: correctly handle short reads on PREALLOC attrs.") Fixes: 17d0774f8068 ("sysfs: correctly handle read offset on PREALLOC attrs") Cc: Signed-off-by: NeilBrown <> Acked-by: Tejun Heo <> Reported-and-tested-by: Miroslav Benes <> Signed-off-by: Greg Kroah-Hartman <>
Diffstat (limited to 'fs/sysfs')
1 files changed, 4 insertions, 2 deletions
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index b803213d1307..39c75a86c67f 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -108,7 +108,7 @@ static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
const struct sysfs_ops *ops = sysfs_file_ops(of->kn);
struct kobject *kobj = of->kn->parent->priv;
- size_t len;
+ ssize_t len;
* If buf != of->prealloc_buf, we don't know how
@@ -117,13 +117,15 @@ static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
if (WARN_ON_ONCE(buf != of->prealloc_buf))
return 0;
len = ops->show(kobj, of->kn->priv, buf);
+ if (len < 0)
+ return len;
if (pos) {
if (len <= pos)
return 0;
len -= pos;
memmove(buf, buf + pos, len);
- return min(count, len);
+ return min_t(ssize_t, count, len);
/* kernfs write callback for regular sysfs files */